ART

THE

CAMBRIDGE NATURAL HISTORY

EDITED BY

S. F. HARMER, Sc.D., F.R.S., Fellow of King's College, Cambridge; Superintendent of the University Museum of Zoology

AND

A. E. SHIPLEY, M.A., F.R.S., Fellow of Christ's College, Cambridge; University Lecturer on the Morphology of Invertebrates

VOLUME I

Macmillan & Co. Publisher's Mark

PROTOZOA

By Marcus Hartog, M.A., Trinity College (D.Sc. Lond.), Professor of Natural History in the Queen's College, Cork

PORIFERA (SPONGES)

By Igerna B. J. Sollas, B.Sc. (Lond.), Lecturer on Zoology at Newnham College, Cambridge

COELENTERATA & CTENOPHORA

By S. J. Hickson, M.A., F.R.S., formerly Fellow and now Honorary Fellow of Downing College, Cambridge; Beyer Professor of Zoology in the Victoria University of Manchester

ECHINODERMATA

By E. W. Macbride, M.A., F.R.S., formerly Fellow of St. John's College, Cambridge; Professor of Zoology in McGill University, Montreal

London
MACMILLAN AND CO., Limited
NEW YORK: THE MACMILLAN COMPANY
1906

All rights reserved

And pitch down his basket before us,

All trembling alive

With pink and grey jellies, your sea-fruit;

You touch the strange lumps,

And mouths gape there, eyes open, all manner

Of horns and of humps.

Browning, The Englishman in Italy

{v}

CONTENTS

PAGE
Scheme of the Classification adopted in this Book ix
PROTOZOA
CHAPTER I
Protozoa—Introduction—Functions of Protoplasm—Cell-division—Animals and Plants 3
CHAPTER II
Protozoa (continued): Spontaneous Generation—Characters of Protozoa—Classification 42
CHAPTER III
Protozoa (continued): Sarcodina 51
CHAPTER IV
Protozoa (continued): Sporozoa 94
CHAPTER V
Protozoa (continued): Flagellata 109
CHAPTER VI
Protozoa (continued): Infusoria (Ciliata and Suctoria) 136
{vi}

PORIFERA (SPONGES)

CHAPTER VII
Porifera (Sponges)—Introduction—History—Description of Halichondria Panicea as an Example of British marine Sponges and of Ephydatia Fluviatilis from Fresh Water—Definition—Position in the Animal Kingdom 165
CHAPTER VIII
Porifera (continued): Forms of Spicules—Calcarea—Homocoela—Heterocoela—Hexactinellida—Demospongiae—Tetractinellida—Monaxonida—Ceratosa—Key to British Genera of Sponges 183
CHAPTER IX
Porifera (continued): Reproduction, Sexual and Asexual—Physiology—Distribution—Flints 226
COELENTERATA
CHAPTER X
Coelenterata—Introduction—Classification—Hydrozoa—Eleutheroblastea—Milleporina—Gymnoblastea—Calyptoblastea—Graptolitoidea—Stylasterina 245
CHAPTER XI
Hydrozoa (continued): Trachomedusae—Narcomedusae—Siphonophora 288
CHAPTER XII
Coelenterata (continued): Scyphozoa = Scyphomedusae 310
CHAPTER XIII
Coelenterata (continued): Anthozoa = Actinozoa—General Characters—Alcyonaria 326
CHAPTER XIV
Anthozoa (continued): Zoantharia 365
{vii}

CTENOPHORA

CHAPTER XV
Ctenophora 412
ECHINODERMATA
CHAPTER XVI
Echinodermata—Introduction—Classification—Anatomy of a Starfish—Systematic Account of Asteroidea 427
CHAPTER XVII
Echinodermata (continued): Ophiuroidea = Brittle Stars 477
CHAPTER XVIII
Echinodermata (continued): Echinoidea = Sea-Urchins 503
CHAPTER XIX
Echinodermata (continued): Holothuroidea = Sea-Cucumbers 560
CHAPTER XX
Echinodermata (continued): Pelmatozoa—Crinoidea = Sea-Lilies—Thecoidea—Carpoidea—Cystoidea—Blastoidea 579
CHAPTER XXI
Echinodermata (continued): Development and Phylogeny 601
INDEX 625
{ix}

SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK

The names of extinct groups are printed in italics.

PROTOZOA (pp. 1, 48).
SARCODINA (p. 51) Rhizopoda (p. 51)

Lobosa (p. 51).

Filosa (p. 52).

Foraminifera (p. 58)

Allogromidiaceae (p. 58).

Astrorhizidaceae (p. 59).

Lituolidaceae (p. 59).

Miliolidaceae (p. 59).

Textulariaceae (p. 59).

Cheilostomellaceae (p. 59).

Lagenaceae (p. 59).

Globigerinidae (p. 59).

Rotaliaceae (p. 59).

Nummulitaceae (p. 59).

Heliozoa (p. 70)

Aphrothoraca (p. 70).

Chlamydophora (p. 71).

Chalarothoraca (p. 71).

Desmothoraca (p. 71).

Radiolaria (p. 75) Porulosa = Holotrypasta (p. 76) Spumellaria = Peripylaea (pp. 76, 77) Collodaria (p. 77)

Colloidea (p. 77).

Beloidea (p. 77).

Sphaerellaria (p. 77)

Sphaeroidea (p. 77).

Prunoidea (p. 77).

Discoidea (p. 77).

Larcoidea (p. 77).

Acantharia = Actipylaea (pp. 76, 78)

Actinelida (p. 78).

Acanthonida (p. 78).

Sphaerophracta (p. 78).

Prunophracta (p. 78).

Osculosa = Monotrypasta (p. 76) Nassellaria = Monopylaea (pp. 76, 78)

Nassoidea (p. 78).

Plectoidea (p. 78).

Stephoidea (p. 78).

Spyroidea (p. 78).

Botryoidea (p. 79).

Cyrtoidea (p. 79).

Phaeodaria = Cannopylaea = Tripylaea (pp. 76, 79)

Phaeocystina (p. 79).

Phaeosphaeria (p. 79).

Phaeogromia (p. 79).

Phaeoconchia (p. 79).

Proteomyxa (p. 88) Myxoidea (p. 89)

Zoosporeae (p. 89).

Azoosporeae (p. 89).

Catallacta (p. 89).
Mycetozoa (p. 90)

Acrasieae (p. 90).

Filoplasmodieae (p. 90).

Myxomycetes (pp. 90, 91).

 
SPOROZOA (p. 94) Telosporidia (p. 97) Gregarinidaceae (pp. 97, 98)

Schizogregarinidae (p. 97).

Acephalinidae (p. 97).

Dicystidae (p. 97).

Coccidiaceae (pp. 97, 99)

Coccidiidae (pp. 97, 99).

Haemosporidae (pp. 97, 102).

Acystosporidae (pp. 97, 102).

Neosporidia (p. 97)

Myxosporidiaceae (pp. 98, 106).

Actinomyxidiaceae (p. 98).

Sarcosporidiaceae (pp. 98, 108).

 
FLAGELLATA (p. 109) Pantostomata (p. 109).
Protomastigaceae (p. 110)

Distomatidae (p. 110).

Oikomonadidae (p. 111).

Bicoecidae (p. 111).

Craspedomonadidae (pp. 111, 121).

Phalansteridae (p. 111).

Monadidae (p. 111).

Bodonidae (p. 111).

Amphimonadidae (p. 111).

Trimastigidae (p. 111).

Polymastigidae (p. 111).

Trichonymphidae (pp. 111, 123).

Opalinidae (pp. 111, 123).

Chrysomonadaceae (pp. 110, 125) Coccolithophoridae (p. 114).
Cryptomonadaceae (p. 110).
Volvocaceae (pp. 110, 111)

Chlamydomonadidae (pp. 111, 125).

Volvocidae (pp. 111, 126).

Chloromonadaceae (p. 110).

Euglenaceae (pp. 110, 124).

Silicoflagellata (pp. 110, 114).

Cystoflagellata (pp. 110, 132).

Dinoflagellata (pp. 110, 130).

 

INFUSORIA (p. 136)

Ciliata (p. 137)

Gymnostomaceae (pp. 137, 152).

Aspirotrichaceae (pp. 137, 153).

Heterotrichaceae (pp. 137, 153).

Oligotrichaceae (pp. 137, 155).

Hypotrichaceae (pp. 137, 138).

Peritrichaceae (pp. 138, 155).

Suctoria = Tentaculifera (p. 158).
PORIFERA (p. 163).
Class. Sub-Class. Order. Family. Sub-Family.
MEGAMASTICTORA (pp. 183, 184) Calcarea (p. 184) Homocoela (p. 185)

Leucosoleniidae (p. 185).

Clathrinidae (p. 185).

Heterocoela (p. 187)

Sycettidae (p. 187).

Grantiidae (p. 192).

Heteropidae (p. 192).

Amphoriscidae (p. 192).

Pharetronidae (p. 192)

Dialytinae (p. 192).

Lithoninae (p. 193).

Astroscleridae (p. 194).
 
MICROMASTICTORA (pp. 183, 195) Myxospongiae (p. 196).
Hexactinellida (p. 197)

Amphidiscophora (p. 203).

Hexasterophora (p. 203).

Receptaculitidae (p. 207).

Octactinellida (p. 208).

Heteractinellida (p. 208).

Demospongiae (p. 209) Tetractinellida (pp. 211, 212)

Choristida (p. 212).

Lithistida (pp. 212, 215).

Monaxonida (pp. 211, 216)

Halichondrina (p. 217).

Spintharophora (p. 217).

Ceratosa (pp. 211, 220) Dictyoceratina (p. 220)

Spongidae (p. 220).

Spongelidae (p. 220).

Dendroceratina (pp. 220, 221).
COELENTERATA (p. 243).
Class. Order. Sub-Order. Family. Sub-Family.
HYDROZOA (p. 249)

Eleutheroblastea (p. 253).

Milleporina (p. 257).

Gymnoblastea (Anthomedusae) (p. 262)

Bougainvilliidae (p. 269).

Podocorynidae (p. 270).

Clavatellidae (p. 270).

Cladonemidae (p. 270).

Tubulariidae (p. 271).

Ceratellidae (p. 271).

Pennariidae (p. 272).

Corynidae (p. 272).

Clavidae (p. 272).

Tiaridae (p. 273).

Corymorphidae (p. 273).

Hydrolaridae (p. 273).

Monobrachiidae (p. 274).

Myriothelidae (p. 274).

Pelagohydridae (p. 274).

Calyptoblastea (Leptomedusae) (p. 275)

Aequoreidae (p. 278).

Thaumantiidae (p. 278).

Cannotidae (p. 278).

Sertulariidae (p. 278).

Plumulariidae (p. 279)

Eleutheroplea (p. 279).

Statoplea (p. 279).

Hydroceratinidae (p. 279).

Campanulariidae (p. 280).

Eucopidae (p. 280).

Dendrograptidae (p. 281).

Graptolitoidea (p. 281)

Monoprionidae (p. 282).

Diprionidae (p. 282).

Retiolitidae (p. 282).

Stromatoporidae (p. 283).
Stylasterina (p. 283) Stylasteridae (p. 285).
Trachomedusae (p. 288)

Olindiidae (p. 291).

Petasidae (p. 294).

Trachynemidae (p. 294).

Pectyllidae (p. 294).

Aglauridae (p. 294).

Geryoniidae (p. 295).

Narcomedusae (p. 295)

Cunanthidae (p. 296).

Peganthidae (p. 296).

Aeginidae (p. 296).

Solmaridae (p. 296).

Siphonophora (p. 297) Calycophorae (p. 305) Monophyidae (p. 306)

Sphaeronectinae (p. 306).

Cymbonectinae (p. 306).

Diphyidae (p. 306)

Amphicaryoninae (p. 306)

Prayinae (p. 306)

Desmophyinae (p. 307)

Stephanophyinae (p. 307)

Oppositae (p. 306)

Galeolarinae (p. 307)

Diphyopsinae (p. 307)

Abylinae (p. 307)

Superpositae (p. 307)
Polyphyidae (p. 307).
Physophorae (p. 307) Physonectidae (p. 307)

Agalminae (p. 307).

Apoleminae (p. 307).

Physophorinae (p. 308).

Auronectidae (p. 308).

Rhizophysaliidae (p. 308).

Chondrophoridae (p. 308).

 
SCYPHOZOA = SCYPHOMEDUSAE (pp. 249, 310) Cubomedusae (p. 318)

Charybdeidae (p. 318).

Chirodropidae (p. 319).

Tripedaliidae (p. 319).

Stauromedusae (p. 320)

Lucernariidae (p. 320).

Depastridae (p. 321).

Stenoscyphidae (p. 321).

Coronata (p. 321)

Periphyllidae (p. 322).

Ephyropsidae (p. 322).

Atollidae (p. 322).

Discophora (p. 323) Semaeostomata (p. 323)

Pelagiidae (p. 323).

Cyanaeidae (p. 324).

Ulmaridae (p. 324).

Rhizostomata (p. 324) Cassiopeidae (p. 324) = Arcadomyaria (p. 324).
Cepheidae (p. 324) = Radiomyaria (p. 324).

Rhizostomatidae (p. 325)

Lychnorhizidae (p. 325)

Leptobrachiidae (p. 325)

Catostylidae (p. 325)

= Cyclomyaria (p. 325).
Class. Sub-Class. Grade. Order. Sub-Order. Family.
ANTHOZOA = ACTINOZOA (pp. 249, 326) Alcyonaria (p. 329) Protoalcyonacea (p. 342) Haimeidae (p. 342).
Synalcyonacea (p. 342) Stolonifera (p. 342)

Cornulariidae (p. 344).

Clavulariidae (p. 344).

Tubiporidae (p. 344).

Favositidae (p. 344).

Coenothecalia (p. 344)

Heliolitidae (p. 346).

Helioporidae (p. 346).

Coccoseridae (p. 346).

Thecidae (p. 346).

Chaetetidae (p. 346).

Alcyonacea (p. 346)

Xeniidae (p. 348).

Telestidae (p. 348).

Coelogorgiidae (p. 349).

Alcyoniidae (p. 349).

Nephthyidae (p. 349).

Siphonogorgiidae (p. 349).

Gorgonacea (p. 350) Pseudaxonia (p. 350)

Briareidae (p. 350).

Sclerogorgiidae (p. 351).

Melitodidae (p. 351).

Coralliidae (p. 352).

Axifera (p. 353)

Isidae (p. 353).

Primnoidae (p. 354).

Chrysogorgiidae (p. 355).

Muriceidae (p. 355).

Plexauridae (p. 356).

Gorgoniidae (p. 356).

Gorgonellidae (p. 357).

Pennatulacea (p. 358) Pennatuleae (p. 361)

Pteroeididae (p. 361).

Pennatulidae (p. 361).

Virgulariidae (p. 362).

Spicatae (p. 362)

Funiculinidae (p. 362).

Anthoptilidae (p. 362).

Kophobelemnonidae (p. 362).

Umbellulidae (p. 362).

Verticilladeae (p. 363)
Renilleae (p. 363) Renillidae (p. 363).
Veretilleae (p. 364)
Zoantharia (pp. 329, 365) Edwardsiidea (p. 375)

Edwardsiidae (p. 377).

Protantheidae (p. 377).

Actiniaria (p. 377) Actiniina (p. 380)

Halcampidae (p. 380).

Actiniidae (p. 381).

Sagartiidae (p. 381).

Aliciidae (p. 382).

Phyllactidae (p. 382).

Bunodidae (p. 382).

Minyadidae (p. 383).

Stichodactylina (p. 383)

Corallimorphidae (p. 383).

Discosomatidae (p. 383).

Rhodactidae (p. 383).

Thalassianthidae (p. 383).

Madreporaria (p. 384)

Cyathophyllidae (p. 394).

Cyathaxoniidae (p. 394).

Cystiphyllidae (p. 394).

Entocnemaria (p. 394)

Madreporidae (p. 395).

Poritidae (p. 396).

Cyclocnemaria (p. 397)

Aporosa (p. 397).

Turbinoliidae (p. 398)

Oculinidae (p. 399)

Astraeidae (p. 399)

  A. Gemmantes (p. 400)

  A. Fissiparantes (p. 400)

  Trochosmiliacea [Sub-Fam.] (p. 401)

Pocilloporidae (p. 401)

Fungacea (p. 402).

Plesiofungiidae (p. 403)

Fungiidae (p. 403)

Cycloseridae (p. 404)

Plesioporitidae (p. 404)

Eupsammiidae (p. 404)

Zoanthidea (p. 404)

Zoanthidae (p. 404).

Zaphrentidae (p. 406).

Antipathidea = Antipatharia (p. 407)

Antipathidae (p. 408).

Leiopathidae (p. 409).

Dendrobrachiidae (p. 409).

Cerianthidea (p. 409).
CTENOPHORA (p. 412).
Class. Order. Family.
TENTACULATA (p. 417) Cydippidea (p. 417)

Mertensiidae (p. 417).

Callianiridae (p. 417).

Pleurobrachiidae (p. 418).

Lobata (p. 418)

Lesueuriidae (p. 419).

Bolinidae (p. 419).

Deiopeidae (p. 419).

Eurhamphaeidae (p. 419).

Eucharidae (p. 420).

Mnemiidae (p. 420).

Calymmidae (p. 420).

Ocyroidae (p. 420).

Cestoidea (p. 420) Cestidae (p. 420).
Platyctenea (p. 421)

Ctenoplanidae (p. 421).

Coeloplanidae (p. 422).

 
NUDA (p. 423) Beroidae (p. 423).
ECHINODERMATA (p. 425).
Sub-Phylum. Class. Order. Sub-Order. Family. Sub-Family.
ELEUTHEROZOA (p. 430) Asteroidea (pp. 430, 431) Spinulosa (pp. 461, 462)

Echinasteridae (p. 462).

Solasteridae (p. 462).

Asterinidae (p. 463).

Poraniidae (p. 464).

Ganeriidae (p. 464).

Mithrodiidae (p. 464).

Velata (pp. 461, 464)

Pythonasteridae (p. 464).

Myxasteridae (p. 464).

Pterasteridae (p. 466).

Paxillosa (pp. 461, 466)

Archasteridae (p. 466).

Astropectinidae (p. 467).

Porcellanasteridae (p. 470).

Valvata (pp. 461, 471)

Linckiidae (p. 471).

Pentagonasteridae (p. 471).

Gymnasteridae (p. 471).

Antheneidae (p. 471).

Pentacerotidae (p. 471).

Forcipulata (pp. 462, 473)

Asteriidae (p. 473).

Heliasteridae (p. 474).

Zoroasteridae (p. 474).

Stichasteridae (p. 474).

Pedicellasteridae (p. 474).

Brisingidae (p. 474).

Ophiuroidea (pp. 431, 477) Streptophiurae (p. 494)
Zygophiurae (pp. 494, 495)

Ophiolepididae (p. 495).

Amphiuridae (p. 497).

Ophiocomidae (p. 499).

Ophiothricidae (p. 499).

Cladophiurae (pp. 494, 500)

Astroschemidae (p. 501).

Trichasteridae (p. 501).

Euryalidae (p. 501).

Echinoidea (pp. 431, 503) Endocyclica (pp. 529, 530)

Cidaridae (p. 533).

Echinothuriidae (p. 535).

Saleniidae (p. 537).

Arbaciidae (p. 538).

Diadematidae (p. 538).

Echinidae (p. 539)

Temnopleurinae (p. 539).

Echininae (p. 539).

Clypeastroidea (pp. 529, 542) Protoclypeastroidea (p. 548).
Euclypeastroidea (p. 549)

Fibularidae (p. 549).

Echinanthidae = Clypeastridae (p. 549).

Laganidae (p. 549).

Scutellidae (p. 549).

Spatangoidea (pp. 529, 549)

Echinonidae (p. 553)

Nucleolidae (p. 554)

Cassidulidae (p. 554)

Asternata (p. 554).

Ananchytidae (p. 554)

Palaeostomatidae (p. 554)

Spatangidae (p. 554)

Brissidae (p. 556)

Sternata (p. 554).

Archaeocidaridae (p. 557).

Melonitidae (p. 557).

Tiarechinidae (p. 557).

Holectypoidea (p. 558).

Echinoconidae (p. 558).

Collyritidae (p. 559).

Holothuroidea (pp. 431, 560)

Aspidochirota (p. 570).

Elasipoda (p. 571).

Pelagothuriida (p. 572).

Dendrochirota (p. 572).

Molpadiida (p. 575).

Synaptida (p. 575).

 
PELMATOZOA (pp. 430, 579) Crinoidea (p. 580)

Hyocrinidae (p. 590).

Rhizocrinidae (p. 590).

Pentacrinidae (p. 591).

Holopodidae (p. 592).

Comatulidae (p. 594).

Inadunata (p. 595).

Articulata (p. 595).

Camerata (p. 595).

Thecoidea = Edrioasteroidea (pp. 580, 596).

Carpoidea (pp. 580, 596).

Cystoidea (pp. 580, 597).

Blastoidea (pp. 580, 599).

PROTOZOA

BY

MARCUS HARTOG, M.A., Trinity College (D.Sc. Lond.)

Professor of Natural History in the Queen's College, Cork.

{3}

CHAPTER I

PROTOZOA—INTRODUCTION—FUNCTIONS OF PROTOPLASM—CELL-DIVISION—ANIMALS AND PLANTS

The Free Amoeboid Cell.—If we examine under the microscope a fragment of one of the higher animals or plants, we find in it a very complex structure. A careful study shows that it always consists of certain minute elements of fundamentally the same nature, which are combined or fused into "tissues." In plants, where these units of structure were first studied, and where they are easier to recognise, each tiny unit is usually enclosed in an envelope or wall of woody or papery material, so that the whole plant is honeycombed. Each separate cavity was at first called a "cell"; and this term was then applied to the bounding wall, and finally to the unit of living matter within, the envelope receiving the name of "cell-wall." In this modern sense the "cell" consists of a viscid substance, called first in animals "sarcode" by Dujardin (1835), and later in plants "protoplasm"[1] by Von Mohl (1846). On the recognition of its common nature in both kingdoms, largely due to Max Schultze, the latter term prevailed; and it has passed from the vocabulary of biology into the domain of everyday life. We shall now examine the structure and behaviour of protoplasm and of the cell as an introduction to the detailed study of the Protozoa, or better still Protista,[2] the lowest types of living beings, and of Animals at large.

{4}

It is not in detached fragments of the tissues of the higher animals that we can best carry on this study: for here the cells are in singularly close connexion with their neighbours during life; the proper appointed work of each is intimately related to that of the others; and this co-operation has so trained and specially modified each cell that the artificial severance and isolation is detrimental to its well-being, if not necessarily fatal to its very life. Again, in plants the presence of a cell-wall interferes in many ways with the free behaviour of the cell. But in the blood and lymph of higher animals there float isolated cells, the white corpuscles or "leucocytes" of human histology, which, despite their minuteness (1⁄3000 in. in diameter), are in many respects suitable objects. Further, in our waters, fresh or salt, we may find similar free-living individual cells, in many respects resembling the leucocytes, but even better suited for our study. For, in the first place, we can far more readily reproduce under the microscope the normal conditions of their life; and, moreover, these free organisms are often many times larger than the leucocyte. Such free organisms are individual Protozoa, and are called by the general term "Amoebae." A large Amoeba may measure in its most contracted state 1⁄100 in. or 250 µ in diameter,[3] and some closely allied species (Pelomyxa, see p. 52) even twelve times this amount. If we place an Amoeba or a leucocyte under the microscope (Fig. 1), we shall find that its form, at first spherical, soon begins to alter. To confine our attention to the external changes, we note that the outline, from circular, soon becomes "island-shaped" by the outgrowth of a promontory here, the indenting of a bay there. The promontory may enlarge into a peninsula, and thus grow until it becomes a new mainland, while the old mainland dwindles into a mere promontory, and is finally lost. In this way a crawling motion is effected.[4] The promontories are called "pseudopodia" (= {5}"false-feet"), and the general character of such motion is called "amoeboid."[5]

fig1

Fig. 1.—Amoeba, showing clear ectoplasm, granular endoplasm, dark nucleus, and lighter contractile vacuole. The changes of form, a-f, are of the A. limax type; g, h, of the A. proteus type. (From Verworn.)

The living substance, protoplasm,[6] has been termed a "jelly," a word, however, that is quite inapplicable to it in its living state. It is viscid, almost semi-fluid, and may well be compared to very soft dough which has already begun to rise. It resembles it in often having a number of spaces, small or large, filled with liquid (not gas). These are termed "vacuoles" or "alveoles," according to their greater or their lesser dimensions. In some cases a vacuole is traversed by strands of plasmic substance, just as we may find such strands stretching across the larger spaces of a very light loaf; but of course in the living cell these are constantly undergoing changes. If we "fix" a cell (i.e. kill it by {6}sudden heat or certain chemical coagulants),[7] and examine it under the microscope, the intermediate substance between the vacuoles that we have already seen in life is again found either to be finely honeycombed or else resolved into a network like that of a sponge. The former structure is called a "foam" or "alveolar" structure, the latter a "reticulate" structure. The alveoles are about 1 µ in diameter, and spheroidal or polygonal by mutual contact, elongated, however, radially to any free surface, whether it be that of the cell itself or that of a larger alveole or vacuole. The inner layer of protoplasm ("endoplasm," "endosarc") contains also granules of various nature, reserve matters of various kinds, oil-globules, and particles of mineral matter[8] which are waste products, and are called "excretory." In fixed specimens these granules are seen to occupy the nodes of the network or of the alveoli, that is, the points where two or three boundaries meet.[9] The outermost layer ("ectoplasm" or "ectosarc") appears in the live Amoeba structureless and hyaline, even under conditions the most favourable for observation. The refractive index of protoplasm, when living, is always well under 1.4, that of the fixed and dehydrated substance is slightly over 1.6.

Again, within the outer protoplasm is found a body of slightly higher refractivity and of definite outline, termed the "nucleus" (Figs. 1, 2). This has a definite "wall" of plasmic nature, and a substance so closely resembling the outer protoplasm in character, that we call it the "nucleoplasm" (also "linin"), distinguishing the outer plasm as "cytoplasm"; the term "protoplasm" including both. Within the nucleoplasm are granules of a substance that stains well with the commoner dyes, especially the "basic" ones, and which has hence been called "chromatin." The linin is {7}usually arranged in a distinct network, confluent into a "parietal layer" within the nuclear wall; the meshes traversing a cavity full of liquid, the nuclear sap, and containing in their course the granules; while in the cavity are usually found one or two droplets of a denser substance termed "nucleoles." These differ slightly in composition from the chromatin granules[10] (see p. 24 f.).

The movements of the leucocyte or Amoeba are usually most active at a temperature of about 40° C. or 100° F., the "optimum." They cease when the temperature falls to a point, the "minimum," varying with the organism, but never below freezing-point; they recommence when the temperature rises again to the same point at which they stopped. If now the temperature be raised to a certain amount above 40° they stop, but may recommence if the temperature has not exceeded a certain point, the "maximum" (45° C. is a common maximum). If it has been raised to a still higher point they will not recommence under any circumstances whatever.

Again, a slight electric shock will determine the retraction of all processes, and a period of rest in a spherical condition. A milder shock will only arrest the movements. But a stronger shock may arrest them permanently. We may often note a relation of the movements towards a surface, tending to keep the Amoeba in contact with it, whether it be the surface of a solid or that of an air-bubble in the liquid (see also p. 20).

fig2

Fig. 2.—Ovum of a Sea-Urchin, showing the radially striated cell-membrane, the cytoplasm containing yolk-granules, the large nucleus (germinal vesicle), with its network of linin containing chromatin granules, and a large nucleole (germinal spot). (From Balfour's Embryology, after Hertwig.)

If a gentle current be set up in the water, we find that the movements of the Amoeba are so co-ordinated that it moves upstream; this must of course be of advantage in nature, as keeping the being in its place, against the streams set up by larger creatures, etc. (see also p. 21).

If substances soluble in water be introduced the Amoeba will, {8}as a rule, move away from the region of greater concentration for some substances, but towards it (provided it be not excessive) for others. (See also pp. 22, 23.) We find, indeed, that there is for substances of the latter category a minimum of concentration, below which no effect is seen, and a maximum beyond which further concentration repels. The easiest way to make such observations is to take up a little strong solution in a capillary tube sealed at the far end, and to introduce its open end into the water, and let the solution diffuse out, so that this end may be regarded as surrounded by zones of continuously decreasing strength. In the process of inflammation (of a Higher Animal) it has been found that the white corpuscles are so attracted by the source of irritation that they creep out of the capillaries, and crowd towards it.

We cannot imagine a piece of dough exhibiting any of these reactions, or the like of them; it can only move passively under the action of some one or other of the recognised physical forces, and that only in direct quantitative relation to the work that such forces can effect; in other words, the dough can have work done on it, but it cannot do work. The Amoeba or leucocyte on the contrary does work. It moves under the various circumstances by the transformation of some of its internal energy from the "potential" into the "kinetic" state, the condition corresponding with this being essentially a liberation of heat or work, either by the breaking down of its internal substances, or by the combination of some of them with oxygen.[11] Such of these changes as involve the excretion of carbonic acid are termed "respiratory."

This liberation of energy is the "response" to an action of itself inadequate to produce it; and has been compared not inaptly to the discharge of a cannon, where foot-tons of energy are liberated in consequence of the pull of a few inch-grains on the trigger, or to an indefinitely small push which makes electric contact: the energy set free is that which was stored up in the charge. This capacity for liberating energy stored up within, in response to a relatively small impulse from without, is termed "irritability"; the external impulse is termed the "stimulus." The responsive act has been termed "contractility," because it so often means an obvious contraction, but is better termed {9}"motility "; and irritability evinced by motility is characteristic of all living beings save when in the temporary condition of "rest."

Again, in the case of the cannon, the gunner after its discharge has to replenish it for future action with a fresh cartridge; the Amoeba or leucocyte can replenish itself—it "feeds." When it comes in contact with a fragment of suitable material, it enwraps it by its pseudopodia (Fig. 3), and its edges coalesce where they touch on the far side as completely as we can join up the edges of dough round the apple in a dumpling. It dissolves all that can be dissolved—i.e. it "digests" it, and then absorbs the dissolved material into its substance, both to replace what it has lost by its previous activity and to supply fuel for future liberation of energy; this process is termed "nutrition," and is another characteristic of living beings.

fig3

Fig. 3.—Amoeba devouring a plant cell; four successive stages of ingestion. (From Verworn.)

Again, as a second result of the nutrition, part of the food taken in goes to effect an increase of the living protoplasm, and that of every part, not merely of the surface—it is "assimilated"; while the rest of the food is transformed into reserves, or consumed and directly applied to the liberation of energy. The increase in bulk due to nutrition is thus twofold: part is the increase of the protoplasm itself—"assimilative growth," part is the storage of reserves—"accumulative growth": these reserves being available in turn by digestion, whether for future true growth or for consumption to liberate energy for the work of the cell.

We can conceive that our cannon might have an automatic feed for the supply of fresh cartridges after each shot; but not that it could make provision for an increase of its own bulk, so as to gain in calibre and strength, nor even for the restoration {10}of its inner surface constantly worn away by the erosion of its discharges. Growth—and that growth "interstitial," operating at every point of the protoplasm, not merely at its surface—is a character of all living beings at some stage, though they may ultimately lose the capacity to grow. Nothing at all comparable to interstitial growth has been recognised in not-living matter.[12]

fig4

Fig. 4.—Amoeba polypodia in successive stages of equal fission; nucleus dark, contractile vacuole clear. (From Verworn, after F. E. Schulze.)

Again, when an Amoeba has grown to a certain size, its nucleus divides into two nuclei, and its cytoplasmic body, as we may term it, elongates, narrows in the middle so as to assume the shape of a dumb-bell or finger-biscuit, and the two halves, crawling in opposite directions, separate by the giving way of the connecting waist, forming two new Amoebas, each with its nucleus (Fig. 4). This is a process of "reproduction"; the special case is one of "equal fission" or "binary division." The original cell is termed the "mother," with respect to the two new ones, and these are of course with respect to it the "daughters," and {11}"sisters" to one another. We must bear in mind that in this self-sacrificing maternity the mother is resolved into her children, and her very existence is lost in their production. The above phenomena, IRRITABILITY, MOTILITY, DIGESTION, NUTRITION, GROWTH, REPRODUCTION, are all characteristic of living beings at some stage or other, though one or more may often be temporarily or permanently absent; they are therefore called "vital processes."

If, on the other hand, we violently compress the cell, if we pass a very strong electric shock through it, or a strong continuous current, or expose it to a temperature much above 45° C., or to the action of certain chemical substances, such as strong acids or alkalies, or alcohol or corrosive sublimate, we find that all these vital processes are arrested once and for all; henceforward the cell is on a par with any not-living substance. Such a change is called "DEATH," and the "capacity for death" is one of the most marked characters of living beings. This change is associated with changes in the mechanical and optical properties of the protoplasm, which loses its viscidity and becomes opaque, having undergone a process of de-solution; for the water it contained is now held only mechanically in the interstices of a network, or in cavities of a honeycomb (as we have noted above, p. 5), while the solid forming the residuum has a refractive index of a little over 1.6. Therefore, it only regains its full transparency when the water is replaced by a liquid of high refractive index, such as an essential oil or phenol. A similar change may be effected by pouring white of egg into boiling water or absolute alcohol, and is attended with the same optical results. The study of the behaviour of coagulable colloids has been recently studied by Fischer and by Hardy, and has been of the utmost service in our interpretation of the microscopical appearances shown in biological specimens under the microscope.[13]

{12}

The death of the living being finds a certain analogy in the breaking up or the wearing out of a piece of machinery; but in no piece of machinery do we find the varied irritabilities, all conducive to the well-being of the organism (under ordinary conditions), or the so-called "automatic processes"[14] that enable the living being to go through its characteristic functions, to grow, and as we shall see, even to turn conditions unfavourable for active life and growth to the ultimate weal of the species (see p. 32). At the same time, we fully recognise that for supplies of matter and energy the organism, like the machine, depends absolutely on sources from without. The debtor and creditor sheet, in respect of matter and energy, can be proved to balance between the outside world and Higher Organisms with the utmost accuracy that our instruments can attain; and we infer that this holds for the Lower Organisms also. Many of the changes within the organism can be expressed in terms of chemistry and physics; but it is far more impossible to state them all in such terms than it would be to describe a polyphase electrical installation in terms of dynamics and hydraulics. And so far at least we are justified in speaking of "vital forces."

The living substance of protoplasm contains a large quantity of water, at least two-thirds its mass, as we have seen, in a state of physical or loose chemical combination with solids: these on death yield proteids and nucleo-proteids.[15] The living protoplasm {13}has an alkaline reaction, while the liquid in the larger vacuoles, at least, is acid, especially in Plant-cells.[16]

Metabolism.—The chemical processes that go on in the organism are termed metabolic changes, and were roughly divided by Gaskell into (1) "anabolic," in which more complex and less stable substances are built up from less complex and more stable ones with the absorption of energy; and (2) "catabolic" changes in which the reverse takes place. Anabolic processes, in all but the cells containing plastids or chromatophores (see p. 36) under the influence of light, necessarily imply the furnishing of energy by concurrent catabolic changes in the food or reserves, or in the protoplasm itself.

Again, we have divided anabolic processes into "accumulative," where the substances formed are merely reserves for the future use of the cell, and "assimilative," where the substances go to the building of the protoplasm itself, whether for the purpose of growth or for that of repair.

Catabolic processes may involve (1) the mere breaking of complex substances into simpler ones, or (2) their combination with oxygen; in either case waste products are formed, which may either be of service to the organism as "secretions" (like the bile in Higher Animals), or of no further use (like the urine). When nitrogenous substances break down in this way they give rise to "excretions," containing urea, urates, and allied substances; other products of catabolism are carbon dioxide, water, and mineral salts, such as sulphates, phosphates, carbonates, oxalates, etc., which if not insoluble must needs be removed promptly from the organism, many of them being injurious or even poisonous. The energy liberated by the protoplasm being derived through the breakdown of another part of the same or of the {14}food-materials or stored reserves, must give rise to waste products. The exchange of oxygen from without for carbonic acid formed within is termed "respiration," and is distinguished from the mere removal of all other waste products called "excretion." In the fresh-water Amoeba both these processes can be studied.

Respiration,[17] or the interchange of gases, must, of course, take place all over the general surface, but in addition it is combined in most fresh-water Protista with excretion in an organ termed the "contractile" or "pulsatile vacuole" (Figs. 1, 4, etc.). This particular vacuole is exceptional in its size and its constancy of position. At intervals, more or less regular, it is seen to contract, and to expel its contents through a pore; at each contraction it completely disappears, and reforms slowly, sometimes directly, sometimes by the appearance of a variable number of small "formative" vacuoles that run together, or as in Ciliata, by the discharge into it of so-called "feeding canals." As this vacuole is filled by the water that diffuses through the substance, and when distended may reach one-third the diameter of the being, in the interval between two contractions an amount of water must have soaked in equal to one-twenty-seventh the bulk of the animal, to be excreted with whatever substances it has taken up in solution, including, not only carbon dioxide, but also, it has been shown, nitrogenised waste matters allied to uric acid.[18]

That the due interchanges may take place between the cell and the surrounding medium, it is obvious that certain limits to the ratio between bulk and surface must exist, which are disturbed by growth, and which we shall study hereafter (p. 23 f.).

The Protista that live in water undergo a death by "diffluence" or "granular disintegration" on being wounded, crushed, or sometimes after an excessive electric stimulation, or contact with alkalies or with acids too weak to coagulate them. In this process the protoplasm breaks up from the surface inwards into a mass of granules, the majority of which themselves finally dissolve. If the injury be a local rupture of the external pellicle or {15}cuticle, a vacuole forms at the point, grows and distends the overlying cytoplasm, which finally ruptures: the walls of the vacuole disintegrate; and this goes on as above described. Ciliate Infusoria are especially liable to this disintegration process, often termed "diffluence," which, repeatedly described by early observers, has recently been studied in detail by Verworn. Here we have death by "solution," while in the "fixing" of protoplasm for microscopic processes we strive to ensure death by "desolution," so as to retain as much of the late living matter as possible. It would seem not improbable that the unusual contact with water determines the formation of a zymase that acts on the living substance itself.

We have suggested[19] that one function of the contractile vacuole, in naked fresh-water Protists, is to afford a regular means of discharge of the water constantly taken up by the crystalloids in the protoplasm, and so to check the tendency to form irregular disruptive vacuoles and death by diffluence. This is supported by the fact that in the holophytic fresh-water Protista, as well as the Algae and Fungi, a contractile vacuole is present in the young naked stage (zoospore), but disappears as soon as an elastic cell-wall is formed to counterbalance by its tension the internal osmotic pressure.

Digestion is always essentially a catabolic process, both as regards the substance digested and the formation of the digesting substance by the protoplasm. The digesting substance is termed a "zymase" or "chemical ferment," and is conjectured to be produced by the partial breakdown of the protoplasm. In presence of suitable zymases, many substances are resolved into two or more new substances, often taking up the elements of water at the same time, and are said to be "dissociated" or "hydrolysed" as the case may be. Thus proteid substances are converted into the very soluble substances, "proteoses" and "peptones," often with the concurrent or ultimate formation of such relatively simple bodies as leucin, tyrosin, and other amines, etc. Starch and glycogen are converted into dextrins and sugars; fats are converted into fatty acids and glycerin. It is these products of digestion, and not the actual food-materials (save certain very simple sugars), that are really taken up by the protoplasm, {16}whether for assimilation, for accumulation, or for the direct liberation of energy for the vital processes of the organism.

Not only food from without, but also reserves formed and stored by the protoplasm itself, must be digested by some zymase before they can be utilised by the cell. In all cases of the utilisation of reserve matter that have been investigated, it has been found that a zymase is formed by the cell itself (or sometimes, in complex organisms, by its neighbours); for, after killing the cell in which the process is going on by mechanical means or by alcohol, the process of digestion can be carried on in the laboratory.[20] The chief digestion of all the animal-feeding Protista is of the same type as in our own stomachs, known as "peptic" digestion: this involves the concurrent presence of an acid, and Le Dantec and Miss Greenwood have found the contents of food-vacuoles, in which digestion is going on, to contain acid liquid. The ferment-pepsin itself has been extracted by Krukenberg from the Myxomycete, "Flowers of tan" (Fuligo varians, p. 92), and by Professor Augustus Dixon and the author from the gigantic multinucleate Amoeba, Pelomyxa palustris (p. 52).[21] The details of the prehension of food will be treated of under the several groups.

The two modes of Anabolism—true "assimilation" in the strictest sense and "accumulation"—may sometimes go on concurrently, a certain proportion of the food material going to the protoplasm, and the rest, after allowing for waste, being converted into reserves.

 

Movements all demand catabolic changes, and we now proceed to consider these in more detail.

The movements of an Amoeboid[22] cell are of two kinds: "expansion," leading to the formation and enlargement of {17}outgrowths, and "contraction," leading to their diminution and disappearance within the general surface.[23] Expansion is probably due to the lessening of the surface-tension at the point of outgrowth, contraction to the increase of surface-tension. Verworn regards these as due respectively to the combination of the oxygen in the medium with the protoplasm in diminishing surface-tension, and the effect of combination with substances from within, especially from the nucleus in increasing it. Besides these external movements, there are internal movements revealed by the contained granules, which stream freely in the more fluid interior. Those Protista that, while exhibiting amoeboid movements, have no clear external layer, such as the Radiolaria, Foraminifera, Heliozoa, etc., present this streaming even at the surface, the granules travelling up and down the pseudopodia at a rate much greater than the movements of these organs themselves. In this case the protoplasm is wetted by the medium, which it is not where there is a clear outer layer: for that behaves like a greasy film.

Motile organs.—Protoplasm often exhibits movements much more highly specialised than the simple expansion or retraction of processes, or the general change of form seen in Amoeba. If we imagine the activities of a cell concentrated on particular parts, we may well suppose that they would be at once more precise and more energetic than we see them in Amoeba or the leucocyte. In some free-swimming cells, such as the individual cells known as "Flagellata," the reproductive cells of the lower Plants, or the male cells ("spermatozoa") of Plants as high as Ferns, and even of the Highest Animals, there is an extension of the cell into one or more elongated lash-like processes, termed "flagella," which, by beating the water in a reciprocating or a spiral rhythm, cause the cell to travel through it; or, if the cell be attached, they produce currents in the water that bring food particles to the surface of the cell for ingestion. Such flagella may, indeed, be seen in some cases to be modified pseudopodia. In other cases part, or the whole, of the surface of the cell may be covered with regularly arranged short filaments of similar activity (termed "cilia," from their resemblance to a diminutive eyelash), which, however, instead of whirling round, bend sharply {18}down to the surface and slowly recover; the movement affects the cilia successively in a definite direction in waves, and produces, like that of flagella, either locomotion of the cell or currents in the medium. We can best realise their action by recalling the waves of bending and recovery of the cornstalks in a wind-swept field; if now the haulms of the corn executed these movements of themselves, they would determine in the air above a breeze-like motion in the direction of the waves (Fig. 5).[24] Such cilia are not infrequent on those cells of even the Highest Animals that, like a mosaic, cover free surfaces ("epithelium cells"). In ourselves such cells line, for instance, the windpipe. One group of the Protozoa, the "Ciliata," are, as their name implies, ciliated cells pure and simple.

fig5

Fig. 5.—Motion of a row of cilia, in profile. (From Verworn.)

The motions of cilia and of flagella are probably also due to changes of surface tension—alternately on one side and the other in the cilium, but passing round in circular succession in the flagellum,[25] giving rise to a conical rotation like that of a weighted string that is whirled round the head. This motion is, however, strongest at the thicker basal part, which assumes a spiral form like a corkscrew of few turns, while the thin lash at the tip may seem even to be quietly extended like the point of the corkscrew. If the tip of the flagellum adhere, as it sometimes does, to any object, the motions induce a jerking motion, which in this case is reciprocating, not rotatory. When the organism is free, the flagellum is usually in advance, and the cell follows, rotating at the same time round its longitudinal axis; such an anterior flagellum, called a "tractellum," is the common form in Protista that possess a single one (Figs. 29, 7, 8; 30, C). In the spermatozoa of Higher Animals (and some Sporozoa) the flagellum is posterior, and is called a "pulsellum."

The cilium or flagellum may often be traced a certain distance into the substance of the cytoplasm to end in a dot of denser, {19}readily-staining plasm, which corresponds to a "centrosome" or centre of plasmic forces (see below, pp. 115, 121, 141); it has been termed a "blepharoplast."[26]

Again, the cytoplasm may have differentiated in it definite streaks of specially contractile character; such streaks within its substance are called "myonemes"; they are, in fact, muscular fibrils. A "muscle-cell," in the Higher Animals, is one whose protoplasm is almost entirely so modified, with the exception of a small portion of granular cytoplasm investing the nucleus, and having mainly a nutritive function.

Definite muscular fibrils in action shorten, and at the same time become thicker. It seems probable that they contain elongated vacuoles, and that the contents of these vary, so that when they have an increased osmotic equivalent, the vacuoles absorb water, enlarge, and tend to become more spherical, i.e. shorter and thicker, and so the fibril shortens as a whole. The relaxation would be due to the diffusion outwards of the solution of the osmotically active substances which induced expansion.[27]

The Motile Reactions of the Protozoa[28] require study from another point of view: they are either (1) "spontaneous" or "arbitrary," as we may say, or (2) responsive to some stimulus. The latter kind we will take first, as they are characteristic of all free cells. The stimuli that induce movements of a responsive character are as follows:—(i.) MECHANICAL: such as agitation and contact; (ii.) force of GRAVITY, or CENTRIFUGAL FORCE; (iii.) CURRENTS in the water; (iv.) RADIANT ENERGY (LIGHT); (v.) changes in the TEMPERATURE of the medium; (vi.) ELECTRIC CURRENTS through the medium; (vii.) the presence of CHEMICAL SUBSTANCES in the medium.

These, or some of them, may induce one of three different results, or a combination thereof: (1) a single movement or an arrest of motion; (2) the assumption of a definite position; (3) movement of a definite character or direction.

{20}

(i.) Mechanical stimuli.—Any sudden touch with another body tends to arrest all motion; and if the shock be protracted or severe, the retraction of the pseudopodia follows. It is to this reaction that we must ascribe the retracted condition of the pseudopodia of most Rhizopods when first placed on the slide and covered for microscopic examination. Free-swimming Protista may, after hitting any body, either remain in contact with it, or else, after a pause, reverse their movement, turn over and swim directly away. This combination of movements is characteristic as a reaction of what we may term "repellent" stimuli in general.[29] Another mechanical reaction is that to continuous contact with a solid; and the surface film of water, either at the free surface or round an air-bubble, may play the part of a solid in exciting it; we term it "thigmotaxy" or "stereotaxy." When positive it determines a movement on to the surface, or a gliding movement along it, or merely the arrest of motion and prolongation of contact; when negative, a contact is followed by the retreat of the being. Thus Paramecium (Fig. 55, p. 151) and many other Ciliates are led to aggregate about solid particles or masses of organic débris in the water, which indeed serve to supply their food. On contact, the cell ceases to move its cilia except those of the oral groove; as these lash backwards, they hold the front end in close contact with the solid, at the same time provoking a backward stream down the groove, which may bring in minute particles from the mass.

(ii.) Most living beings are able to maintain their level in water by floating or crawling against Gravity, and they react in virtue of the same power against centrifugal force. This mode of irritability is termed (negative) "geotaxy" or "barotaxy." We can estimate the power of resisting such force by means of a whirling machine, since when the acceleration is greater than the resistance stimulated thereby in the beings, they are passively sent to the sides of the vessel. The Flagellates, Euglena and Chlamydomonas, begin to migrate towards the centre when exposed to a centrifugal force about equal to ½ G (G = 32.2 feet or 982 cm. per second); they remain at the centre until the centrifugal force is increased to 8 G; above that they yield to the force, and are driven passively to the sides. The reaction ceases or is reversed at high temperatures.

{21}

(iii.) Rheotaxy.—This is the tendency to move against the stream in flowing water. It is shown by most Protists, and can be conveniently studied in the large amoeboid plasmodia of the Myxomycetes, which crawl against the stream along wet strips of filter paper, down which water is caused to flow. Most animals, even of the highest groups, tend to react in the same way; the energetic swimming of Fishes up-stream being in marked contrast with their sluggishness the other way; and every student of pond-life knows how small Crustacea and Rotifers, no less than Ciliates, swim away from the inrush of liquid into the dipping-tube, and so evade capture. (See Vol. II. p. 216.)

(iv.) The movements of many Protozoa are affected greatly by Light. These movements have been distinguished into "photopathic," i.e. to or from the position of greatest luminosity; and "phototactic," along the direct path of the rays.[30] Those Protozoa that contain a portion of their cytoplasm, known as a "plastid" or "chromatophore" (see pp. 36, 39), coloured by a green or yellow pigment are usually "phototactic." They mostly have at the anterior end a red pigment spot, which serves as an organ of sight, and is known as an "eye-spot." In diffused light of low intensity they do not exhibit this reaction, but in bright sunlight they rise to the surface and form there a green or yellow scum.

Most of the colourless Protista are negatively phototactic or photopathic; but those which are parasitic on the coloured ones are positively phototactic, like their hosts.

Here, as in the case of other stimuli,[31] the absolute intensity of the light is of importance; for as it increases from a low degree, different organisms in turn cease to be stimulated, and {22}then are repelled instead of being attracted. The most active part of the spectrum in determining reactions of movement are the violet and blue rays of wave-length between 40 µ/10 and 49 µ/10, while the warmer and less refractive half of the spectrum is inert save in so far as it determines changes in the temperature of the medium.

(v.) The movements of many Protozoa are rendered sluggish by cold, and active by a rise of Temperature up to what we may term the "optimum"; the species becomes sluggish again as the temperature continues to rise to a certain point when the movements are arrested, and the being is said to be in a state of "heat-rigor." Most Protozoa, again, tend to move in an unequally heated medium to the position nearest to their respective optimum temperature. This is called "thermotaxy." The temperature to which Amoeba is thermotactic is recorded as 35° C. (95° F.); that of Paramecium is 28° C. (82° F.).

(vi.) Most active Protozoa tend to take up a definite position in respect to a current of Electricity passing through the medium, and in the majority of cases, including most Ciliates, Amoeba, and Trachelomonas, they orient their long diameters in the direction of the lines of force and swim along these to assemble behind the cathode. The phenomenon is called "galvanotaxy," and this particular form is "negative." Opalina (Fig. 41, p. 123), however, and most Flagellates are "positively galvanotactic," and move towards the anode. H. H. Dale[32] has shown that the phenomenon may be possibly in reality a case of chemiotaxy, for the direction of motion varies with the nature and concentration of the medium. It would thus be a reaction to the "ion" liberated in contact with the one or other extremity of the being. Induction shocks, as we have seen, if slight, arrest the movements of Protozoa, or if a little stronger determine movements of contraction; if of sufficient intensity they kill them. No observation seems to have been made on the behaviour of Protista in an electric field. A magnetic field of the highest intensity appears to be indifferent to all Protista.

(vii.) We have already referred to the effect of dissolved Chemical Substances present in the water. If the substance is in itself not harmful, and the effect varies with the concentration, we term the reaction one of "tonotaxy," which combines {23}with that of "chemiotaxy" for substances that in weak solution are attractive or repellent to the being. Paramecium, which feeds on bacteria, organisms of putrefaction, is positively chemiotactic to solutions of carbon dioxide, and as it gives this off in its own respiration, it is attracted to its fellows. The special case of reaction to gases in solution is termed "aerotaxy," or "pneumotaxy," according as the gas is oxygen or carbon dioxide. We find that in this respect there are degrees, so that a mixed culture of Flagellates in an organic infusion sorts itself out, under the cover of a microscopic preparation, into zones of distinct species, at different distances from the freely aerated edge, according to the demands of each species for oxygen and CO2 respectively.

Finally, we must note that the apparently "spontaneous movements" of Protists can hardly be explained as other than due either to external stimuli, such as we have just studied, or to internal stimuli, the outcome of internal changes, such as fatigue, hunger, and the like. Of the latter kind are the movements that result in REPRODUCTION.

Reproduction.—We have noted above that the growth of an organism which retains its shape alters the ratio of the surface area to the whole volume, so necessary for the changes involved in life. For the volume of an organism varies as the cube of any given diameter, whereas the surface varies with the square only. Without going into the arithmetical details, we may say that the ratio of surface to volume is lessened to roughly four-fifths of the original ratio when the cell doubles its bulk. As Herbert Spencer and others have pointed out, this must reduce the activities of the cell, and the due ratio is restored by the division of the cell into two.[33] This accounts for what we must look on as the most primitive mode of reproduction, as it is the simplest, and which we term "fission" at Spencer's "limit of {24}growth." Other modes of reproduction will be studied later (p. 30), after a more detailed inquiry into the structure of the nucleus and of its behaviour in cell-division. All cell-division is accompanied by increased waste, and is consequently catabolic in character, though the anabolic growth of living protoplasm, at the expense of the internal reserves, may be concurrent therewith.

Cell-Division

In ordinary cases of fission of an isolated cell the cell elongates, and as it does so, like other viscid bodies, contracts in the middle, which becomes drawn out into a thread, and finally gives way. In some cases (e.g. that of the Amoeba, Fig. 4) the nucleus previously undergoes a similar division by simple constriction, which is called direct or "amitotic" division. But usually the division of the nucleus prior to cell-division is a more complex process, and involves the co-operation of the cytoplasm; and we must now study in detail the nucleus and its structure in "rest" and in fission.[34]

We have noted above (p. 6, Fig. 2) the structure of the so-called "resting nucleus,"[35] when the cell is discharging the ordinary functions of its own life, with its wall, network of linin, chromatin-granules, and nucleole or nucleoles. The chromatin-granules are most abundant at two periods in the life of the cell, (1) when it is young and fresh from division, and (2) at the term of its life, when it is itself preparing for division. In the interim they are fewer, smaller, and stain less intensely. In many Protista the whole or greater part of the chromatin is densely aggregated into a central "nuclein-mass" or karyosome {25}suspended in the linin network (long regarded as a mere nucleole). Such a nucleus is often termed a "vesicular nucleus".[36]

fig6

Fig. 6.—Changes in nucleus and cell in indirect (mitotic) nuclear division. A, resting nucleus with two centrioles[37] in single centrosphere (c); B, centrosphere divided, spindle and two asters (a) forming; C, centrospheres separated, nuclear wall disappearing; D, resolution of nucleus into chromosomes; E, mature plasmic spindle, with longitudinal fission of chromosomes; F, chromosomes forming equatorial plate (ep) of spindle. (From Wilson.)

{26}

When cell-division is about to take place the linin, or at least the greater part of it, assumes the character of a number of distinct threads, and the whole of the chromatin granules are distributed at even distances along these (Fig. 6, A, B, C), so as to appear like so many strings of beads. Each such thread is called a "chromosome." Then each bead divides longitudinally into two. The thread flattens into a ribbon, edged by the two lines of chromatin beads. Finally, the ribbon splits longitudinally into two single threads of beads (Fig. 6, E). During these changes the nucleole or nucleoles diminish, or even disappear, as if they had contributed their matter to the growth of the chromatin proper. In Higher Animals and Plants the nuclear wall next disappears, and certain structures become obvious, especially in the cytoplasm of Metazoa. Two minute spheres of plasm (themselves often showing a concentric structure), the "centrosomes,"[38] which hitherto lay close together at the side of the nuclear wall, now separate; but they remain connected by a spindle of clear plasmic threads (Fig. 6, B-E) which, as the centres diverge, comes to lie across the spot the nucleus occupied, and now the chromosomes lie about the equator of this spindle (Fig. 6, F). Moreover, the surrounding cytoplasm shows a radiating structure, diverging from the centrosome, so that spindle and external radiations together make up a "strain-figure," like that of the "lines of force" in relation to the poles of a magnet. Such we can demonstrate in a plane by spreading or shaking iron filings on a piece of paper above the poles of a magnet, or in space by suspending finely divided iron in a thick liquid, such as mucilage or glycerin, and bringing the vessel with the mixture into a strong magnetic field;[39] the latter mode has the advantage {27}of enabling us to watch the changes in the distribution of the lines under changing conditions or continued strain.

fig7

Fig. 7.—Completion of mitotic cell-division. G, splitting of equatorial plate (ep); H, recession of daughter chromosomes; I, J, reconstitution of these into new nuclei, fission of the centrioles and of the cytoplasm. if, Central fibres of spindle; n, remains of old nucleole. (From Wilson.)

The chromosomes are now completely split, each into its two daughter-segments, which glide apart (Fig. 7, G, ep), and pass each to its own pole of the spindle, stopping just short of the centrosome (I). Thus, on the inner side of either centrosome is found an aggregation of daughter-segments, each of which is sister to one at the opposite pole, while the number at either pole is identical with that of the segments into which the old nucleus had resolved itself at the outset. The daughter-segments shorten and thicken greatly as they diverge to the poles, and on their arrival crowd close together.

A distinct wall now forms around the aggregated {28}daughter-chromosomes (J), so as to combine them into a nucleus for the daughter-cell. The reorganisation of the young nucleus certainly varies in different cases, and has been ill-studied, probably because of the rapidity of the changes that take place. The cytoplasm now divides, either tapering into a "waist" which finally ruptures, or constricting by the deepening of a narrow annular groove so as to complete the formation and isolation of the daughter-cells.

We might well compare the cell-division to the halving of a pumpkin or melon, of which the flesh as a whole is simply divided into two by a transverse cut, while the seeds and the cords that suspend them are each singly split to be divided evenly between the two halves of the fruit; the flesh would represent the cytoplasm, the cords the linin threads of the nucleus, and the seeds the chromatin granules. In this way the halving of the nucleus is much more complete and intimate than that of the cytoplasm; and this is the reason why many biologists have been led to regard the nuclear segments, and especially their chromatic granules, as the seat of the hereditary properties of the cell, properties which have to be equally transmitted on its fission to each daughter-cell.[40] But we must remember that the linin is also in great part used up in the formation of these segments, like the cords of our supposed melon; and it is open to us to regard the halving in this intimate way of the "linin" as the essence of the process, and that of the chromatin as accessory, or even as only part of the necessary machinery of the process. The halving or direct splitting lengthwise of a viscid thread is a most difficult problem from a physical point of view; and it may well be that the chromatin granules have at least for a part of their function the facilitation of this process. If such be the case, we can easily understand the increase in number, and size and staining power of these granules as cell-division approaches, and their atrophy or partial disappearance during their long intervening periods of active cell life. Hence we hesitate to accept the views so commonly maintained that the chromatin represents a {29}"germ-plasm" or "idioplasm" of relatively great persistence, which gives the cell its own racial qualities.[41]

The process we have just examined is called "mitosis," "karyomitosis," or "karyokinesis"; and the nucleus is said to undergo "indirect" division, as compared to "direct" division by mere constriction. In an intermediate mode, common to many Protista, the nuclear wall persists throughout the whole process, though a spindle is constituted within, and chromosomes are formed and split: the division of the nucleus takes place, however, by simple constriction, as seen in the Filose Rhizopod Euglypha (Fig. 8).

fig8

Fig. 8.—Fission with modified karyokinesis in the Filose Rhizopod Euglypha. A, outgrowth of half of the cytoplasm, passage of siliceous plates for young shell outwards; B, completion of shell of second cell, formation of intra-nuclear spindle; C, D, further stages. (From Wilson, after Schewiakoff.)

In many Sarcodina and some Sporozoa the nucleus gives off small fragments into the cytoplasm, or is resolved into them; {30}they have been termed "chromidia" by E. Hertwig. New nuclei may be formed by their growth and coalescence, the original nucleus sometimes disappearing more or less completely.

In certain cases the division of the nucleus is not followed by that of the cytoplasm, so that a plurinucleate mass of protoplasm results: this is called an "apocyte"; and we find transitional forms between this and the uninucleate or true cell. Thus in one species of Amoeba (A. binucleata) there are always two nuclei, which divide simultaneously to provide for the outfit of the daughter-cells on fission. Again, we find in some cases that similar multinucleate masses may be formed by the union of two or more cells by their cytoplasm only: such a union is termed "permanent plastogamy," and the plurinucleate mass a "plasmodium."[42] Here again we find intermediate forms between plasmodium and apocyte, for the nuclei of the former may divide and so increase in number, without division of the still growing mass. Both kinds of plurinucleate organisms are termed "coenocytes" without reference to their mode of origin.

The rhythm of cell-life that we have just studied is called the "Spencerian" rhythm. Each cell in turn grows from half the bulk of its parent at the time it was formed to the full size of that parent, when it divides in its own turn. Rest is rare, and assumed only when the cell is exposed to such unfavourable external conditions as starvation, drought, etc.; it has no necessary relation to fission.

Multiple fission or brood-formation.—We may now turn to a new rhythm, in strong contrast to the former: a cell after having attained a size, often notably greater than its parents, divides: without any interval for growth, the daughter-cells again divide, and this may be repeated as many as ten times, or even more, so as to give rise to a number of small cells—4, 8, 16—1024,[43] etc., respectively. Such an assemblage of small cells so formed is called a brood, and well deserves this name, for they never separate until the whole series of divisions is completed. By this process the number of individuals is rapidly {31}increased, hence it has received the name of "sporulation." The term spores is especially applied to the reproductive bodies of Cryptogams, such as Mosses, Fungi, etc.: the resulting cells are called "spores," "zoospores" if active ("amoebulae" if provided with pseudopodia, "flagellulae" if flagellate), "aplanospores," if motionless. We prefer to call them by the general term "brood-cells," the original cell the "brood-mother-cell," and the process, "multiple fission" or "brood-formation." As noted, the brood-mother-cell usually attains an exceptionally large size, and it in most cases passes into a state of rest before entering on division: thus brood-formation is frequently the ultimate term of a long series of Spencerian divisions. Two contrasting periods of brood-formation may occur in the life cycle of some beings, notably the Sporozoa.[44]

Colonial union.—In certain cases, the brood-cells instead of separating remain together to form a "colony"; and this may enlarge itself again by binary division of its individual cells at their limit of growth. Here, certain or all of the cells may (either after separation, or in their places) undergo brood-formation: such cells are often termed "reproductive cells" in contrast with the "colonial cells."

Some such colonial Protista must have been the starting-points for the Higher Animals and Plants; probably apocytial Protista were the starting-points of the Fungi. In the Higher Animals and Plants, the spermatozoa and the oospheres (the male and female pairing-cells) are alike the offspring of brood-formation: and the coupled-cell (fertilised egg) starts its new life by segmentation, which is a brood-formation in which the cells do not separate, but remain in colonial union, to differentiate in due course into the tissue-cells of the organism.

Retarded brood-formation.—The nuclear divisions may alternate with cell-divisions, as above stated, or the former may be {32}completed before the cytoplasm divides; thus the brood-mother-cell becomes temporarily an apocyte,[45] which is then resolved simultaneously into the 1-nucleate brood-cells.

A temporary apocytial condition is often passed through in the formation of the brood of cells by repeated divisions without any interval for enlargement; for the nuclear divisions may go on more rapidly than those of the cytoplasm, or be completed before any cell-division takes place (Figs. 31, 34, 35, pp. 95, 101, 104), the nuclear process being "accelerated" or the cytoplastic being "retarded," whichever we prefer to say and to hold. Thus as many as thirty-two nuclei may have been formed by repeated binary subdivisions before any division of the cytoplasm takes place to resolve the apocyte into true 1-nucleate cells.

In many cases of brood-formation the greater part of the food-supply of the brood-mother-cell has been stored as reserve-products, which accumulate in quantity in the cell; this is notably seen in the ovum or egg of the Higher Animals. How great such an accumulation may be is indeed well seen in the enormous yolk of a bird's egg, gorged as it were to repletion. When a cell has entered on such course of "miserly" conduct, it may lose all power of drawing on its own supplies, and finally that of accumulating more, and passes into the state of "rest." To resume activity there is needed either a change in the internal conditions—demanding the lapse of time—or in the external conditions, or in both.[46] We may call this resumption "germination."

Very often in the study of a large and complex organism we are able to find processes in action on a large scale which, depending as they must do on the protoplasmic activities of its individual cells, reveal the nature of similar processes in simple unicellular beings: such a clue to the utilisation of reserves by a cell which has gorged itself with them so as to pass into a state of rest is to be found in that common multicellular organism, the Potato. This stores up reserves in its underground stems (tubers); if we plant these immediately on the completion of their growth, they will not start at once, even under what would outwardly seem to be most appropriate conditions. A certain lapse of time is an essential factor for sprouting. It would appear that in the Potato the starch can only be digested by a definite ferment, which does not exist when it is dug, but which is only formed very slowly, and not at all until a certain time has supervened; and that sprouting can only {33}take place when soluble material has been provided in this way for the growth of the young shoots. We have also reason to believe that these ferments are only formed by the degradation of the protoplasm itself. Now obviously this degradation must be very slow in a resting organism; and any external stimulus that will tend to protoplasmic activity will thereby tend to form at the same time the digestive ferments and dissolve the stored supplies, to render them available for the life-growth and reproduction of the being. We now see why inactive "miserly" cells so often pass into a resting state before dividing, and why they go on dividing again and again when once they re-enter upon an active life, the living protoplasm growing at the expense of the reserves.[47] Resting cells of this type occur of course only at relatively rare intervals in the animal-feeding Protozoa, that have to take into their substance the food they require for their growth and life-work, and cannot therefore store up much reserves. For they are constantly producing in the narrow compass of their body those very ferments that would dissolve the reserves when formed. Internal parasites and "saprophytes," that is, beings which live on dead and decayed organic matter, on the other hand, live surrounded by a supply of dissolved food; and rarely do we find larger cells, richer in reserves, than in the parasitic Sporozoa, which owe their name to the importance of brood-formation in their life-history. In Radiolaria (p. 75 f.) a central capsule separates off an inner layer of protoplasm; the outer layer is the one in which digestion is performed, while the inner layer stores up reserves; and here brood-formation appears to be the rule. But the largest cells of all are the eggs of the Metazoa, which in reality lead a parasitic life, being nurtured by the animal as a whole, and contributing nothing to the welfare of it as an individual. Their activity is reduced to a minimum, and the consequent need for a high ratio of surface to volume is also reduced, which accounts for their inordinate size. But directly the reserve materials are rendered available by the formation of a digestive ferment, then protoplasmic growth takes place, and the need for an extended surface is felt; cell-division follows cell-division with scarcely an interval in the process of segmentation.

Syngamy.[48]—The essence of typical syngamy is, that two cells ("pairing-cells," "gametes") of the same species approach one another, and fuse, cytoplasm with cytoplasm, and nucleus with nucleus, to form a new cell ("coupled-cell," "zygote "). This process is called also "conjugation" or "cytogamy." In the simplest cases the two cells are equal and attract one another equally ("isogamy"), and have frequently the character of zoospores.

In an intermediate type, the one cell is larger and more sluggish (female), "megagamete," "oogamete," "oosphere," "egg"; the other smaller, more active (male), "microgamete," "spermogamete," "spermatozoon," "sperm"; and in the most specialised {34}cases which prevail among the Higher Animals and Plants, the larger cell is motionless, and the smaller is active, ciliate, flagellate, or amoeboid: the coupled-cell or zygote is here termed the "oosperm."[49] It encysts immediately in most Protista except Infusoria, Acystosporidae, Haemosporidae, and Trypanosomatidae.

As the size of the two gametes is so disproportionate in most cases that the oosphere may be millions of times bigger than the sperm, and the latter at its entrance into the oosphere entirely escape unaided vision, the term "egg" is applied in lax speech, both (1) to the female cell, and (2) to the oosperm, the latter being distinguished as the "fertilised egg," a survival from the time when the union of two cells, as the essence of the process, was not understood.

We know that in many cases, and have a right to infer that in all, chemiotaxy plays an important part in attracting the pairing-cells to one another. In Mammals and Sauropsida there seems also to be a rheotactic action of the cilia lining the oviducts, which work downwards, and so induce the sperms to swim upwards to meet the ovum, a condition of things that was most puzzling until the nature of rheotaxy was understood. A remarkable fact is that equal gametes rarely appear to be attracted by members of the same brood, though they are attracted by those of any other brood of the same species.[50] It may well be that each brood has its own characteristic secretion, or "smell," as it were, slightly different from that of other broods, just as every dog has his, so easily recognisable by other dogs; and that the cells only react to different "smells" to their own. Such a secretion from the surface of the female cell would lessen its surface tension, and thereby render easier the penetration of the sperm into its substance.

As a rule, one at least of the pair-cells is fresh from division, and it would thus appear that the union of the nuclei is facilitated when one at least of them is a "young" one. Of the final mechanism of the union of the nuclei, we know nothing: they may unite in any of the earlier phases of mitosis, or even in the "resting state." A fibrillation of the cytoplasm during the process, radiating around a centrosome or two centrosomes indicates a strained condition.[51]

{35}

Regeneration.—Finally, experiments have been made by several observers as to the effects of removing parts of Protozoa, to see how far regeneration can take place. The chief results are as follows:—

1. A nucleated portion may regenerate completely, if of sufficient size. Consequently, multinucleate forms, such as Actinosphaerium (Heliozoa, Fig. 19, p. 72), may be cut into a number of fragments, and regenerate completely. In Ciliata, such as Stentor (Fig. 59, p. 156), each fragment must possess a portion of the meganucleus, and at least one micronucleus (p. 145), and, moreover, must possess a certain minimum size. A Radiolarian "central-capsule" (p. 75) with its endoplasm and nucleus may regenerate its ectoplasm, but the isolated ectoplasm being non-nucleate is doomed. A "central capsule" of one species introduced into the ectoplasm of another, closely allied, did well. All non-nucleate pieces may exhibit characteristic movements, but appear unable to digest; and they survive only a short time.[52]

"Animals" and "Plants"

Hitherto we have discussed the cell as if it were everywhere an organism that takes in food into its substance, the food being invariably "organic" material, formed by or for other cells; such nutrition is termed "holozoic." There are, however, limits to the possibilities in this direction, as there are to the fabled capacities of the Scillonians of gaining their precarious livelihood by taking in one another's washing. For part of the food material taken in in this way is applied to the supply of the energies of the cell, and is consequently split up or oxidised into simpler, more stable bodies, no longer fitted for food; and of the matter remaining to be utilised for building up the organism, a certain proportion is always wasted in by-products. Clearly, then, the supply of food under such conditions is continually lessening in the universe, and we have to seek for a manufactory of food-material from inorganic materials: this is to be found in those cells that are known as "vegetal," in the widest sense of {36}the word. In this, sense, vegetal nutrition is the utilisation of nitrogenous substances that are more simple than proteids or peptones, together with suitable organic carbon compounds, etc., to build up proteids and protoplasm. The simplest of organisms with a vegetal nutrition are the Schizomycetes, often spoken of loosely as "bacteria" or "microbes," in which the differentiation of cytoplasm and nucleus is not clearly recognisable. Some of these can build up their proteids from the free uncombined nitrogen of the atmosphere, carbon dioxide, and inorganic salts, such as sulphates and phosphates. But the majority of vegetal feeders require the nitrogen to be combined at least in the form of a nitrate or an ammonium salt—nay, for growth in the dark, they require the carbon also to be present in some organic combination, such as a tartrate, a carbohydrate, etc. Acetates and oxalates, "aromatic" compounds[53] and nitriles are rarely capable of being utilised, and indeed are often prejudicial to life. In many vegetal feeders certain portions of the protoplasm are specialised, and have the power of forming a green, yellow, or brown pigment; these are called "plastids" or "chromatophores." They multiply by constriction within the cell, displaying thereby a certain independent individuality. These plastids have in presence of light the extraordinary power of deoxidising carbon dioxide and water to form carbohydrates (or fats, etc.) and free oxygen; and from these carbohydrates or fats, together with ammonium salts or nitrates, etc., the vegetal protoplasm at large can build up all necessary food matter. So that in presence of light of the right quality[54] and adequate intensity, such coloured vegetal beings have the capacity for building up their bodies and reserves from purely inorganic materials. Coloured vegetal nutrition, then, is a process involving the absorption of energy; the source from which this is derived in the bacteria being very obscure at present. Nutrition by means of coloured plastids is {37}distinguished as "holophytic," and that from lower substances, which, however, contain organically combined carbon, as "saprophytic," for such are formed by the death and decomposition of living beings. The third mode of nutrition (found in some bacteria) from wholly inorganic substances, including free nitrogen, has received no technical name. All three modes are included in the term "autotrophic" (self-nourishing).

Vegetal feeders have a great tendency to accumulate reserves in insoluble forms, such as starch, paramylum, and oil-globules on the one hand, and pyrenoids, proteid crystals, aleurone granules on the other.

When an animal-feeding cell encysts or surrounds itself with a continuous membrane, this is always of nitrogenous composition, usually containing the glucosamide "chitin." The vegetal cell-wall, on the contrary, usually consists, at least primarily, of the carbohydrate "cellulose"—the vegetal cell being richly supplied with carbohydrate reserves, and drawing on them to supply the material for its garment. This substance is what we are all familiar with in cotton or tissue-paper.

Again, not only is the vegetal cell very ready to surround itself with a cell-wall, but its food-material, or rather, speaking accurately, the inorganic materials from which that food is to be manufactured, may diffuse through this wall with scarcely any difficulty. Such a cell can and does grow when encysted: it grows even more readily in this state, since none of its energies are absorbed by the necessities of locomotion, etc. Growth leads, of course, to division: there is often an economy of wall-material by the formation of a mere party-wall dividing the cavity of the old cell-wall at its limit of growth into two new cavities of equal size. Thus the division tends to form a colonial aggregate, which continues to grow in a motionless, and, so far, a "resting" state. We may call this "vegetative rest," to distinguish it from "absolute rest," when all other life-processes (as well as motion) are reduced to a minimum or absolutely suspended.

The cells of a plant colony are usually connected by very fine threads of protoplasm, passing through minute pores where the new party-wall is left incomplete after cell-division.[55] In a few plants, such as most Fungi, the cell-partitions are {38}in abeyance for the most part, and there is formed an apocyte with a continuous investment, sometimes, however, chambered at intervals by partitions between multinucleate units of protoplasm. We started with a purely physiological consideration, and we have now arrived at a morphological distinction, very valid among higher organisms.

Higher Plants consist of cells for the most part each isolated in its own cell-cavity, save for the few slender threads of communication.

Higher Animals consist of cells that are rarely isolated in this way, but are mostly in mutual contact over the greater part of their surface.

Again, Plants take in either food or else the material for food in solution through their surface, and only by diffusion through the cell-wall. Insectivorous Plants that have the power of capturing and digesting insects have no real internal cavity. Animal-feeding Protista take in their food into the interior of their protoplasm and digest it therein, and the Metazoa have an internal cavity or stomach for the same purpose. Here again there are exceptions in the case of certain internal parasites, such as the Tapeworms and Acanthocephala (Vol. II. pp. 74, 174), which have no stomachs, living as they do in the dissolved food-supplies of their hosts, but still possessing the general tissues and organs of Metazoa.

Corresponding with the absence of mouth, and the absorption instead of the prehension of food, we find that the movements of plant-beings are limited. In the higher Plants, and many lower ones, the colonial organism is firmly fixed or attached, and the movements of its parts are confined to flexions. These are produced by inequalities of growth; or by inequalities of temporary distension of cell-masses, due to the absorption of liquid into their vacuoles, while relaxation is effected by the cytoplasm and cell-wall becoming pervious to the liquid. We find no case of a differentiation of the cytoplasm within the cell into definite muscular fibrils. In the lower Plants single naked motile cells disseminate the species; and the pairing-cells, or at least the males, have the same motile character. In higher Cryptogams, Cycads, and Ginkgo (the Maiden-hair Tree), the sperms alone are free-swimming; and as we pass to Flowering Plants, the migratory character of the male cells is restricted to the smallest limits. {39}though never wholly absent. Intracellular movements of the protoplasm are, however, found in all Plants.

In Plants we find no distinct nervous system formed of cells and differentiated from other tissues with centres and branches and sense-organs. These are more or less obvious in all Metazoa, traces being even found in the Sponges.

We may then define Plants as beings which have the power of manufacturing true food-stuffs from lower chemical substances than proteids, often with the absorption of energy. They have the power of surrounding themselves with a cell-wall, usually of cellulose, and of growing and dividing freely in this state, in which animal-like changes of form and locomotion are impossible; their colonies are almost always fixed or floating; free locomotion is only possible in the case of their naked reproductive cells, and is transitory even in these. The movements of motile parts of complex plant-organisms are due to the changes in the osmotic powers of cells as a whole, and not to the contraction of differentiated fibrils in the cytoplasm of individual cells. Plants that can form carbohydrates with liberation of free oxygen have always definite plastids coloured with a lipochrome[56] pigment, or else (in the Phycochromaceae) the whole plasma is so coloured. Solid food is never taken into the free plant-cell nor into an internal cavity in complex Plants. If, as in insectivorous Plants, it is digested and absorbed, it is always in contact with the morphological external surface. In the complex Plants apocytes and syncytes are rare—the cells being each invested with its own wall, and, at most, only communicating by minute threads with its neighbours. No trace of a central nervous system with differentiated connexions can be made out.

Animals all require proteid food; their cyst-walls are never formed of cellulose; their cells usually divide in the naked condition only, or if encysted, no secondary party-walls are formed between the daughter-cells to unite them into a vegetative colony. Their colonies are usually locomotive, or, if fixed, their parts largely retain their powers of relative motion, and are often provided on their free surfaces with cilia or flagella; and many cells have differentiated in their cytoplasm contractile muscular fibrils. Their food (except in a few parasitic groups) is always taken {40}into a distinct digestive cavity. A complex nervous system, of many special cells, with branched prolongations interlacing or anastomosing, and uniting superficial sense-organs with internal centres, is universally developed in Metazoa. All Metazoa fulfil the above conditions.

But when we turn to the Protozoa we find that many of the characters evade us. There are some Dinoflagellates (see p. 130) which have coloured plastids, but which differ in no other respect (even specific) from others that lack them: the former may have mouths which are functionless, the latter have functional mouths. Some colourless Flagellates are saprophytic and absorb nutritive liquids, such as decomposing infusions of organic matter, possibly free from all proteid constituents; while others, scarcely different, take in food after the fashion of Amoeba. Sporozoa in the persistence of the encysted stage are very plant-like, though they are often intracellular and are parasitic in living Animals. On the other hand, the Infusoria for the most part answer to all the physiological characters of the Animal world, but are single cells, and by the very perfection of their structure, all due to plasmic not to cellular differentiation, show that they lie quite off the possible track of the origin of Metazoa from Protozoa. Indeed, a strong natural line of demarcation lies between Metazoa and Protista. Of the Protozoa, certain groups, like the Foraminifera and Radiolaria and the Ciliate and Suctorial Infusoria are distinctly animal in their chemical activities or metabolism, their mode of nutrition, and their locomotive powers. When we turn to the Proteomyxa, Mycetozoa, and the Flagellates we find that the distinction between these and the lower Fungi is by no means easy, the former passing, indeed, into true Fungi by the Chytridieae, which it is impossible to separate sharply from those Flagellates and Proteomyxa which Cienkowsky and Zopf have so accurately studied under the name of "Monadineae." Again, many of the coloured Flagellates can only (if at all) be distinguished from Plants by the relatively greater prominence and duration of the mobile state, though classifiers are generally agreed in allotting to Plants those coloured Flagellates which in the resting state assume the form of multicellular or apocytial filaments or plates.

On these grounds we should agree with Haeckel in distinguishing the living world into the Metazoa, or Higher Animals, which {41}are sharply marked off; the Metaphyta, or Higher Plants, which it is not so easy to characterise, but which unite at least two or more vegetal characters; and the Protista, or organisms, whose differentiation is limited to that within the cell (or apocyte), and does not involve the cells as units of tissues. These Protista, again, it is impossible to separate into animal and vegetal so sharply as to treat adequately of either group without including some of the other: thus it is that every text-book on Zoology, like the present work, treats of certain Protophyta. The most unmistakably animal group of the Protista, the Ciliata, is, as we have seen, by the complex differentiation of its protoplasm, widely removed from the Metazoa with their relatively simple cells but differentiated cell-groups and tissues. The line of probable origin of the Metazoa is to be sought, for Sponges at least, among the Choanoflagellates (pp. 121 f. 181 f.).

{42}

CHAPTER II

PROTOZOA (CONTINUED): SPONTANEOUS GENERATION—CHARACTERS OF PROTOZOA—CLASSIFICATION

The Question of Spontaneous Generation

From the first discovery of the Protozoa, their life-history has been the subject of the highest interest: yet it is only within our own times that we can say that the questions of their origin and development have been thoroughly worked out. If animal or vegetable matter of any kind be macerated in water, filtered, or even distilled, various forms of Protista make their appearance; and frequently, as putrefaction advances, form after form makes its appearance, becomes abundant, and then disappears to be replaced by other species. The questions suggested by such phenomena are these: (1) Do the Protista arise spontaneously, that is, by the direct organisation into living beings of the chemical substances present, as a crystal is organised from a solution: (2) Are the forms of the Protista constant from one generation to another, as are ordinary birds, beasts, and fishes?

The question of the "spontaneous generation" of the Protista was readily answered in the affirmative by men who believed that Lice bred directly from the filth of human skins and clothes;[57] and that Blow-flies, to say nothing of Honey-bees, arose in rotten flesh: but the bold aphorism of Harvey "omne vivum ex ovo" at once gained the ear of the best-inspired men of science, and set them to work in search of the "eggs" that gave rise to the organisms of putrefaction. Redi (ob. 1699) showed that Blow-flies never arise save when other Blow-flies gain access to meat and deposit their very visible eggs thereon. Leeuwenhoek, his {43}contemporary, in the latter half of the seventeenth century adduced strong reasons for ascribing the origin of the organisms of putrefaction to invisible air-borne eggs. L. Joblot and H. Baker in the succeeding half-century investigated the matter, and showed that putrefaction was no necessary antecedent of the appearance of these beings: that, as well as being air-borne, the germs might sometimes have adhered to the materials used for making the infusion; and that no organisms were found if the infusions were boiled long enough, and corked when still boiling. These views were strenuously opposed by Needham in England, by Wrisberg in Germany, and by Buffon, the great French naturalist and philosopher, whose genius, unballasted by an adequate knowledge of facts, often played him sad tricks. Spallanzani made a detailed study of what we should now term the "bionomical" or "oecological" conditions of Protistic life and reproduction in a manner worthy of modern scientific research, and not attained by some who took the opposite side within living recollection. He showed that infusions kept sufficiently long at the boiling-point in hermetically sealed vessels developed no Protistic life. As he had shown that active Protists are killed at much lower temperatures, he inferred that the germs must have much higher powers of resistance; and, by modifying the details of his experiments, he was able to meet various objections of Needham.

Despite this good work, the advocates of spontaneous generation were never really silenced; and the widespread belief in the inconstancy of species in Protista added a certain amount of credibility to their cause. In 1845 Pineau put forward these views most strongly; and from 1858 to 1864 they were supported by the elder Pouchet. Louis Pasteur, who began life as a chemist, was led from a study of alcoholic fermentation to that of the organisms of fermentation and of putrefaction and disease. He showed that in infusions boiled sufficiently long and sealed while boiling, or kept at the boiling-point in a sealed vessel, no life manifested itself: objections raised on the score of the lack of access of fresh air were met by the arrangement, so commonly used in "pure cultures" at the present day, of a flask with a tube attached plugged with a little cotton-wool, or even merely bent repeatedly into a zigzag. The former attachment filtered off all germs or floating solid particles from the air, the latter brought about the settling of such particles in the elbows {44}or on the sides of the tube: in neither case did living organisms appear, even after the lapse of months. Other observers succeeded in showing that the forms and characters of species were as constant as in Higher Animals and Plants, allowing, of course, for such regular metamorphoses as occur in Insects, or alternations of generations paralleled in Tapeworms and Polypes. The regular sequences of such alternations and metamorphoses constitute, indeed, a strong support of the "germ-theory"—the view that all Protista arise from pre-existing germs. It is to the Rev. W. H. Dallinger and the late Dr. Charles Drysdale that we owe the first complete records of such complex life-histories in the Protozoa as are presented by the minute Flagellates which infest putrefying liquids (see below, p. 116 f.). The still lower Schizomycetes, the "microbes" of common speech, have also been proved by the labours of Ferdinand Cohn, von Koch, and their numerous disciples, to have the same specific constancy in consecutive generations, as we now know, thanks to the methods first devised by De Bary for the study of Fungi, and improved and elaborated by von Koch and his school of bacteriologists.

And so to-day the principle "omne vivum ex vivo" is universally accepted by men of science. Of the ultimate origin of organic life from inorganic life we have not the faintest inkling. If it took place in the remote past, it has not been accomplished to the knowledge of man in the history of scientific experience, and does not seem likely to be fulfilled in the immediate or even in the proximate future.[58]

PROTOZOA

Organisms of various metabolism, formed of a single cell or apocyte, or of a colony of scarcely differentiated cells, whose organs are formed by differentiations of the protoplasm, and its secretions and accretions; not composed of differentiated multicellular tissues or organs.[59]

{45}

This definition, as we have seen, excludes Metazoa (including Mesozoa, Vol. II. p. 92) sharply from Protozoa, but leaves an uncertain boundary on the botanical side; and, as systematists share with nations the desire to extend their sphere of influence, we shall here follow the lead of other zoologists and include many beings that every botanist would claim for his own realm. Our present knowledge of the Protozoa has indeed been largely extended by botanists,[60] while the study of protoplasmic physiology has only passed from their fostering care into the domain of General Biology within the last decade. The study of the Protozoa is little more than two centuries old, dating from the school of microscopists of whom the Dutchman Leeuwenhoek is the chief representative: and we English may feel a just pride in the fact that his most important publications are to be found in the early records of our own Royal Society.

Baker, in the eighteenth century, and the younger Wallich, Carter, Dallinger and Drysdale, Archer, Saville Kent, Lankester, and Huxley, in the last half-century, are our most illustrious names. In France, Joblot, almost as an amateur, like our own Baker, flourished in the early part of the eighteenth century. Dujardin in the middle of the same century by his study of protoplasm, or sarcode as he termed it, did a great work in laying the foundations of our present ideas, while Balbiani, Georges Pouchet, Fabre-Domergue, Maupas, Léger, and Labbé in France, have worthily continued and extended the Gallic traditions of exact observation and careful deduction. Otto Friedrich Müller, the Dane, in the eighteenth century, was a pioneer in the exact study and description of a large number of forms of these, as of other microscopic forms of life. The Swiss collaborators, Claparède and Lachmann, in the middle of the nineteenth century, added many facts and many descriptions; and illustrated them by most valuable figures of the highest merit from every point of view. Germany, with her large population of students and her numerous universities, has given many names to our list; among these, Ehrenberg and von Stein have added {46}the largest number of species to the roll. Ehrenberg about 1840 described, indeed, an enormous number of forms with much care, and in detail far too elaborate for the powers of the microscope of that date: so that he was led into errors, many and grave, which he never admitted down to the close of a long and honoured life. Max Schultze did much good work on the Protozoa, as well as on the tissues of the Metazoa, and largely advanced our conceptions of protoplasm. His work was continued in Germany by Ernst Haeckel, who systematised our knowledge of the Radiolaria, Greeff, Richard Hertwig, Fritz Schaudinn, and especially Bütschli, who contributed to Bronn's Thier-Reich a monograph of monumental conception and scope, and of admirable execution, on which we have freely drawn. Cienkowsky, a Russian, and James-Clark and Leidy, both Americans, have made contributions of high quality.

Lankester's article in the Encyclopædia Britannica was of epoch-making quality in its philosophical breadth of thought.

Delage and Hérouard have given a full account of the Protozoa in their Traité de Zoologie Concrète, vol. i. (1896); and A. Lang's monograph in his Vergleichende Anatomie, 2nd ed. (1901), contains an admirable analysis of their general structure, habits, and life-cycles, together with full descriptions of well-selected types. Calkins has monographed "The Protozoa" in the Columbia University Biological series (1901). These works of Bütschli, Delage, Lang, and Calkins contain full bibliographies. Doflein has published a most valuable systematic review of the Protozoa parasitic on animals under the title Die Protozoen als Parasiten und Krankheitserreger (1901); and Schaudinn's Archiv für Protistenkunde, commenced only four years ago, already forms an indispensable collection of facts and views.

The protoplasm of the Protozoa (see p. 5 f.) varies greatly in consistency and in differentiation. Its outer layer may be granular and scarcely altered in Proteomyxa, the true Myxomycetes, Filosa, Heliozoa, Radiolaria, Foraminifera, etc.; it is clear and glassy in the Lobose Rhizopods and the Acrasieae; it is continuous with a firm but delicate superficial pellicle of membranous character in most Flagellates and Infusoria; and this pellicle may again be consolidated and locally thickened in some members of both groups so as to form a coat of mail, even with definite spines and hardened polygonal plates (Coleps, Fig. 54, {47}p. 150). Again, it may form transitory or more or less permanent pseudopodia,[61] (1) blunt or tapering and distinct, with a hyaline outer layer, or (2) granular and pointed, radiating and more or less permanent, or (3) branching and fusing where they meet into networks or perforated membranes. Cilia or flagella are motile thread-like processes of active protoplasm which perforate the pellicle; they may be combined into flattened platelets or firm rods, or transformed into coarse bristles or fine motionless sense-hairs. The skeletons of the Protozoa, foreign to the cytoplasm, will be treated of under the several groups.

Most of the fresh-water and brackish forms (and some marine ones) possess one or more contractile vacuoles, often in relation to a more or less complex system of spaces or canals in Flagellates and Ciliates.

The Geographical Distribution of Protozoa is remarkable for the wide, nay cosmopolitan, distribution of the terrestrial and fresh-water forms;[62] they owe this to their power of forming cysts, within which they resist drought, and can be disseminated as "dust." Very few of them can multiply at a temperature approaching freezing-point; the Dinoflagellates can, however, and therefore present Alpine and Arctic forms. The majority breed most freely at summer temperatures; and, occurring in small pools, can thus achieve their full development in such as are heated by the sun during the long Arctic day as readily as in the Tropics. In infusions of decaying matter, the first to appear are those that feed on bacteria, the essential organisms of putrefaction. These, again, are quickly followed and preyed upon by carnivorous species, which prefer liquids less highly charged with organic matters, and do not appear until the liquid, hitherto cloudy, has begun to clear. Thus we have rather to do with "habitat" than with "geographical {48}distribution," just as with the fresh-water Turbellaria and the Rotifers (vol. ii. pp. 4 f., 226 f.). We can distinguish in fresh-water, as in marine Protista, "littoral" species living near the bank, among the weeds; "plankton," floating at or near the surface; "zonal" species dwelling at various depths; and "bottom-dwellers," mostly "limicolous" (or "sapropelic," as Lauterborn terms them), and to be found among the bottom ooze. Many species are "epiphytic" or "epizoic," dwelling on plants or animals, and sometimes choice enough in their preference of a single genus or species as host. Others again are "moss-dwellers," living among the root-hairs of mosses and the like, or even "terrestrial" and inhabiting damp earth. The Sporozoa are internal parasites of animals, and so are many Flagellates, while many Proteomyxa are parasitic in plant-cells. The Foraminifera (with the exception of most Allogromidiaceae) are marine, and so are the Radiolaria; while most Heliozoa inhabit fresh water. Concerning the distribution in time we shall speak under the last two groups, the only ones whose skeletons have left fossil remains.

Classification.—The classification of the Protozoa is no easy task. We omit here, for obvious reasons, the unmistakable Plant Protists that have a holophytic or saprophytic nutrition; that are, with the exception of a short period of locomotion in the young reproductive cells, permanently surrounded with a wall of cellulose or fungus-cellulose, and that multiply and grow freely in this encysted state: to these consequently we relegate the Chytridieae, which so closely allied to the Proteomyxa and the Phycomycetous Fungi; and the Confervaceae, which in the resting state form tubular or flattened aggregates and are allied to the green Flagellates; besides the Schizophyta. At the opposite pole stand the Infusoria in the strict sense, with the most highly differentiated organisation found in our group, culminating in the possession of a nuclear apparatus with nuclei of two kinds, and exhibiting a peculiar form of conjugation, in which the nuclear apparatus is reorganised. The Sporozoa are clearly marked off as parasites in living animals, which mostly begin life as sickle-shaped cells and have always at least two alternating modes of brood-formation, the first giving rise to aplanospores, wherein is formed the second brood of sickle-shaped, wriggling zoospores. The remainder, comprising the Sarcodina, or Rhizopoda in the old wide sense (including all {49}that move by pseudopodia during the great part of their active life), and the Flagellata in the widest sense, are not easy to split up; for many of the former have flagellate reproductive cells, and many of the latter can emit pseudopodia with or without the simultaneous retraction of their flagella. The Radiolaria are well defined by the presence in the body plasm of a central capsule marking it off into a central and a peripheral portion, the former containing the nucleus, the latter emitting the pseudopodia. Again, on the other hand, we find that we can separate as Flagellata in the strict sense the not very natural assemblage of those Protista that have flagella as their principle organs of movement or nutrition during the greater part of their active life. The remaining groups (which with the Radiolaria compose the Sarcodina of Bütschli), are the most difficult to treat. The Rhizopoda, as we shall limit them, are naked or possess a simple shell, never of calcium carbonate, have pseudopodia that never radiate abundantly nor branch freely, nor coalesce to form plasmatic networks, nor possess an axial rod of firmer substance. The Foraminifera have a shell, usually of calcium carbonate, their pseudopodia are freely reticulated, at least towards the base; and (with the exception of a few simple forms) all are marine. The Mycetozoa are clearly united by their tendency to aggregate more or less completely into complex resting-groups (fructifications), and by reproducing by unicellular zoospores, flagellate or amoeboid, which are not known to pair. The Heliozoa resemble the Radiolaria in their fine radiating pseudopodia, but have an axial filament always present in each, and lack the central capsule; and are, for the most part, fresh-water forms. Finally, the Proteomyxa forms a sort of lumber-room for beings which are intermediate between the Heliozoa, Rhizopoda, and Flagellata, usually passing through an amoeboid stage, and for the most part reproducing by brood-formation. Zoospores that possess flagella are certainly known to occur in some forms of Foraminifera, Rhizopoda, Heliozoa, and Radiolaria, though not by any means in all of each group.[63]

{50}
A. Pseudopodia the principal means of locomotion and feeding; flagella absent or transitory I. Sarcodina
(1) Plastogamy only leading to an increase in size, never to the formation of "fructifications."
(a) Pseudopodia never freely coalescing into a network nor fine to the base Rhizopoda.
(*) Ectoplasm clear, free from granules; pseudopodia, usually blunt Rhizopoda Lobosa
(**) Ectoplasm finely granular; pseudopodia slender, branching, but not forming a network, passing into the body by basal dilatation Rhizopoda Filosa
(b) Pseudopodia branching freely and coalescing to form networks; ectoplasm granular; test usually calcareous or sandy Foraminifera
(c) Pseudopodia fine to the very base; radiating, rarely coalescing.
(i.) Pseudopodia with a central filament Heliozoa
(ii.) Pseudopodia without a central filament.
(*) Body divided into a central and a peripheral part by a "central capsule" Radiolaria
(**) Body without a central capsule Proteomyxa
(2) Cells aggregating or fusing into plasmodia before forming a complex "fructification" Mycetozoa
B. Cells usually moving by "euglenoid" wriggling or by excretion of a trail of viscid matter; reproduction by alternating modes of brood-formation, rarely by Spencerian fission II. Sporozoa
C. Flagella (rarely numerous) the chief or only means of motion and feeding III. Flagellata
D. Cilia the chief organs of motion, in the young state at least; nuclei of two kinds IV. Infusoria
{51}

CHAPTER III

PROTOZOA (CONTINUED): SARCODINA

I. Sarcodina.

Protozoa performing most of their life-processes by pseudopodia; nucleus frequently giving off fragments (chromidia) which may play a part in nuclear reconstitution on division; sometimes with brood-cells, which may be at first flagellate; but never reproducing in the flagellate state.[64]

1. Rhizopoda

Sarcodina of simple form, whose pseudopodia never coalesce into networks (1),[65] nor contain an axial filament (2), which commonly multiply by binary fission (3), though a brood-formation may occur; which may temporarily aggregate, or undergo temporary or permanent plastogamic union, but never to form large plasmodia or complex fructifications as a prelude to spore-formation (4); test when present gelatinous, chitinous, sandy, or siliceous, simple and 1-chambered (5).

Classification.[66]

I. Ectoplasm distinct, clear; pseudopodia blunt or tapering, but not branching at the apex Lobosa
Amoeba, Auctt.; Pelomyxa, Greeff; Trichosphaerium, A. Schneid.; Dinamoeba, Leidy; Amphizonella, Greeff; Centropyxis, Stein; Arcella, {52}Ehr.; Difflugia, Leclercq; Lecqueureusia, Schlumberger; Hyalosphenia, Stein; Quadrula, F. E. Sch.; Heleopera, Leidy; Podostoma, Cl. and L.; Arcuothrix, Hallez.
II. Ectoplasm undifferentiated, containing moving granules; pseudopodia branching freely towards the tips Filosa
Euglypha, Duj.; Paulinella, Lauterb.; Cyphoderia, Schlumb.; Campascus, Leidy; Chlamydophrys, Cienk.; Gromia, Duj. = Hyalopus, M. Sch.

We have defined this group mainly by negative characters, as such are the only means for their differentiation from the remaining Sarcodina; and indeed from Flagellata, since in this group zoospores are sometimes formed which possess flagella. Moreover, indeed, in a few of this group (Podostoma, Arcuothrix), as in some Heliozoa, the flagellum or flagella may persist or be reproduced side by side with the pseudopodia. The subdivision of the Rhizopoda is again a matter of great difficulty, the characters presented being so mixed up that it is hard to choose: however, the character of the outer layer of the cytoplasm is perhaps the most obvious to select. In Lobosa there is a clear layer of ectosarc, which appears to be of a greasy nature at its surface film, so that it is not wetted. In the Filosa, as in most other Sarcodina, this film is absent, and the ectoplasm is not marked off from the endoplasm, and may have a granular surface. Corresponding to this, the pseudopodia of the Lobosa are usually blunt, never branching and fraying out, as it were, at the tip, as in the Filosa; nay, in the normal movements of Amoeba limax (Fig. 1, p. 5) the front of the cell forms one gigantic pseudopodium, which constantly glides forward. Apart from this distinction the two groups are parallel in almost every respect.

There may be a single contractile vacuole, or a plurality; or none, especially in marine and endoparasitic species. The nucleus may remain single or multiply without inducing fission, thus leading to apocytial forms. It often gives off "chromidial" fragments, which may play an important part in reproduction.[67] In Amoeba binucleata there are constantly two nuclei, both of which divide as an antecedent to fission, each giving a separate nucleus to either daughter-cell. Pelomyxa palustris, the giant of the group, attaining a diameter of 1‴ (2 mm.), has very blunt pseudopodia, an enormous number of nuclei, and no contractile vacuole, though {53}it is a fresh-water dweller, living in the bottom ooze of ponds, etc., richly charged with organic débris. It is remarkable also for containing symbiotic bacteria, and brilliant vesicles with a distinct membranous wall, containing a solution of glycogen.[68] Few, if any, of the Filosa are recorded as plurinuclear.

The simplest Lobosa have no investment, nor indeed any distinction of front or back. In some forms of Amoeba, however, the hinder part is more adhesive, and may assume the form of a sucker-like disc, or be drawn into a tuft of short filaments or villi, to which particles adhere. Other species of Lobosa and all Filosa have a "test," or "theca," i.e. an investment distinct from the outermost layer of the cell-body. The simplest cases are those of Amphizonella, Dinamoeba, and Trichosphaerium, where this is gelatinous, and in the two former allows the passage of food particles through it into the body by mere sinking in, like the protoplasm itself, closing again without a trace of perforation over the rupture. In Trichosphaerium (Fig. 9) the test is perforated by numerous pores of constant position for the passage of the pseudopodia, closing when these are retracted; and in the "A" form of the species (see below) it is studded with radial spicules of magnesium carbonate. Elsewhere the test is more consistent and possesses at least one aperture for the emission of pseudopodia and the reception of food; to avoid confusion we call this opening not the mouth but the "pylome": some Filosa have two symmetrically placed pylomes. When the test is a mere pellicle, it may be recognised by the limitation of the pseudopodia to the one pylomic area. But the shell is often hard. In Arcella (Fig. 10, C), a form common among Bog-mosses and Confervas, it is chitinous and shagreened, circular, with a shelf running in like that of a diving-bell around the pylome: there are two or more contractile vacuoles, and at least two nuclei. Like some other genera, it has the power of secreting carbonic acid gas in the form of minute bubbles in its cytoplasm, so as to enable it to float up to the surface of the water. The chitinous test shows minute hexagonal sculpturing, the expression of vertical partitions reaching from the inner to the outer layer.

{54}
fig9

Fig. 9.—Trichosphaerium sieboldii. 1, Adult of "A" form; 2, its multiplication by fission and gemmation; 3, resolution into 1-nucleate amoeboid zoospores; 4, development (from zoospores of "A") into "B" form (5); 6, its multiplication by fission and gemmation; 7, its resolution after nuclear bipartition into minute 2-flagellate zoospores or (exogametes); 8, liberation of gametes; 9, 10, more highly magnified pairing of gametes of different origin; 11, 12, zygote developing into "A" form. (After Schaudinn.)

Several genera have tests of siliceous or chitinous plates, formed in the cytoplasm in the neighbourhood of the nucleus, and connected by chitinous cement. Among these Quadrula (Fig. 10, A) is Lobose, with square plates, Euglypha (Fig. 8, p. 29), and Paulinella[69] are Filose, with hexagonal plates. In the latter they are in five longitudinal rows, with a pentagonal oral plate, perforated by the oval pylome. In other genera again, such as Cyphoderia (Filosa), the plates are merely {55}chitinous. Again, the shell may be encrusted with sand-grains derived directly from without, or from ingested particles, as shown in Centropyxis, Difflugia (Fig. 10, D), Heleopera, and Campascus when supplied with powdered glass instead of sand. The cement in Difflugia is a sort of organic mortar, infiltrated with ferric oxide (more probably ferric hydrate). In Lecqueureusia spiralis (formerly united with Difflugia) the test is formed of minute sausage-shaped granules, in which may be identified the partly dissolved valves of Diatoms taken as food; it is spirally twisted at the apex, as if it had enlarged after its first formation, a very rare occurrence in this group. The most frequent mode of fission in the testaceous Rhizopods (Figs. 8, 10) is what Schaudinn aptly terms "bud-fission," where half the protoplasm protrudes and accumulates at the mouth of the shell, and remains till a test has formed for it, while the other half retains the test of the original animal. The materials for the shell, whether sand-granules or plates, pass from the depths of the original shell outwards into the naked cell, and through its cytoplasm to the surface, where they become connected by cementing matter into a continuous test. The nucleus now divides into two, one of which passes into the external animal; after this the two daughter-cells separate, the one with the old shell, the other, larger, with the new one.

fig10

Fig. 10.—Test-bearing Rhizopods. A, Quadrula symmetrica: B, Hyalosphenia lata; C, Arcella vulgaris; D, Difflugia pyriformis. (From Lang's Comparative Anatomy.)

If two individuals of the shelled species undergo bud-fission in close proximity, the offspring may partially coalesce, so that a monstrous shell is produced having two pylomes.

{56}

Reproduction by fission has been clearly made out in most members of the group; some of the multinucleate species often abstrict a portion, sometimes at several points simultaneously, so that fission here passes into budding[70] (Fig. 9, 2, 6).

Brood-division, either by resolution in the multinucleate species, or preceded by multiple nuclear division in the habitually 1-nucleate, though presumably a necessary incident in the life-history of every species, has only been seen, or at least thoroughly worked out, in a few cases, where it is usually preceded by encystment, and mostly by the extrusion into the cyst of any undigested matter.[71]

In Trichosphaerium (Fig. 9) the cycle described by Schaudinn is very complex, and may be divided into two phases, which we may term the A and the B subcycles. The members of the A cycle are distinguished by the gelatinous investment being armed with radial spicules, which are absent from the B form. The close of the A cycle is marked by the large multinucleate body resolving itself into amoeboid zoospores (3), which escape from the gelatinous test, and develop into the large multinucleate adults of the B form. These, like the A form, may reproduce by fission or budding. At the term of growth, however, they retract their pseudopodia, expel the excreta, and multiply their nuclei by mitosis (7). Then the body is resolved into minute 2-flagellate microzoospores (8), which are exogamous gametes, i.e. they will only pair with similar zoospores from another cyst. The zygote (9-11) resulting from this conjugation is a minute amoeboid; its nucleus divides repeatedly, a gelatinous test is formed within which the spicules appear, and so the A form is reconstituted. In many of the test-bearing forms, whether Lobose or Filose, plastogamic unions occur, and the two nuclei may remain distinct, leading to plurinucleate monsters in their offspring by fission, or they may fuse and form a giant nucleus, a process which has here no relation to normal syngamy, as it is not associated with any marked change in the alternation of feeding and fission, etc. In Trichosphaerium also plastogamic unions between small individuals have for their only result the increase of size, enabling the produce to deal with {57}larger prey. Temporary encystment in a "hypnocyst" is not infrequent in both naked and shelled species, and enables them to tide over drought and other unfavourable conditions.

Schaudinn has discovered and worked out true syngamic processes, some bisexual, some exogamous, in several other Rhizopods. In Chlamydophrys stercorea the pairing-cells are equal, and are formed by the aggregation of the chromidia into minute nuclei around which the greater part of the cytoplasm aggregates, while the old nucleus (with a little cytoplasm) is lost. These brood-cells are 2-flagellate pairing-cells, which are exogamous: the zygote is a brown cyst; if this be swallowed by a mammal, the original Chlamydophrys appears in its faeces.[72]

Centropyxis aculeata, a species very common in mud or moss, allied to Difflugia, also forms a brood by aggregation around nuclei derived from chromidia. The brood-cells are amoeboid, and secrete hemispherical shells like those of Arcella; some first divide into four smaller ones, before secreting the shell. Pairing takes place between the large and the small forms; and the zygote encysts. Weeks or months afterwards the cyst opens and its contents creep out as a minute Centropyxis. Finally, Amoeba coli produces its zygote in a way recalling that of Actinosphaerium (pp. 73-75, Fig. 21): the cell encysts; its nucleus divides, and each daughter divides again into two, which fuse reciprocally. Thus the cyst contains two zygote nuclei. After a time each of these divides twice, so that the mature cyst contains eight nuclei. Probably when swallowed by another animal they liberate a brood of eight young amoebae. Thus in different members of this group we have exogamy, both equal and bisexual, and endogamy.

Most of the Rhizopoda live among filamentous Algae in pools, ponds, and in shallow seas, etc.; some are "sapropelic" or mud-dwellers (many species of Amoeba, Pelomyxa, Difflugia, etc.), others frequent the roots of mosses. Amoeba coli is often found as a harmless denizen of the large intestine of man. Amoeba histolytica, lately distinguished therefrom by Schaudinn, is the cause of tropical dysentery. It multiplies enormously in the gut, and is found extending into the tissues, and making its way into the abscesses that so frequently supervene in the liver and other organs. Chlamydophrys stercorea is found in the {58}faeces of several mammals. The best monograph of this group is that of Penard.[73]

2. Foraminifera[74]

Sarcodina with no central capsule or distinction of ectosarc; the pseudopodia fine, branching freely, and fusing where they meet to form protoplasmic networks, or the outermost in the pelagic forms radiating, but without a central or axial filament: sometimes dimorphic, reproducing by fission and by rhizopod or flagellate germs in the few cases thoroughly investigated: all marine (with the exception of some of the Allogromidiaceae), and usually provided with a test of carbonate of lime ("vitreous" calcite, or "porcellanous" aragonite?), or of cemented particles of sand ("arenaceous"); test-wall continuous, or with the walls perforated by minute pores or interstices for the protrusion of pseudopodia.

The classification of Carpenter (into Vitreous or Perforate, Porcellanous or Imperforate, and Arenaceous), according to the structure of the shell, had proved too artificial to be used by Brady in the great Monograph of the Foraminifera collected by the "Challenger" Expedition,[75] and has been modified by him and others since then. We reproduce Lister's account of Brady's classification.[76] We must, however, warn the tyro that its characterisations are not definitions (a feature of all other recent systems), for rigid definitions are impossible: here as in the case, for instance, of many Natural Orders of Plants, transitional forms making the establishment of absolute boundaries out of the question. In the following classification we do not think it, therefore, necessary to complete the characterisations by noting the extremes of variation within the orders:—

1. Allogromidiaceae: simple forms, often fresh-water and similar to Rhizopoda; test 0, or chitinous, gelatinous, or formed of cemented particles, whether secreted platelets or ingested granules. Biomyxa, Leidy = Gymnophrys, Cienk.; {59}Diaphorodon, Archer; Allogromia, Rhumbl. (= Gromia, auctt.[77] nec Duj.) (Fig. 14, 1); Lieberkühnia, Cl. and Lachm. (Fig. 12); Microgromia, R. Hertw. (Fig. 11); Pamphagus, Bailey.

2. Astrorhizidaceae: test arenaceous, often large, never truly chambered, or if so, asymmetrical. Astrorhiza, Sandahl; Haliphysema, Bowerb.; Saccammina, M. Sars (Fig. 13, 1); Loftusia, Brady.

3. Lituolidaceae: test arenaceous, often symmetrical or regularly spiral, isomorphous with calcareous forms: the chambers when old often "labyrinthine" by the ingrowth of wall-material. Lituola, Lam.; Reophax, Montf.; Ammodiscus, Reuss; Trochammina, Parker and Jeffreys.

4. Miliolidaceae: test porcellanous, imperforate, spirally coiled or cyclic, often chambered except in Cornuspira: simple in Squamulina. Cornuspira, Max Sch.; Peneroplis, Montf.; Miliolina, Lam. (incl. Biloculina (Fig. 15), Triloculina, Quinqueloculina (Figs. 14, 4; 15, B), Spiroloculina (Fig. 13, 5) of d'Orb.); Alveolina, d'Orb.; Hauerina, d'Orb.; Calcituba, Roboz; Orbitolites, Lam.; Orbiculina, Lam.; Alveolina, Park. and Jeffr.; Nubecularia, Def.; Squamulina, Max Sch. (Fig. 14, 3).

5. Textulariaceae: test calcareous, hyaline, perforated; chambers increasing in size in two alternating rows, or three, or passing into a spiral. Textularia, Def.; Bulimina, d'Orb.; Cassidulina, d'Orb.

6. Cheilostomellaceae: test vitreous, delicate, finely perforated, chambered, isomorphic with the spiral forms of the Miliolidaceae. Cheilostomella, Reuss.

7. Lagenaceae: Test vitreous, very finely perforate, chambers with a distinct pylome projecting (ectosolenial), or turned in (entosolenial), often succeeding to form a necklace-like shell. Lagena, Walker and Boys (Fig. 13, 2); Nodosaria, Lam. (Fig. 13, 3); Cristellaria, Lam.; Frondicularia, Def. (Fig. 13, 4); Polymorphina, Lam.; Ramulina, Wright.

8. Globigerinidae: test vitreous, perforate; chambers few, dilated, and arranged in a flat or conical spiral, usually with a crescentic pylome to the last. Globigerina, d'Orb. (Figs. 13, 6; 16, 2); Hastigerina, Wyv. Thoms.; Orbulina, d'Orb. (Fig. 16, 1).

9. Rotaliaceae; test vitreous, perforate, usually a conical spiral (like a snail), chambers often subdivided into chamberlets, and with a proper wall, and intermediate skeleton traversed by canals. Rotalia, Lam. (Fig. 14, 2); Planorbulina, d'Orb. (Fig. 13, 9); Polytrema, Risso; Spirillina, Ehr. (non-septate); Patellina, Will.; Discorbina, P. and J. (Fig. 13, 7).

10. Nummulitaceae: test usually a complex spiral, the turns completely investing their predecessors: wall finely tubular, often with a proper wall and intermediate skeleton. Fusulina, Fisch.; Polystomella, Lam.; Nummulites, d'Orb. (Fig. 13, 11); Orbitoides, d'Orb.

The Allogromidiaceae are a well-marked and distinct order, on the whole resembling the Rhizopoda Filosa, and are often found with them in fresh water, while all other Foraminifera are marine. The type genus, Allogromia (Fig. 14, 1), has an oval chitinous shell. Microgromia socialis (Fig. 11) is often found in aggregates, the pseudopodia of neighbours fusing where they meet into a {60}common network. This is due to the fact that one of the two daughter-cells at each fission, that does not retain the parent shell, remains in connexion with its sister that does: sometimes, however, it retracts its pseudopodia, except two which become flagella, wherewith it can swim off. The test of Pamphagus is a mere pellicle. In Lieberkühnia (Fig. 12) it is hardly that; though the body does not give off the fine pseudopodia directly, but emits a thick process or "stylopodium"[78] comparable to the protoplasm protruded through the pylome of its better protected allies; and from this, which often stretches back parallel to the elongated body, the reticulum of pseudopodia is emitted. Diaphorodon has a shell recalling that of Difflugia (Fig. 10, D, p. 55), formed of sandy fragments, but with interstices between them through which as well as through the two pylomes the pseudopodia pass. In all of these the shell is formed as in the Rhizopods once for all, and does not grow afterwards; and the fresh-water forms, which are the majority, have one or more contractile vacuoles; in Allogromia they are very numerous, scattered on the expanded protoplasmic network.

fig11

Fig. 11.—Microgromia socialis. A, entire colony; B, single zooid; C, zooid which has undergone binary fission, with one of the daughter-cells creeping out of the shell; D, flagellula. c.vac, Contractile vacuole; nu, nucleus; sh, shell. (From Parker and Haswell, after Hertwig and Lesser.)

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fig12

Fig. 12.—Lieberkühnia, a fresh-water Rhizopod, from the egg-shaped shell of which branched pseudopodial filaments protrude. (From Verworn.)

The remaining marine families may all be treated of generally, before noting their special characters. Their marine habitat is variable, but in most cases restricted. A few extend up the brackish water of estuaries: a large number are found between tide-marks, or on the so-called littoral shelf extending to deep water; they are for the most part adherent to seaweeds, or lie among sand or on the mud. Other forms, again, are pelagic, such as Globigerina (Figs. 13, 6, 16, 17) and its allies, and float as part of the plankton, having the surface of their shells extended by delicate spines, their pseudopodia long and radiating, and the outer part of their cytoplasm richly vacuolated ("alveolate"), and probably containing a liquid lighter than sea water, as in the Radiolaria. Even these, after their death and the decay of the protoplasm, must sink to the bottom (losing the fine spines by solution as they fall); and they accumulate there, to form a light oozy mud, the "Globigerina-ooze" of geographers, at depths where the carbonic acid under pressure is not adequate to dissolve the more solid calcareous matter. Grey Chalk is such an ooze, consolidated by {62}the lapse of time and the pressure of superincumbent layers. Some Foraminifera live on the sea bottom even at the greatest depths, and of course their shell is not composed of calcareous matter. Foraminifera may be obtained for examination by carefully washing sand or mud, collected on the beach at different levels between tide-marks, or from dredgings, or by carefully searching the surface of seaweeds, or by washing their roots, or, again, by the surface or deep-sea tow-net. The sand used to weight sponges for sale is the ready source of a large number of forms, and may be obtained for the asking from the sponge-dealers to whom it is a useless waste product. If this sand is dried in an oven, and then poured into water, the empty shells, filled with air, will float to the surface, and may be sorted by fine silk or wire gauze.

From the resemblance of the shells of many of them to the Nautilus they were at first described as minute Cephalopods, or Cuttlefish, by d'Orbigny,[79] and their true nature was only elucidated in the last century by the labours of Williamson, Carpenter, Dujardin, and Max Schultze. At first they possess only one nucleus, but in the adult stage may become plurinucleate without dividing, and this is especially the case in the "microsphaeric" states exhibited by many of those with a complex shell; the nucleus is apt to give off fragments (chromidia) which lie scattered in the cytoplasm. At first, too, in all cases, the shell has but a single chamber, a state that persists through life in some. When the number of chambers increases, their number has no relation to that of the nuclei, which remains much smaller till brood-formation sets in.

The shell-substance, if calcareous, has one of the two types, porcellanous or vitreous, that we have already mentioned, but Polytrema, a form of very irregular shape, though freely perforated, is of a lovely pink colour. In the calcareous shells sandy particles may be intercalated, forming a transition to the Arenacea. In these the cement has an organic base associated with calcareous or ferruginous matter; in some, however, the cement is a phosphate of iron. The porcellanous shells are often deep brown by transmitted light.

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fig13

Fig. 13.—Shells of Foraminifera. In 3, 4, and 5, a shows the surface view, and b a section; 8a is a diagram of a coiled cell without supplemental skeleton; 8b of a similar form with supplemental skeleton (s.sk); and 10 of a form with overlapping whorls; in 11a half the shell is shown in horizontal section; b is a vertical section; a, aperture of the shell; 1-15, successive chambers, 1 being always the oldest or initial chamber. (From Parker and Haswell, after other authors.)

Despite the apparent uniformity of the protoplasmic body in this group, the shell is infinitely varied in form. As Carpenter writes, in reference to the Arenacea, "There is nothing more wonderful in nature than the building up of these elaborate and symmetrical structures by mere jelly-specks, presenting no traces {64}whatever of that definite organisation which we are accustomed to regard as necessary to the manifestations of conscious life.... The tests (shells) they construct when highly magnified bear comparison with the most skilful masonry of man. From the same sandy bottom one species picks up the coarsest quartz grains, unites them together with a ferruginous cement, and thus constructs a flask-shaped test, having a short neck and a single large orifice; another picks up the finer grains and puts them together with the same cement into perfectly spherical tests of the most extraordinary finish, perforated with numerous small pores disposed at pretty regular intervals. Another species selects the minutest sand grains and the terminal portions of sponge-spicules, and works them up together—apparently with no cement at all, but by the mere laying of the spicules—into perfect white spheres like homoeopathic globules, each showing a single-fissured orifice. And another, which makes a straight, many-chambered test, the conical mouth of each chamber projecting into the cavity of the next, while forming the walls of its chambers of ordinary sand grains rather loosely held together, shapes the conical mouths of the chambers by firmly cementing together the quartz grains which border it." The structure of the shell is indeed variable. The pylome may be single or represented by a row of holes (Peneroplis, Orbitolites), or, again, there may be several pylomes (Calcituba); and, again, there are in addition numerous scattered pores for the protrusion of pseudopodia elsewhere than from the stylopodium, in the whole of the "Vitrea" and in many "Arenacea"; and, as we shall see, this may exercise a marked influence on the structure of the shell.

In some cases the shell is simple, and in Cornuspira and Spirillina increases so as to have the form of a flat coiled tube. In Calcituba the shell branches irregularly in a dichotomous way, and the older parts break away as the seaweed on which they grow is eaten away, and fall to the bottom, while the younger branches go on growing and branching. The fallen pieces, if they light on living weed, attach themselves thereto and repeat the original growth; if not, the protoplasm crawls out and finds a fresh weed and forms a new tube. In the "Polythalamia" new chambers are formed by the excess of the protoplasm emerging and surrounding itself with a shell, organically united with the existing chamber or chambers, and in a space-relation which follows definite laws characteristic of the species or of its stage of growth, so as to give rise to circular, spiral, or irregular complexes (see Fig. 13).

{65}
fig14

Fig. 14.—Various forms of Foraminifera. In 4, Miliola, a, shows the living animal; b, the same killed and stained; a, aperture of shell; f, food particles; nu, nucleus; sh, shell. (From Parker and Haswell, after other authors.)

In most {66}cases the part of the previously existing chamber next the pylome serves as the hinder part of the new chamber, and the old pylome becomes the pore of communication. But in some of the "Perforata" each new chamber forms a complete wall of its own ("proper wall," Fig. 13, 8b), and the space between the two adjacent walls is filled with an intermediate layer traversed by canals communicating with the cavities of the chambers ("intermediate skeleton"), while an external layer of the same character may form a continuous covering. The shell of the Perforata may be adorned with pittings or fine spines, which serve to increase the surface of support in such floating forms as Globigerina, Hastigerina, and the like (Fig. 17). In the "Imperforata" the outer layer is often ornamented with regular patterns of pits, prominences, etc., which are probably formed by a thin reflected external layer of protoplasm. In some of the "Arenacea" a "labyrinthine" complex of laminae is formed.

A very remarkable point which has led to great confusion in the study of the Foraminifera, is the fact that the shell on which we base our characters of classification, may vary very much, even within the same individual. Thus in the genus Orbitolites the first few chambers of the shell have the character of a Milioline, in Orbiculina of a Peneroplis. The arrangements of the Milioline shell, known as Triloculine, Quinqueloculine, and Biloculine respectively, may succeed one another in the same shell (Figs. 14 4, 15). A shell may begin as a spiral and end by a straight continuation: again, the spherical Orbulina (Fig. 16 1) is formed as an investment to a shell indistinguishable from Globigerina, which is ultimately absorbed. In some cases, as Rhumbler has pointed out, the more recent and higher development shows itself in the first formed chambers, while the later, younger chambers remain at a lowlier stage, as in the case of the spiral passing into a straight succession; but the other cases we have cited show that this is not always the case. In Lagena (Fig. 13 2) the pylome is produced into a short tube, which may protrude from the shell or be turned into it, so that for the latter form the genus Entosolenia was founded. Shells identical in minute sculpture are, however, found with either form of neck, and, moreover, the polythalamial shells (Nodosaria, Fig. 13 3), formed of a nearly straight succession of Lagena-like chambers, may have these chambers with their {67}communications on either type. Rhumbler goes so far as to suggest that all so-called Lagena shells are either the first formed chamber of a Nodosaria which has not yet become polythalamian by the formation of younger ones, or are produced by the separation of an adult Nodosaria into separate chambers.

fig15

Fig. 15.—A, Megalospheric; B, microspheric shell of Biloculina. c, The initial chamber. The microspheric form begins on the Quinqueloculina type. (From Calkins' Protozoa.)

Many of the chambered species show a remarkable dimorphism, first noted by Schlumberger, and finally elucidated by J. J. Lister and Schaudinn. It reveals itself in the size of the initial chamber; accordingly, the two forms may be distinguished as "microspheric" and "megalospheric" respectively (Fig. 15), the latter being much the commoner. The microspheric form has always a plurality of nuclei, the megalospheric a single one, except at the approach of reproduction. Chromidial masses are, however, present in both forms. The life-history has been fully worked out in Polystomella by Schaudinn, and in great part in Polystomella, Orbitolites, etc., by Lister; and the same scheme appears to be general in the class, at least where the dimorphism noted occurs. The microspheric form gives birth only to the megalospheric, but the latter may reproduce megalospheric broods, or give rise to swarmers, which by their (exogamous) {68}conjugation produce the microspheric young. The microspheric forms early become multinucleate, and have also numerous chromidia detached from the nuclei, which they ultimately replace. These collect in the outer part of the shell and aggregate into new nuclei, around which the cytoplasm concentrates, to separate into as many amoeboid young "pseudopodiospores" as there are nuclei. These escape from the shell or are liberated by its disintegration, and invest themselves with a shell to form the initial large central chamber or megalosphere.

fig16

Fig. 16.—1, Orbulina universa. Highly magnified. 2, Globigerina bulloides. Highly magnified. (From Wyville Thomson, after d'Orbigny.)

In the ordinary life of the megalospheric form the greater part of the chromatic matter is aggregated into a nucleus, some still remaining diffused. At the end of growth the nucleus itself disintegrates, and the chromidia concentrate into a number of small vesicular nuclei, each of which appropriates to itself a small surrounding zone of thick plasm and then divides by mitosis twice; and the 4-nucleate cells so formed are resolved into as many 1-nucleate, 2-flagellate swarmers, which conjugate {69}only exogamously.[80] The fusion of their nuclei takes place after some delay: ultimately the zygote nucleus divides into two, a shell is formed, and we have the microsphere, which is thus pluri-nucleate ab initio. As we have seen, the nuclei of the microsphere are ultimately replaced by chromidia, and the whole plasmic body divides into pseudopodiospores, which grow into the megalospheric form.

fig17

Fig. 17.—Shell of Globigerina bulloides, from tow-net, showing investment of spines. (From Wyville Thomson.)

In the Perforate genera, Patellina and Discorbina, plastogamy precedes brood formation, the cytoplasms of the 2-5 pairing individuals contracting a close union; and then the nuclei proceed to break up without fusion, while the cytoplasm aggregates around the young nuclei to form amoebulae, which acquire a shell and separate. In both cases it is the forms with a single nucleus, corresponding to megalospheric forms that so pair, and the brood-formation is, mutatis mutandis, the same as in these forms. Similar individuals may reproduce in the same way, in both genera, without this plastogamic pairing, which is therefore, though probably advantageous, not essential. If pseudopodiospores form their shells while near one another, they may coalesce to form monsters, as often happens in Orbitolites.[81]

The direct economic uses of the Foraminifera are perhaps greater than those of any other group of Protozoa. The Chalk is {70}composed largely of Textularia and allied forms, mixed with the skeletons of Coccolithophoridae (pp. 113-114), known as Coccoliths, etc. The Calcaire Grossier of Paris, used as a building stone, is mainly composed of the shells of Miliolines of Eocene age; the Nummulites of the same age of the Mediterranean basin are the chief constituent of the stone of which the Pyramids of Egypt are built. Our own Oolitic limestones are composed of concretions around a central nucleus, which is often found to be a minute Foraminiferous shell.

The palaeontology of the individual genera is treated of in Chapman's and Lister's recent works. They range from the Lower Cambrian characterised by perforated hyaline genera, such as Lagena, to the present day. Gigantic arenaceous forms, such as Loftusia, are among the Tertiary representatives; but the limestones formed principally of their shells commence at the Carboniferous. The so-called Greensands contain greenish granules of "glauconite," containing a ferrous silicate, deposited as a cast in the chambers of Foraminifera, and often left exposed by the solution of the calcareous shell itself. Such granules occur in deep-sea deposits of the present day.[82]

3. Heliozoa

Sarcodina with radiate non-anastomosing pseudopodia of granular protoplasm, each with a stiff axial rod passing into the body plasma; no central capsule, nor clear ectoplasm; skeleton when present siliceous; nucleus single or multiple; contractile vacuole (or vacuoles) in fresh-water species, superficial and prominent at the surface in diastole; reproduction by fission or budding in the active condition, or by brood-formation in a cyst, giving rise to resting spores; conjugation isogamous in the only two species fully studied; habitat floating or among weeds, mostly fresh water.

1. Naked or with an investment only when encysted.

Aphrothoraca.—Actinolophus F.E. Sch.; Myxastrum Haeck.; Gymnosphaera Sassaki; Dimorpha (Fig. 37, 5, p. 112) Gruber; Actinomonas Kent; Actinophrys Ehrb.; Actinosphaerium St.; Camptonema Schaud; Nuclearia Cienk.

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2. Invested with a gelatinous layer, sometimes traversed by a firmer elastic network.

Chlamydophora.—Heterophrys Arch.; Mastigophrys Frenzel; Acanthocystis, Carter.

3. Ectoplasm with distinct siliceous spicules.

Chalarothoraca.—Raphidiophrys Arch.

4. Skeleton a continuous, fenestrated shell, sometimes stalked.

Desmothoraca.—Myriophrys Penard; Clathrulina Cienk.; Orbulinella Entz.

This class were at first regarded and described as fresh-water Radiolaria, but the differences were too great to escape the greatest living specialist in this latter group, Ernst Haeckel, who in 1866 created the Heliozoa for their reception. We owe our knowledge of it mainly to the labours of Cienkowsky, the late William Archer, F. E. Schulze, R. Hertwig, Lesser, and latterly to Schaudinn, who has monographed it for the "Tierreich" (1896); and Penard has published a more recent account.

fig18

Fig. 18.—Actinophrys sol. a, Axial filament of pseudopod; c.v, contractile vacuole; n, nucleus. (From Lang's Comparative Anatomy, after Grenacher.)

Actinophrys sol Ehrb. (Fig. 18) is a good and common type. It owes its name to its resemblance to a conventional drawing of the sun, with a spherical body and numerous close-set diverging rays. The cytoplasm shows a more coarsely vacuolated outer layer, sometimes called the ectosarc, and a denser internal layer the endosarc. In the centre of the figure is the large nucleus, to which the continuations of the rays may be seen to converge; the pseudopodia contain each a stiffish axial filament,[83] which is covered by the fine granular plasm, showing currents of the granules. The axial filament disappears when the pseudopodia are retracted or bent, and is regenerated afterwards. This bending occurs when a living prey touches and adheres to a ray, all its neighbours bending in like the tentacles of a Sundew. The prey is carried down to the surface of the ectoplasm, and {72}sinks into it with a little water, to form a nutritive vacuole. Fission is the commonest mode of reproduction, and temporary plastogamic unions are not uncommon. Arising from these true conjugations occur, two and two, as described by Schaudinn. A gelatinous cyst wall forms about the two which are scarcely more than in contact with their rays withdrawn. Then in each the nucleus divides into two, one of which passes to the surface, and is lost (as a "polar body"), while the other approaches the corresponding nucleus of the mate, and unites with it, while at the same time the cytoplasms fuse. Within the gelatinous cyst the zygote so formed divides to produce two sister resting spores, from each of which, after a few days, a young Actinophrys escapes, as may take place indeed after encystment of an ordinary form without conjugation.

fig19

Fig. 19.—Actinosphaerium eichornii. A, entire animal with two contractile vacuoles (c.vac); B, a portion much magnified, showing alveolate cytoplasm, pseudopodia with axial rods, non-nucleate cortex (cort), multiple nuclei (nu) of endoplasm (med), and food-vacuole (chr). (From Parker and Haswell.)

The axial rods of the pseudopodia may pass either to the circumference of the nucleus or to a central granule, corresponding, it would appear, to a centrosome or blepharoplast; or again, {73}in the plurinucleate marine genus Camptonema, each rod abuts on a separate cap on the outer side of each nucleus. The nucleus is single in all but the genera Actinosphaerium, Myxastrum, Camptonema, and Gymnosphaera. The movements of this group are very slow, and are not well understood. A slow rolling over on the points of the rays has been noted, and in Camptonema they move very decidedly to effect locomotion, the whole body also moving Amoeba-fashion; but of the distinct movements of the species when floating no explanation can be given. The richly vacuolate ectoplasm undoubtedly helps to sustain the cell, and the extended rays must subserve the same purpose by so widely extending the surface. Dimorpha (Fig. 37, 5, p. 112) has the power of swimming by protruding a pair of long flagella from the neighbourhood of the eccentric nucleus; and Myriophrys has an investment of long flagelliform cilia. Actinomonas has a stalk and a single flagellum in addition to the pseudopodia; these genera form a transition to the Flagellata.

Several species habitually contain green bodies, which multiply by bipartition, and are probably Zoochlorellae, Chlamydomonadidae of the same nature as we shall find in certain Ciliata (pp. 154, 158) in fresh-water Sponges (see p. 175), in Hydra viridis (p. 256), and the marine Turbellarian Convoluta (Vol. II. p. 43).

Reproduction by fission is not rare, and in some cases (Acanthocystis) the cell becomes multinuclear, and buds off 1-nucleate cells. In such cases the buds at first lack a centrosome, and a new one is formed first in the nucleus, and passes out into the cytoplasm. These buds become 2-flagellate before settling down. In Clathrulina the formation of 2-flagellate zoospores has long been known (Fig. 20, 3). In Actinosphaerium (Figs. 19, 21), a large species, differing from Actinophrys only in the presence of numerous nuclei in its endoplasm, a peculiar process, which we have characterised as endogamy, results in the formation of resting spores. The animal retracts its rays and encysts; and the number of nuclei is much reduced by their mutual fusion, or by the solution of many of them, or by a combination of the two processes. The body then breaks up into cells with a single nucleus, and each of these surrounds itself with a wall to form a cyst of the second order.

{74}
fig20

Fig. 20.—Various forms of Heliozoa. In 3, a is the entire animal and b the flagellula; c.vac, contractile vacuole; g, gelatinous investment; nu, nucleus; psd, pseudopodia; sk, siliceous skeleton; sp, spicules. (From Parker and Haswell, after other authors.)

Each of these divides, and the two sister cells then conjugate after the same fashion as in Actinophrys, but the nuclear divisions to form the coupling nucleus are two in number, i.e. the nucleus divides into two, one of which goes to the surface as the first polar body, and the sister of this again divides to form a second polar body (which also passes to {75}the surface) and a pairing nucleus.[84] The two cells then fuse completely, and surround themselves with a second gelatinous cyst wall, separated from the outer one by a layer of siliceous spicules. The nucleus appears to divide at least twice before the young creep out, to divide immediately into as many Actinophrys-like cells as there were nuclei; then each of these multiplies its nuclei, to become apocytial like the adult form.

fig21

Fig. 21.—Diagram illustrating the conjugation of Actinosphaerium. 1, Original cell; 2, nucleus divides to form two, N2N2; 3, each nucleus again divides to form two, N3 and n3, the latter passing out with a little cytoplasm as an abortive cell; 4, repetition of the same process as in 3; 5, the two nuclei N4 have fused in syngamy to form the zygote nucleus Nz.

Schaudinn admits 24 genera (and 7 doubtful) and 41 species (and 18 doubtful). None are known fossil. Their geographical distribution is cosmopolitan, as is the case with most of the minute fresh-water Protista; 8 genera are exclusively marine, and Orbulinella has only been found in a salt-pond; Actinophrys sol is both fresh-water and marine, and Actinolophus has 1 species fresh-water, the other marine. One of the 14 species of Acanthocystis is marine; the remaining genera and species are all inhabitants of fresh water.[85]

4. Radiolaria

Sarcodina with the protoplasm divided by a perforated chitinous central capsule into a central mass surrounding the nucleus, and an outer layer; the pseudopodia radiate, never anastomosing enough to form a marked network; skeleton either siliceous, of spicules, or perforated; or of definitely arranged spicules of proteid matter (acanthin), sometimes also coalescing into a latticed shell; reproduction by fission and by zoospores formed in the central capsule. Habitat marine, suspended at the surface (plankton), at varying depths (zonarial), or near the bottom (abyssal).

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fig22

Fig. 22.—Collozoum inerme. A, B, C, three forms of colony; D, small colony with central capsules (c.caps), containing nuclei, and alveoli (vac) in ectoplasm; E, isospores, with crystals (c); F, anisospores; nu, nucleus. (From Parker and Haswell.)

The following is Haeckel's classification of the Radiolaria:—

I. Porulosa (Holotrypasta).—Homaxonic, or nearly so. Central capsule spherical in the first instance; pores numerous, minute, scattered; mostly pelagic.

A. Spumellaria (Peripylaea).—Pores evenly scattered; skeleton of solid siliceous spicules, or continuous, and reticulate or latticed, rarely absent; nucleus dividing late, as an antecedent to reproduction.

B. Acantharia (Actipylaea).—Pores aggregated into distinct areas; skeleton of usually 20 centrogenous, regularly radiating spines of acanthin, whose branches may coalesce into a latticed shell; nucleus dividing early.

II. Osculosa (Monotrypasta).—Monaxonic; pores of central capsule limited to the basal area (osculum), sometimes accompanied by two (or more) smaller oscula at apical pole, mostly zonarial or abyssal.

C. Nassellaria (Monopylaea).—Central capsule ovoid, of a single layer; pores numerous on the operculum or basal field; skeleton siliceous, usually with a principal tripod or calthrop-shaped spicule passing, by branching, into a complex ring or a latticed bell-shaped shell; nucleus eccentric, near apical pole.

D. Phaeodaria (Cannopylaea, Haeck.; Tripylaea, Hertw.).—Central capsule spheroidal, of two layers, in its outer layer an operculum, with radiate ribs and a single aperture, beyond which protrudes the outer layer; osculum basal, a dependent tube (proboscis); accessory oscula, when present, simpler, usually two placed symmetrically about the apical pole; skeleton siliceous, with a combination of organic matter, often of hollow spicules; nucleus sphaeroidal, eccentric; extracapsular protoplasm containing an accumulation of dusky pigment granules ("phaeodium").

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fig23

Fig. 23.—Actinomma asteracanthion. A, the shell with portions of the two outer spheres broken away; B, section showing the relations of the skeleton to the animal, cent.caps, Central capsule; ex.caps.pr, extra-capsular protoplasm: nu, nucleus; sk.1, outer, sk.2, middle, sk.3, inner sphere of skeleton. (From Parker and Haswell, after Haeckel and Hertwig.)

A. Spumellaria.

Sublegion (1). Collodaria.[86]—Skeleton absent or of detached spicules; colonial or simple.

Order i. Colloidea.—Skeleton absent. (Families 1, 2.) Thalassicolla Huxl.; Thalassophysa Haeck.; Collozoum Haeck.; Collosphaera J. Müll.; Actissa Haeck.

Order ii. Beloidea.—Skeleton spicular. (Families 3, 4.)

Sublegion (2). Sphaerellaria.—Skeleton continuous, latticed or spongy, reticulate.

Order iii. Sphaeroidea.—Skeleton of one or several concentric spherical shells; sometimes colonial. (Families 5-10.) Haliomma Ehrb.; Actinomma Haeck. (Fig. 23).

Order iv. Prunoidea.—Skeleton a prolate sphaeroid or cylinder, sometimes constricted towards the middle, single or concentric. (Families 11-17.)

Order v. Discoidea.—Shell flattened, of circular plan, simple or concentric, rarely spiral. (Families 18-23.)

Order vi. Larcoidea.—Shell ellipsoidal, with all three axes unequal or irregular, sometimes becoming spiral. (Families 24-32.)[87]

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fig24

Fig. 24.—Xiphacantha (Acantharia). From the surface. The skeleton only, × 100, (From Wyville Thomson.)

B. Acantharia.

Order vii. Actinelida.—Radial spines numerous, more than 20, usually grouped irregularly. (Families 33-35.) Xiphacantha Haeck.

Order viii. Acanthonida.—Radial spines equal. (Families 36-38.)

Order ix. Sphaerophracta.—Radial spines 20, with a latticed spherical shell, independent of, or formed from the reticulations of the spines. (Families 39-41.) Dorataspis Haeck. (Fig. 25, A).

Order x. Prunophracta.—Radial spines 20, unequal; latticed shell, ellipsoidal, lenticular, or doubly conical. (Families 42-44.)

C. Nassellaria.

Order xi. Nassoidea.—Skeleton absent. (Family 45.)

Order xii. Plectoidea.—Skeleton of a single branching spicule, the branches sometimes reticulate, but never forming a latticed shell or a sagittal ring. (Families 46-47.)

Order xiii. Stephoidea.—Skeleton with a sagittal ring continuous with the branched spicule, and sometimes other rings or branches. (Families 48-51.) Lithocercus Théel (Fig. 26, A).

Order xiv. Spyroidea.—Skeleton with a latticed shell developed around the sagittal ring (cephalis), and constricted in the sagittal plane, with a lower chamber (thorax) sometimes added. (Families 52-55.)

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Order xv. Botryoidea.—As in Spyroidea, but with the cephalis 3-4 lobed; lower chambers, one or several successively formed. (Families 56-58.)

Order xvi. Cyrtoidea.—Shell as in the preceding orders, but without lobing or constrictions. (Families 59-70.) Theoconus Haeck. (Fig. 25, B).

D. Phaeodaria.

Order xvii. Phaeocystina.—Skeleton 0 or of distinct spicules; capsule centric. (Families 71-73.) Aulactinium Haeck. (Fig. 26, B).

Order xviii. Phaeosphaeria.—Skeleton a simple or latticed sphere, with no oral opening (pylome); capsule central. (Families 74-77.)

Order xix. Phaeogromia.—Skeleton a simple latticed shell with a pylome at one end of the principal axis; capsule excentric, sub-apical. (Families 78-82.) Pharyngella Haeck.; Tuscarora Murr.; Haeckeliana Murr. (Fig. 28).

Order xx. Phaeoconchia.—Shell of two valves, opening in the plane ("frontal") of the three openings of the capsule. (Families 83-85.)

We exclude Haeckel's Dictyochida, with a skeleton recalling that of the Stephoidea, but of the impure hollow substance of the Phaeodaria (p. 84). They rank now as Silicoflagellates (p. 114).

The Radiolarian is distinguished from all other Protozoa by the chitinous central capsule, so that its cytoplasm is separated into an outer layer, the extracapsular protoplasm (ectoplasm), and a central mass, the intracapsular, containing the nucleus.[88]

The extracapsular layer forms in its substance a gelatinous mass, of variable reaction, through which the plasma itself ramifies as a network of threads ("sarcodictyum"), uniting at the surface to constitute the foundation for the pseudopodia. This gelatinous matter constitutes the "calymma." It is largely vacuolated, the vacuoles ("alveoli"), of exceptional size, lying in the nodes of the plasmic network, and containing a liquid probably of lower specific gravity than seawater; and they are especially abundant towards the surface, where they touch and become polygonal. On mechanical irritation they disappear, to be formed anew after an interval, a fact that may explain the sinking from the surface in disturbed water. This layer may contain minute pigment granules, but the droplets of oil and of albuminous matter frequent in the central layer are rare here. {80}The "yellow cells" of a symbiotic Flagellate or Alga, Zooxanthella, are embedded in the jelly of all except Phaeodaria, and the whole ectosarc has the average consistency of a firm jelly.

The pseudopodia are long and radiating, with a granular external layer, whose streaming movements are continuous with those of the inner network. In the Acantharia they contain a firm axial filament, like that of the Heliozoa, which is traceable to the central capsule; and occasionally a bundle of pseudopodia may coalesce to form a stout process like a flagellum ("sarcoflagellum"). Here, too, each spine, at its exit from the jelly, is surrounded by a little cone of contractile filaments, the myophrisks, whose action seems to be to pull up the jelly and increase the volume of the spherical body so as to diminish its density.

fig25

Fig. 25.—Skeletons of Radiolaria. A, Dorataspis; B, Theoconus. (After Haeckel.)

The intracapsular protoplasm is free from Zooxanthella except in the Acantharia. It is less abundantly vacuolated, and is finely granular. In the Porulosa it shows a radial arrangement, with pyramidal stretches of hyaline plasma separated by intervals rich in granules. Besides the alveoli with watery contents, others are present with albuminoid matter in solution. Oil-drops, often brilliantly coloured, occur either in the plasma or floating in either kind of vacuole; and they are often luminous at night. Added to these, the intracapsular plasm contains pigment-granules, most frequently red or orange, {81}passing into yellow or brown, though violet, blue, and green also occur. The "phaeodium,"[89] however, that gives its name to the Phaeodaria, is an aggregate of dark grey, green, or brown granules which are probably formed in the endoplasm, but accumulate in the extracapsular plasm of the oral side of the central capsule. Inorganic concretions and crystals are also found in the contents of the central capsule, as well as aggregates of unknown composition, resembling starch-grains in structure.

In the Monopylaea, or Nassellaria (Figs. 25, B, 26, A), the endoplasm is differentiated above the perforated area of the central capsule into a cone of radiating filaments termed the "porocone," which may be channels for the communication between the exoplasm and the endoplasm, or perhaps serve, as Haeckel suggests, to raise, by their contraction, the perforated area: he compares them to the myophane striae of Infusoria. In the Phaeodaria (Fig. 26, B), a radiating laminated cone is seen in the outermost layer of the endoplasm above the principal opening ("astropyle"), and a fibrillar one around the two accessory ones ("parapyles"); and in some cases, continuous with these, the whole outer layer of the endoplasm shows a meridional striation.

The nucleus is contained in the endoplasm, and is always at first single, though it may divide again and again. The nuclear wall is a firm membrane, sometimes finely porous. If there are concentric shells it at first occupies the innermost, which it may actually come to enclose, protruding lobes which grow through the several perforations of the lattice-work, finally coalescing outside completely, so as to show no signs of the joins. In the Nassellaria a similar process usually results in the formation of a lobed nucleus, contained in an equally lobed central capsule. The chromatin of the nucleus may be concentrated into a central mass, or distributed into several "nucleoli," or it may assume the form of a twisted, gut-like filament, or, again, the nuclear plasm may be reticulated, with the chromatin deposited at the nodes of the network.

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fig26

Fig. 26.—A, Lithocercus annularis, with sagittal ring (from Parker and Haswell). B, Aulactinium actinastrum. C, calymma; cent.caps., km, central capsule; Ext.caps.pr., Extracapsular, and Int.caps.pr., intracapsular protoplasm; n, nu, nucleus; op, operculum; ph, phaeodium; psd, pseudopodium; Skel., skeleton; z, Zooxanthella. (From Lang's Comparative Anatomy, after Haeckel.)

The skeleton of this group varies, as shown in our conspectus, in the several divisions.[90] The Acantharia (Figs. 24, 25, A) have a skeleton of radiating spines meeting in the centre of figure of the endoplasm, and forcing the nucleus to one side. The spines are typically 20 in number, and emerge from the surface of the regular spherical forms (from which the others may be readily derived) radially, in five sets of four in the regions corresponding to the equator and the tropics and polar circles of our world. {83}The four rays of adjacent circles alternate, so that the "polar" and "equatorial" rays are on one set of meridians 90° apart, and the "tropical" spines are on the intermediate meridians, as shown in the figures. By tangential branching, and the meeting or coalescence of the branches, reticulate (Figs. 23, 24, 25) and latticed shells are formed in some families, with circles of openings or pylomes round the bases of the spines. In the Sphaerocapsidae the spines are absent, but their original sites are inferred from the 20 circles of pylomes.

In the Spumellaria the simplest form of the (siliceous) skeleton is that of detached spicules, simple or complex, or passing into a latticed shell, often with one or more larger openings (pylomes). Radiating spines often traverse the whole of the cavity, becoming continuous with its latticed wall, and bind firmly the successive zones when present (Fig. 23).

Calcaromma calcarea was described by Wyville Thomson as having a shell of apposed calcareous discs, and Myxobrachia, by Haeckel, as having collections of the calcareous Coccoliths and Coccospheres. In both cases we have to do with a Radiolarian not possessing a skeleton, but retaining the undigested shells of its food, in the former case (Actissa) in a continuous layer, in the latter (Thalassicolla) in accumulations that, by their weight, droop and pull out the lower hemisphere into distinct arms.

The (siliceous) skeleton of the Nassellaria is absent only in the Nassoidea, and is never represented by distinct spicules. Its simplest form is a "tripod" with the legs downward, and the central capsule resting on its apex. The addition of a fourth limb converts the tripod into a "calthrop," the central capsule in this case resting between the upturned leg and two of the lower three regarded as the "anterolateral"; the odd lower leg, like the upturned one, being "posterior." Again, the skeleton may present a "sagittal ring," often branched and spiny (Fig. 26, A), or combined with the tripod or calthrop, or complicated by the addition of one or more horizontal rings. Another type is presented by the "latticed chamber" surrounding the central capsule, with a wide mouth ("pylome") below. This is termed the "cephalis"; it may be combined in various ways with the sagittal ring and the tripod or calthrop; and, again, it may be prolonged by the addition of one, two, or three chambers below, {84}the last one opening by a pylome (Fig. 25, B). These are termed "thorax," "abdomen," and "post-abdomen" respectively.

In the Phaeodaria the skeleton may be absent, spicular (of loose or connected spicules) or latticed, continuous or bivalve. It is composed of silica combined with organic matter, so that it chars when heated, is more readily dissolved, and is not preserved in fossilisation. The spicules or lattice-work are hollow, often with a central filament running in the centre of the gelatinous contents. The latticed structure of the shell of the Challengeridae (Fig. 28) is so fine as to recall that of the Diatomaceae. In the Phaeoconchida the shell is in two halves, parted along the "frontal" plane of the three apertures of the capsule.

fig27

Fig. 27.—Scheme of various possible skeletal forms deposited in the meshes of an alveolar system, most of which are realised in the Radiolaria. (From Verworn, after Dreyer.)

The central capsule (rarely inconspicuous and difficult, if not impossible to demonstrate) is of a substance which resembles chitin, though its chemical reactions have not been fully studied hitherto, and indeed vary from species to species. It is composed of a single layer, except in Phaeodaria, where it is double. The operculum in this group, i.e. the area around the aperture, is composed of an outer layer, which is radially thickened, and a thin inner layer; the former is produced into the projecting tube ("proboscis").

Reproduction in the Radiolaria may be simple fission due to the binary fission of the nucleus, the capsule, and the ectoplasm in succession. If this last feature is omitted we have a colonial organism, composed of the common ectoplasm containing numerous central capsules; and the genera in which this occurs, all belonging to the Peripylaea, were formerly separated (as Polycyttaria) from {85}the remaining Radiolaria (Monocyttaria). They may either lack a skeleton (Collozoidae, Fig. 22), or have a skeleton of detached spicules (Sphaerozoidae), or possess latticed shells (Collosphaeridae) one for each capsule, and would seem therefore to belong, as only differentiated by their colonial habit, to the several groups having these respective characters. Fission has been well studied in Aulacantha (a Phaeodarian) by Borgert.[91] He finds that in this case the skeleton is divided between the daughter-cells, and the missing part is regenerated. In cases where this is impossible one of the daughter-cells retains the old skeleton, and the other escapes as a bud to form a new skeleton.

fig28

Fig. 28.—Shells of Challengeridae: A, Tuscarora; B, Pharyngella; C, Haeckeliana. (From Wyville Thomson.)

Two modes of reproduction by flagellate zoospores have been described (Fig. 22). In the one mode all the zoospores are alike—isospores—and frequently contain a crystal of proteid nature as well as oil-globules. In the Polycyttaria alone has the second mode of spore-formation been seen, and that in the same species in which the formation of isospores occurs. Here "anisospores" are formed, namely, large "mega-," and small "micro-zoospores." They probably conjugate as male and female respectively; but neither has the process been observed, nor has any product of such conjugation (zygote) been recognised. In every case the formation of the zoospores only involves the {86}endoplasm: the nucleus first undergoes brood division, and the plasma within the capsule becomes concentrated about its offspring, and segregates into the spores; the extracapsular plasm disintegrates.[92]

The Yellow Cells (Zooxanthella), so frequently found in the Radiolaria were long thought to be constituents of their body. Cienkowsky found that when the host died from being kept in unchanged water, the yellow cells survived and multiplied freely, often escaping from the gelatinised cell-wall as biflagellate zoospores. The cell-wall is of cellulose. The cell contains two chloroplastids, or plates coloured with the vegetal pigment "diatomin." Besides ordinary transverse fission in the ordinary encysted state in the ectoplasm of the host, when free they may pass into what is known as a "Palmella-state," the cell-walls gelatinising; in this condition they multiply freely, and constitute a jelly in which the individual cells are seen as rounded bodies. They contain starch in two forms—large hollow granules, not doubly refractive, and small solid granules which polarise light. We may regard them as Chrysomonadaceae (p. 113). Similar organisms occur in many Anthozoa (see pp. 261, 339, 373 f., 396). Diatomaceae (yellow Algae with silicified cell-walls) sometimes live in the jelly of certain Collosphaera. Both these forms live in the state known as "symbiosis" with their host; i.e. they are in mutually helpful association, the Radiolarian absorbing salts from the water for the nutrition of both, and the Alga or Flagellate taking up the CO2 due to the respiration of the host, and building up organic material, the surplus of which is doubtless utilised, at least in part, for the nutrition of the host. A similar union between a Fungus and a coloured vegetal ("holophytic") organism is known as a Lichen.

The Suctorian Infusorian Amoebophrya is parasitic in the ectoplasm of certain Acantharia, and in the peculiar genus Sticholonche which appears to be intermediate between this group and Heliozoa.

The Silicoflagellate family Dictyochidae are found temporarily {87}embedded in the ectoplasm of some of the Phaeocystina, and have a skeleton of similar nature. Their true nature was shown by Borgert.

The Amphipod crustacean Hyperia[93] may enter the jelly of the colonial forms, and feed there at will on the host.[94]

Haeckel, in his Monograph of the Radiolaria of the Challenger enumerated 739 genera, comprising 4318 species; and Dreyer has added 6 new genera, comprising 39 species, besides 7 belonging to known genera. Possibly, as we shall see, many of the species may be mere states of growth, for it is impossible to study the life-histories of this group; on the other hand, it is pretty certain that new forms are likely to be discovered and described. The Radiolaria are found living at all depths in the sea, by the superficial or deep tow-net; and some appear to live near the bottom, where the durable forms of the whole range also settle and accumulate. They thus form what is known as Radiolarian ooze, which is distinguished from other shallower deposits chiefly through the disappearance by solution of all calcareous skeletons, as they slowly fell through the waters whereon they originally floated at the same time with the siliceous remains of the Radiolaria. The greatest wealth of forms is found in tropical seas, though in some places in cold regions large numbers of individuals of a limited range of species have been found.

Radiolaria of the groups with a pure siliceous skeleton can alone be fossilised, even the impure siliceous skeleton of the Phaeodaria readily dissolving in the depths at which they live: they have been generally described by Ehrenberg's name Polycystineae. Tripolis (Kieselguhr) of Tertiary ages have been found in many parts of the globe, consisting largely or mainly of Radiolaria, and representing a Radiolarian ooze. That of the Miocene of Barbados contains at least 400 species; that of Gruppe at least 130. In Secondary and Palaeozoic rocks such oozes pass into Radiolarian quartzites (some as recent as the Jurassic). They occur also in fossilised excrement (coprolites), and in flint or chert concretions, as far down as the lowest fossiliferous rocks, {88}the Cambrian. The older forms are simple Sphaerellaria and Nassellaria. From a synopsis of the history of the order in Haeckel's Monograph (pp. clxxxvi.-clxxxviii.) we learn that while a large number of skeletal forms had been described by Ehrenberg, Huxley in 1851 published the first account of the living animal. Since then our knowledge has been extended by the labours of Haeckel, Cienkowsky, R. Hertwig, Karl Brandt, and A. Borgert.

5. Proteomyxa

Sarcodina without a clear ectoplasm, whose active forms are amoeboid or flagellate, or pass from the latter form to the former; multiplying chiefly, if not exclusively, by brood-formation in a cyst. No complete cell-pairing (syngamy) known, though the cytoplasms may unite into plasmodia; pseudopodia of the amoeboid forms usually radiate or filose, but without axial filaments. Saprophytic or parasitic in living animals or plants.

This group is a sort of lumber-room for forms which it is hard to place under Rhizopoda or Flagellata, and which produce simple cysts for reproduction, not fructifications like the Mycetozoa. The cyst may be formed for protection under drought ("hypnocyst"), or as a preliminary to spore-formation ("sporocyst"). The latter may have a simple wall (simple sporocyst), or else two or three formed in succession ("resting cyst"), so as to enable it to resist prolonged desiccation, etc.: both differing from the hypnocyst in that their contents undergo brood formation. On encystment any indigestible food materials are extruded into the cyst, and in the "resting cysts," which are usually of at least two layers, this faecal mass lies in the space between them. The brood-cells escape, either as flagellate-cells, resembling the simpler Protomastigina, called "flagellulae," and which often become amoeboid (Fig. 29); or already furnished with pseudopodia, and called "amoebulae," though they usually recall Actinophrys rather than Amoeba. In Vampyrella and some others the amoebulae fuse, and so attain a greater size, which is most probably advantageous for feeding purposes. But usually it is as a uninucleate cell that the being encysts. They may feed either by ingestion by the pseudopodia, by the whole surface contained in a living host-cell, or by passing a pseudopodium into a host-cell (Fig. 29 5). They may be divided as follows:—

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A. Myxoidea.—Flagella 1-3; zoospores separating at once.

1. Zoosporeae.—Brood-cells escaping as flagellulae, even if they become amoeboid later. Ciliophrys Cienk.; Pseudospora Cienk. (Fig. 29).

2. Azoosporeae.—Cells never flagellate. Protomyxa Haeckel; Plasmodiophora Woronin; Vampyrella Cienk.; Serumsporidium L. Pfeiffer.

B. Catallacta.—Brood-cells of cyst on liberation adhering at the centre to form a spherical colony, multiflagellate; afterwards separating, and becoming amoeboid. Magosphaera Haeckel (marine).[95]

fig29

Fig. 29.—Pseudospora lindstedtii. 1, 2, Flagellate zoospores; 3, young amoebula, with two contractile vacuoles, one being reconstituted by three minute formative vacuoles; 4, 5, an amoebula migrating to a fungus hypha through the wall of which it has sent a long pseudopodium; 6, amoebula full-grown; 7, 8, mature cells rounded off, protruding a flagellum, before encysting; 9, young sporocyst; 10, the nucleus has divided into a brood of eight; 11-14, stages of formation of zoospores. cv, Contractile vacuole; e, mass of faecal granules; fl, flagellum; n, nucleus, × about 750⁄1.

Plasmodiophora infests the roots of Crucifers, causing the disease known as "Hanburies," or "fingers and toes," in turnips, etc. Serumsporidium dwells in the body cavity of small Crustacea. Many of this group were described by Cienkowsky under the name of "Monadineae" (in Arch. Mikr. Anat. i. 1865, p. 203). Zopf has added more than anyone else since then to our knowledge. He monographed them under Cienkowsky's name, as a subordinate group of the Myxomycetes, "Pilzthiere oder Schleimpilze," in Schenk's Handb. d. Bot. vol. iii. pt. ii. (1887). To Lankester (Encycl. Brit., reprint 1891) we owe the name here adopted. Zopf has successfully pursued their study in recent {90}papers in his Beitr. Nied. Org. The Chytridieae, usually ascribed to Fungi, are so closely allied to this group that Zopf proposes to include at least the Synchytrieae herein.

This group is very closely allied to Sporozoa; for the absence of cytogamy, and of sickle-germs,[96] and of the complex spores and cysts of the Neosporidia, are the only absolute distinctions.

6. Mycetozoa (Myxomycetes, Myxogastres)

Sarcodina moving and feeding by pseudopodia, with no skeleton, aggregating more or less completely into complex "fructifications" before forming 1-nucleate resting spores; these may in the first instance liberate flagellate zoospores, which afterwards become amoeboid, or may be amoeboid from the first; zoospores capable of forming hypnocysts from which the contents escape in the original form.

1. Aggregation taking place without plastogamy, zoospores amoeboid, with a clear ectosarc Acrasieae.
Copromyxa Zopf; Dictyostelium Brefeld.
2. Aggregation remaining lax, with merely thread-like connexions, except when encystment is to take place; cytoplasm finely granular throughout; complete fusion of the cytoplasm doubtful Filoplasmodieae
Labyrinthula Cienk.; Chlamydomyxa Archer; Leydenia (?) Schaud.
3. Plasmodium formation complete, eventuating in the formation of a complex fructification often traversed by elastic, hygroscopic threads, which by their contraction scatter the spores; zoospores usually flagellate at first Myxomycetes.
Fuligo Hall.; Chondrioderma Rostaf.; Didymium Schrad. (Fig. 30).

I. The Acrasieae are a small group of saprophytes, often in the most literal sense, though in some cases it has been proved that the actual food is the bacteria of putrefaction. In them, since no cell-division takes place in the fructification, it is certain that the multiplication of the species must be due to the fissions of the amoeboid zoospores, which often have the habit of Amoeba limax (Fig. 1, p. 5).

II. Filoplasmodieae.Chlamydomyxa[97] is a not uncommon inhabitant of the cells of bog-mosses and bog-pools, and its nutrition may be holophytic, as it contains chromoplasts; but it {91}can also feed amoeba-fashion. Labyrinthula is marine, and in its fructification each of the component cells forms four spores. Leydenia has been found in the fluid of ascitic dropsy, associated with malignant tumour.

III. Myxomycetes.—The fructification in this group is not formed by the mere aggregation of the zoospores, but these fuse by their cytoplasm to form a multinucleate body, the "plasmodium," which, after moving and growing (with nuclear division) for some time like a great multinucleate Reticularian, passes into rest, and develops a fructification by the formation of a complex outer wall; within this the contents, after multiplication of the nuclei, resolve themselves into uninucleate spores, each with its own cyst-wall. The fructifications of this group are often conspicuous, and resemble those of the Gasteromycetous fungi (e.g., the Puffballs), whence they were at first called Myxogastres. De Bary first discovered their true nature in 1859, and ever since they have been claimed by botanist and zoologist alike.

The spore on germination liberates its contents as a minute flagellate, with a single anterior lash and a contractile vacuole (Fig. 30, C). It soon loses the lash, becomes amoeboid, and feeds on bacteria, etc. (Fig. 30, D, E). In this state it can pass into hypnocysts, from which, as from the spores, it emerges as a flagellula. After a time the amoeboids, which may multiply by fission, fuse on meeting, so as to form the plasmodium (Fig. 30, F). This contains numerous nuclei, which multiply as it grows, and numerous contractile vacuoles. When it attains full size it becomes negatively hydrotactic, crawls to a dry place, and resolves itself into the fructification. The external wall, and sometimes a basal support to the fruit, are differentiated from the outer layer of protoplasm; while the nuclei within, after undergoing a final bipartition, concentrate each around an independent portion of plasma, which again is surrounded as a spore by a cyst-wall. Often the maturing plasmodium within the wall of the fruit is traversed by a network of anastomosing tubes filled with liquid, the walls of which become differentiated into membrane like the fruit-wall, and are continuous therewith. As the fruit ripens the liquid dries, and the tubes now form a network of hollow threads, the "capillitium," often with external spiral ridges (Fig. 30, A, B). These are very hygroscopic, and by their expansion and contraction {92}determine the rupture of the fruit-wall and the scattering of the spores.

fig30

Fig. 30.—Didymium difforme. A, two sporangia (spg 1 and 2) on a fragment of leaf (l); B, section of sporangium, with ruptured outer layer (a), and threads of capillitium (cp); C, a flagellula with contractile vacuole (c.vac) and nucleus (nu); D, the same after loss of flagellum; b, an ingested bacillus; E, an amoebula; F, conjugation of amoebulae to form a small plasmodium; G, a larger plasmodium accompanied by numerous amoebulae; sp, ingested spores. (After Lister.)

Again, in some cases the plasmodia themselves aggregate in the same way as the amoeboids do in the Acrasieae, and combine to form a compound fruit termed an "aethalium,"[98] with the regions of the separate plasmodia more or less clearly marked off. The species formerly termed Aethalium septicum is now known as Fuligo varians. It is a large and conspicuous species, common on tan, and is a pest in the tanpits. Its aethalia may reach a {93}diameter of a foot and more, and a thickness of two inches. Chondrioderma diffusum, often utilised as a convenient "laboratory type," is common on the decaying haulms of beans in the late autumn. The interest of this group is entirely biological, save for the "flowers of tan."[99]

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CHAPTER IV

PROTOZOA (CONTINUED): SPOROZOA[100]

II. Sporozoa.

Protozoa parasitic in Metazoa, usually intracellular for at least part of their cycle, rarely possessing pseudopodia, or flagella (save in the sperms), never cilia; reproduction by brood-formation, often of alternating types; syngamy leading up to resting spores in which minute sickle-germs are formed, or unknown (Myxosporidiaceae).

This group, of which seven years ago no single species was known in its complete cycle, has recently become the subject of concentrated and successful study, owing to the fact that it has been recognised to contain the organisms which induce such scourges to animals as malarial fevers, and various destructive murrains. Our earliest accurate, if partial knowledge, was due to von Siebold, Kölliker, and van Beneden. Thirty years ago Ray Lankester in England commenced the study of species that dwell in the blood, destined to be of such moment for the well-being of man and the animals in his service; and since then our knowledge has increased by the labours of Manson, Ross and Minchin at home, Laveran, Blanchard, Thélohan, Léger, Cuénot, Mesnil, Aimé Schneider in France, Grassi in Italy, Schaudinn, Siedlecki, L. and R. Pfeiffer, Doflein in Central Europe, and many others.

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fig31

Fig. 31.—Lankesteria ascidiae, showing life-cycle. a, b, c, Sporozoites in digestive epithelium cells of host; d, e, growth stages; f, free gregarine; g, association; h, encystment; i, j, brood-divisions in associated mates; k, pairing-cells; l, syngamy; m, zygote; n, o, p, nuclear divisions in spores; q, cyst with adult spores, each containing 8 sickle-germs. (After Luhe, modified from Siedlecki.)

As a type we will take a simple form of the highest group, the Gregarinidaceae, Monocystis, which inhabits the seminal vesicles of the earthworm. In its youngest state, the "sporozoite," it is a naked, sickle-shaped cell, which probably makes its way from the gut into one of the large radial cells of the seminal funnel, where it attains its full size, and then passes out into the vesicles or reservoirs of the semen, to lie among the sperm morulae and young spermatozoa. The whole interior is formed of the opaque endosarc, which contains a large central nucleus, and is full of refractive granules of paramylum or paraglycogen,[101] a carbohydrate allied to glycogen or animal starch, so common in the liver and {96}muscles of Metazoa; besides these it contains proteid granules which stain with carmine, and oil-drops. The ectosarc is formed of three layers: (1) the outer layer or "cuticle"[102] is, in many cases if not here, ribbed, with minute pores in the furrows, and is always porous enough to allow the diffusion of dissolved nutriment; (2) a clear plasmatic layer, the "sarcocyte"; (3) the "myocyte," formed of "myonemes," muscular fibrils disposed in a network with transverse meshes, which effect the wriggling movements of the cell. The endosarc contains the granules and the large central nucleus. The adult becomes free in the seminal vesicles; here two approximate, and surround themselves with a common cyst: a process which has received the name of "association" (Fig. 31, g-i). Within this, however, the protoplasms remain absolutely distinct. The nucleus undergoes peculiar changes by which its volume is considerably reduced. When this process of "nuclear reduction" is completed, each of the mates undergoes brood-divisions (j), so as to give rise to a large number of rounded naked 1-nucleate cells—the true pairing-cells. These unite two and two, and so form the 1-nucleate spores (k-m), which become oat-shaped, form a dense cyst-wall, and have been termed "pseudonavicellae" from their likeness to the Diatomaceous genus Navicella. Some of the cytoplasm of the original cells remains over unused, as "epiplasm," and ultimately degenerates, as do a certain number of the brood-cells which presumably have failed to pair. It is believed that the brood-cells from the same parent will not unite together. The contents of each spore have again undergone brood-division to form eight sickle-shaped zoospores, or "sporozoites" (n-q), and thus the developmental cycle is completed. Probably the spores, swallowed by birds, pass out in their excrement, and when eaten by an earthworm open in its gut; the freed sickle-germs can now migrate through the tissues to the seminal funnels, in the cells of which they grow, ultimately becoming free in the seminal vesicles.[103]

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We may now pass to the classification of the group.

A. Telosporidia.—Cells 1-nucleate until the onset of brood-formation, which is simultaneous.
1. Gregarinidaceae.—Cells early provided with a firm pellicle and possessing a complex ectosarc; at first intracellular, soon becoming free in the gut or coelom of Invertebrates. Pairing between adults, which simultaneously produce each its brood of gametes, isogamous or bisexual, which pair within the common cyst; zygotospores surrounded by a firm cyst, and producing within a brood of sickle-shaped zoospores.
(i.) Schizogregarinidae.—Multiplying by simple fission in the free state as well as by brood-formation; the brood-cells conjugating in a common cyst, but producing only one pairing nucleus in each mate (the rest aborting), and consequently only one spore.
Ophryocystis A. Schn.
(ii.) Acephalinidae.—Cell one-chambered, usually without an epimerite for attachment.
Monocystis F. Stein; Lankesteria Mingazzini.
(iii.) Dicystidae.—Cell divided by a plasmic partition; epimerite usually present.
Gregarina Dufour; Stylorhynchus A. Schn.; Pterocephalus A. Schn.
2. Coccidiaceae.—Cells of simple structure, intracellular in Metazoa. Pairing between isolated cells usually sexually differentiated as oosphere and sperm, the latter often flagellate. Brood-formation of the adult cell giving rise to sickle-shaped zoospores (merozoites), or progamic and producing the gametes. Oosperm motile or motionless, finally producing a brood of spores, which again give rise to a brood of sickle-spores.
(i.) Coccidiidae.—Cell permanently intracellular, or very rarely coelomic, encysting or not before division; zoospores always sickle-shaped; oosperm encysting at once, producing spores with a dense cell-wall producing sickle-germs.
(ii.) Haemosporidae.—Cells parasitic in the blood corpuscles or free in the blood of cold-blooded animals, encysting before brood-formation; zoospores sickle-shaped; oosperm at first motile.
Lankesterella Labbé; (Drepanidium Lank.;) Karyolysus Labbé; Haemogregarina Danilewski.
(iii.) Acystosporidae.—Cells parasitic in the blood and haematocytes of warm-blooded Vertebrates; never forming a cyst-wall before dividing; zoospores formed in the corpuscles, amoeboid. Gametocytes only forming gametes when taken into the stomach of insects. Oosperm at first active, passing into the coelom, producing naked spores which again produce a large brood of sickle zoospores, which migrate to the salivary gland, and are injected with the saliva into the warm-blooded host.
Haemamoeba Grassi and Feletti; Laverania Grassi and Feletti; Haemoproteus Kruse; Halteridium Labbé.[104]
B. Neosporidia.—Cells becoming multinucleate apocytes before any brood-formation occurs. Brood-formation progressive through the apocyte, not simultaneous.
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1. Myxosporidiaceae.—Naked parasites in cold-blooded animals. Spore-formation due to an aggregation of cytoplasm around a single nucleus to form an archespore, which then produces a complex of cells within which two daughter-cells form the spores and accessory nematocysts.

Myxidium Bütsch.; Myxobolus Bütsch.; Henneguya Thélohan; Nosema Nageli (= Glugea Th.).
2. Actinomyxidiaceae.[105]—Apocyte resolved into a sporange, containing eight secondary sporanges (so-called spores), of ternary symmetry and provided with three polar nematocysts.
3. Sarcosporidiaceae.—Encysted parasites in the muscles of Vertebrates, with a double membrane; spores simple.
Sarcocystis Lankester.
fig32

Fig. 32.—Gregarina blattarum Sieb. A, two cephalonts, embedded by their epimerite (ep), in cells of the gut-epithelium; deu, deutomerite; nu, nucleus; pr, protomerite; B1, B2, two free specimens of an allied genus; the epimerite is falling off in B2, which is on its way to become a sporont; C, cyst (cy) of A, with sporoducts (spd) discharging the spores (sp), surrounded by an external gelatinous investment (g). (From Parker and Haswell.)

Monocystis offers us the simplest type of Gregarinidaceae. In most Gregarines (Figs. 31, 32) the sporozoite enters the epithelium-cell of the gut of an Arthropod, Worm or Mollusc, and as it enlarges protrudes the greater part of its bulk into the lumen, and may become free therein, or pass into the coelom. The attached part is often enlarged into a sort of grapple armed with spines, the "epimerite"; this contains only sarcocyte, the other layers being absent. The freely projecting body is usually divided by an ingrowth of the myocyte into a front segment ("protomerite"), and a rear one ("deutomerite"), with the nucleus usually in the latter. In this state the cell is termed a "cephalont." Conjugation is frequent, but apparently is not always connected with {99}syngamy or spore-formation; sometimes from two to five may be aggregated into a chain or "syzygy." The number of cases in which a syngamic process between two cells has been observed is constantly being increased. In Stylorhynchus (Fig. 33) the conjugation at first resembles that of Monocystis, but the actual pairing-cells are bisexually differentiated into sperms in the one parent, and oospheres in the other; it is remarkable that here the pear-shaped sperms are apparently larger than the oospheres. In Pterocephalus the chief difference is that the sperms are minute.[106] In all cases of spore-formation the epimerite is lost and the septum disappears; in this state the cell is termed a sporont. Sometimes the epiplasm of the sporont forms tubes ("sporoducts"), which project through the cyst-wall and give exit to the spores, as in Gregarina (Fig. 32, C), a parasite in the beetle Blaps.

Gregarines infest most groups of Invertebrates except Sponges and perhaps Coelenterates, the only exception cited being that of Epizoanthus glacialis, a Zoantharian (p. 406). They appear to be relatively harmless and are not known to induce epidemics.

The Coccidiaceae never attain so high a degree of cellular differentiation as the Gregarines, which may be due to their habitat; for in the growing state they are intracellular parasites. Their life-history shows a double cycle, which has been most thoroughly worked out in Coccidiidae by Schaudinn and Siedlecki in parasites of our common Centipedes. We take that of Coccidium schubergi (in Lithobius forficatus[107]), beginning with the sporozoite, which is liberated from the spores taken in with the food, in the gut of the Centipede. This active sickle-shaped cell (Fig. 34, l) enters an epithelial cell of the mid-gut, and grows therein till it attains its full size (a), when it is termed a "schizont"; for it segments (Gk. σχίζω, "I split") superficially into a large number of sickle-shaped zoospores, the "merozoites" (c), resembling the sporozoites. The segmentation is superficial, so that there may remain a large mass of residual epiplasm. The merozoites are set free by the destruction of the epithelium-cell in which they were formed, and which becomes disorganised, like the residual epiplasm. Each merozoite may repeat the {100}behaviour of the sporozoite, so that the disease spreads freely, and becomes acute after several reinfections. After a time the adult parasites, instead of becoming schizonts and simply forming merozoites by division, differentiate into cells that undergo a binary sexual differentiation. Some cells, the "oocytes" (d, e), escape into the gut, and the nucleus undergoes changes by which some of its substance (or an abortive daughter-nucleus) is expelled to the exterior (f), such a cell is now an "oogamete" or oosphere. Others, again, are spermatogones (h): each when full grown on escaping into the gut commences a division (i, j), like that of the schizonts. The products of this division or segment-cells are the flagellate sperms (s): they are more numerous and more minute than the merozoites produced by the schizonts, and are attracted to the oosphere by chemiotaxy (p. 23), and one enters it and fuses with it (g). The oosperm, zygote or fertilised egg, thus formed invests itself with a dense cyst-wall, as a "oospore" (k), its contents form one or more (2, 4, 8, etc.) spores; and each spore forms again one, two, or four sickle-shaped zoospores ("sporozoites"), destined to be liberated for a fresh cycle of parasitic life when the spores are swallowed by another host.

fig33

Fig. 33.—Bisexual pairing of Stylorhynchus. a, Spermatozoon; b-e, fusion of cytoplasm of spermatozoon and oosphere; f, g, fusion of nuclei; h-j, development of wall to zygote; k, l, formation of four sporoblasts; l, side view of spore; m, mature sporozoites in spore. (After Léger.)

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fig34

Fig. 34.—Life-history of Coccidium schubergi. a, Penetration of epithelium-cell of host by sporozoite; b-d, stages of multiple cell-formation in naked state (schizogony); e, f, formation of oogamete; g, conjugation; h-j, formation of sperms (s); k, development of zygote (fertilised ovum) to form four spores; l, formation of two zoospores (or sickle germs) in each spore. (From Calkins's Protozoa, after Schaudinn.)

In some cases the oogametes are at first oblong, like ordinary merozoites, and round off in the gut. The microgametocyte, or spermatogone, has the same character, but is smaller; it applies itself like a cap to one pole of the oogamete, which has rounded off; it then divides into four sperms, whose cytoplasm is not sharply separated; one of these then separates from the common mass, enters the oogamete, and so conjugation is effected, with an oosperm as its result. This latter mode of conjugation is that of Adelea ovata and Coccidium lacazei: the former is probably the more primitive and the commoner. The sperms {102}of Coccidiidae, when free, usually possess two long flagella, either both anterior, or a very long one in front and a short one behind, both turned backwards.

The genus Coccidium affects many animals, and one species in particular, C. cuniculi Rivolta, attacks the liver of young rabbits,[108] giving rise to the disease "coccidiosis." Coccidium may also produce a sort of dysentery in cattle on the Alpine pastures of Switzerland; and cases of human coccidiosis are by no means unknown. Coccidium-like bodies have been demonstrated in the human disease, "molluscum contagiosum," and the "oriental sore" of Asia; similar bodies have also been recorded in smallpox and vaccinia, malignant tumours and even syphilis, but their nature is not certainly known; some of these are now referred to Flagellata (see p. 121).

Closely allied to the Coccidiidae are the Haemosporidae, dwellers in the blood of various cold-blooded Vertebrates,[109] and entering the corpuscles as sporozoites or merozoites to attain the full size, when they divide by schizogony; they are freed like those of the next family by the breaking up of the corpuscle. The merozoites were described by Gaule (1879) as "vermicles" ("Würmchen"), and regarded by him as peculiar segregation-products of the blood; though Lankester had described the same species in the Frog's blood as early as 1871, with a full recognition of its true character. His name, Drepanidium, has had to give way, having been appropriated to another animal, and has been aptly replaced by that of Lankesterella. The sexual process of Karyolysus has been found to take place in a Tick, that of Haemogregarina in a Leech, thus presenting a close analogy to the next group, which only differs in its less definite form in the active state, and in the lack of a cell-wall during brood-formation.

Laveran was the first to describe a member of the {103}Acystosporidae, in 1880, as an organism always to be found in the blood of patients suffering from malarial fever; this received the rather inappropriate name of Plasmodium, which, by a pedantic adherence to the laws of priority, has been used by systematists as a generic name. Golgi demonstrated the coincidence of the stages of the intermittent fever with those of the life-cycle of the parasite in the patient, the maturation of the schizont and liberation of the sporozoites coinciding with the fits of fever. Manson, who had already shown that the Nematodes of the blood that give rise to Filarial haematuria (see Vol. II. p. 149) have an alternating life in the gnats or mosquitos of the common genus Culex,[110] in 1896 suggested to Ronald Ross that the same might apply to this parasite, and thus inspired a most successful work. The hypothesis had old prejudices in its favour, for in many parts there was a current belief that sleeping under mosquito-netting at least helped other precautions against malaria. Ross found early in his investigations that Culex was a good host for the allied genus Haemoproteus or Proteosoma, parasitic in birds, but could neither inoculate man with fever nor be inoculated from man. He found, however, that the malaria germs from man underwent further changes in the stomach of a "dappled-wing mosquito," that is, as we have since learned, a member of the genus Anopheles. Thenceforward the study advanced rapidly, and a number of inquirers, including Grassi, Koch, MacCallum (who discovered the true method of sexual union in Halteridium[111]), and Ross himself, completed his discovery by supplying a complete picture of the life-cycles of the malaria-germs. Unfortunately, there has been a most unhappy rivalry as to the priority of the share in each fragment of the discovery, whose history is summarised by Nuttall, we believe, with perfect fairness.[112]

The merozoite is always amoeboid, and in this state enters the blood corpuscle; herein it attains its full size, as a schizont, becoming filled with granules of "melanin" or black pigment, probably a decomposition product of the red colouring matter (haemoglobin).

{104}
fig35

Fig. 35.—Life-history of Malarial Parasites. A-G, Amoebula of quartan parasite to sporulation; H, its gametocyte; I-M, amoebula of tertian parasite to sporulation; N, its gametocyte; O, T, "crescents" or gametocytes of Laverania; P-S, sperm-formation; U-W, maturation of oosphere; X, fertilisation; Y, zygote. a, Zygote enlarging in gut of Mosquito; b-e, passing into the coelom; f, the contents segmented into naked spores; g, the spores forming sickle-germs or sporozoites; h, sporozoites passing into the salivary glands. (From Calkins's Protozoa, after Ross and Fielding Ould.)

The nucleus of the schizont now divides repeatedly, and then the schizont segments into a flat brood of germs (merozoites), relatively few in the parasite of quartan fever (Haemamoeba malariae, Fig. 35, E-G), many in that of tertian (H. vivax, Fig. 35, M). These brood-cells escape and behave for the most part as before. But after the disease has persisted for some time we find that in the genus Haemamoeba, {105}which induces the common malarial fevers of temperate regions, certain of the full-grown germs, instead of behaving as schizonts, pass, as it were, to rest as round cells; while in the allied genus Laverania, (Haemomenas, Ross) these resting-cells are crescentic, with blunt horns, and are usually termed half-moons (Fig. 35, O, T), characteristic of the bilious or pernicious remittent fevers of the tropics and of the warmer temperate regions in summer. These round or crescent-shaped cells are the gametocytes, which only develop further in the drawn blood, whether under the microscope, protected against evaporation, or in the stomach of the Anopheles: the crescents become round, and then they, like the already round ones of Haemamoeba, differentiate in exactly the same way as the corresponding cells of Coccidium schubergi. The female cell only exhibits certain changes in its nucleus to convert it into an oosphere: the male emits a small number of sperms, long flagellum-like bodies, each with a nucleus; and these, by their wriggling, detach themselves from the central core, no longer nucleated. The male gametogonium with its protruded sperms was termed the "Polymitus form," and was by some regarded as a degeneration-form, until MacCallum discovered that a "flagellum" regularly undergoes sexual fusion with an oosphere in Halteridium, as has since been found in the other genera. The oosperm (Y) so formed is at first motile ("ookinete"), as it is in Haemosporidae, and passes into the epithelium of the stomach of the gnat and then through the wall, acquiring a cyst-wall and finally projecting into the coelom (a-e). Here it segments into a number of spheres ("zygotomeres" of Ross) corresponding to the Coccidian spores, but which never acquire a proper wall (f). These by segmentation produce at their surface an immense quantity of elongated sporozoites (the "zygotoblasts" or "blasts" of Ross, Fig. 35, g), these are ultimately freed by the disappearance of the cyst-wall of the oosperm, pass through the coelom into the salivary gland (h), and are discharged with its secretion into the wound that the gnat inflicts in biting. In the blood the blasts follow the ordinary development of merozoites in the blood corpuscle, and the patient shows the corresponding signs of fever. This has been completely proved by rearing the insect from the egg, feeding it on the blood of a patient in whose blood there were ascertained to be the germs of a definite species of {106}Haemamoeba, sending it to England, where it was made to bite Dr. Manson's son, who had never had fever and whose blood on repeated examination had proved free from any germs. In the usual time he had a well-defined attack of the fever corresponding to that germ, and his blood on examination revealed the Haemamoeba of the proper type. A few doses of quinine relieved him of the consequences of his mild martyrdom to science. Experiments of similar character but of less rigorous nature had been previously made in Italy with analogous results. Again, it has been shown that by mere precautions against the bites of Anopheles, and these only, all residents who adopted them during the malarious season in the most unhealthy districts of Italy escaped fever during a whole season; while those who did not adopt the precautions were badly attacked.[113]

Anopheles flourishes in shallow puddles, or small vessels such as tins, etc., the pools left by dried-up brooks and torrents, as well as larger masses of stagnant water, canals, and slow-flowing streams. Sticklebacks and minnows feed freely on the larvae and keep down the numbers of the species; where the fish are not found, the larvae may be destroyed by pouring paraffin oil on the surface of the water and by drainage. A combination of protective measures in Freetown (Sierra Leone) and other ports on the west coast of Africa, Ismailia, and elsewhere, has met with remarkable success during the short time for which it has been tried; and it seems not improbable, that as the relatively benign intermittent fevers have within the last century been banished from our own fen and marsh districts, so the Guinea coast may within the next decade lose its sad title of "The White Man's Grave."

So closely allied to this group in form, habit, and life-cycle are some species of the Flagellate genus Trypanosoma, that in their less active states they have been unhesitatingly placed here (see p. 119). Schaudinn has seen Trypanosomic characters in the "blasts" of this group, which apparently is the most primitive of the Sporozoa and a direct offshoot of the Flagellates.

The Myxosporidiaceae (Fig. 36) are parasitic in various {107}cold-blooded animals. They are at least binucleate in the youngest free state, and become large and multinucleate apocytes, which may bud off outgrowths as well as reproduce by spores. The spores of the apocyte are not produced by simultaneous breaking up, but by successive differentiation. A single nucleus aggregates around itself a limited portion of the cytoplasm, and this again forms a membrane, becoming an archespore or a "pansporoblast," destined to produce two spores; within this, nuclear division takes place so as to form about eight nuclei, two of which are extruded as abortive, and of the other six, three are used up in the formation of each of the two spores. Of these three nuclei in each spore, two form nematocysts, like those of a Coelenterate (p. 246 f.), at the expense of the surrounding plasm; while the third nucleus divides to form the two final nuclei of the reproductive body. The whole aggregate of the reproductive body and the two nematocysts is enveloped in a bivalve shell. In what we may call germination, the nematocysts eject a thread that serves for attachment, the valves of the shell open, and the binucleate mass crawls out and grows afresh. Nosema bombycis Nägeli (the spore of which has a single nematocyst) is the organism of the "Pébrine" of the silkworm, which was estimated to have caused a total loss in France of some £40,000,000 before Pasteur investigated the malady and prescribed the effectual cure, or rather precaution against its spread. This consisted in crushing each mother in water after it had laid its eggs and seeking for pébrine germs. If the mother proved to be infected, her eggs were destroyed, as the eggs she had laid were certain to be also tainted. Balbiani completed the study of the organism from a morphological standpoint. Some Myxosporidiaceae produce destructive epidemics in fish.

fig36

Fig. 36.—A, Myxidium lieberkühnii, amoeboid phase; B, Myxobolus mülleri, spore with discharged nematocysts (ntc); C, spores (psorosperms) of a Myxosporidian. ntc, nematocysts. (From Parker and Haswell.)

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The Dolichosporidia or Sarcosporidiaceae are, in the adult state, elongated sacs, often found in the substance of the voluntary muscles, and known as "Rainey's" or "Miescher's Tubes"; they are at first uninucleate, then multinucleate, and then break up successively into uninucleate cells, the spores, in each of which, by division, are formed the sickle-shaped zoospores.[114]

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CHAPTER V

PROTOZOA (CONTINUED): FLAGELLATA

III. Flagellata.

Protozoa moving (and feeding in holozoic forms) by long flagella: pseudopodia when developed usually transitory: nucleus single or if multiple not biform: reproduction occurring in the active state and usually by longitudinal fission, sometimes alternating with brood-formation in the cyst or more rarely in the active state: form usually definite: a firm pellicle or distinct cell-wall often present.

The Flagellates thus defined correspond to Bütschli's group of the Mastigophora. The lowest and simplest forms, often loosely called "Monads," are only distinguishable from Sarcodina (especially Proteomyxa) and Sporozoa by the above characters: their artificial nature is obvious when we remember that many of the Sarcodina have a flagellate stage, and that the sperms of bisexual Sporozoa are flagellate (as are indeed those of all Metazoa except Nematodes and most Crustacea). Even as thus limited the group is of enormous extent, and passes into the Chytridieae and Phycomycetes Zoosporeae on the one hand, and by its holophytic colonial members into the Algae, on the other.[115]

Classification.

A. Fission usually longitudinal (transverse only in a cyst), or if multiple, radial and complete: pellicle absent, thin, or if armour-like, with not more than two valves.
I. Food taken in at any part of the body by pseudopodia 1. PANTOSTOMATA
Multicilia Cienk.; Mastigamoeba F. E. Sch. (Fig. 37, 4).
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II. Food taken in at a definite point or points, or by absorption, or nutrition holophytic.

1. No reticulate siliceous shell. Diameter under 500 µ (1⁄50").
* Contractile vacuole simple (one or more).
(α) Colourless: reserves usually fat: holozoic, saprophytic or parasitic 2. Protomastigaceae
(β) Plastids yellow or brown: reserves fat or proteid: nutrition variable: body naked, often amoeboid in active state (C. nudae), or with a test, sometimes containing calcareous discs ("coccoliths," "rhabdoliths") of peculiar form (C. loricatae) 3. Chrysomonadaceae
Chromulina Cienk.; Chrysamoeba Klebs; Hydrurus Ag. Dinobryon Ehrb. (Fig. 37, 11); Syncrypta Ehrb. (Fig. 37, 12); Zooxanthella Brandt; Pontosphaera Lohm.; Coccolithophora Lohm.; Rhabdosphaera Haeck.
(γ) Green, (more rarely yellow or brown) or colourless: reserves starch: fission longitudinal 4. Cryptomonadaceae
Cryptomonas Ehrb. (Fig. 37, 9); Paramoeba Greeff.
(δ) Green (rarely colourless): fission multiple, radial 5. Volvocaceae
** System of contractile vacuoles complex, with accessory formative vacuoles or reservoir, or both.
(ε) Pellicle delicate or absent: pseudopodia often emitted: excretory pore distinct from flagellar pit: reserves fat 6. Chloromonadaceae
Chloramoeba Lagerheim; Thaumatomastix, Lauterborn.
(ζ) Pellicle dense, tough or hard, often wrinkled or striate: contractile vacuole discharging by the flagellar pit. Nutrition variable 7. Euglenaceae
Euglena Ehrb.; Astasia Duj. (Fig. 37, 3); Anisonema Duj.; Eutreptia Perty (Fig. 42, p. 124); Trachelomonas Ehrb. (Fig. 37, 1); Cryptoglena Ehrb.
2. Skeleton an open network of hollow siliceous spicules. Plastids yellow. Diameter under 500 µ. 8. Silicoflagellata
Dictyocha Ehrb.
3. Diameter over 500 µ. Mouth opening into a large reticulate endoplasm: flagella 1, or 2, very unequal. 9. Cystoflagellata
Noctiluca Suriray (Fig. 48); Leptodiscus R. Hertw.
B. Fission oblique or transverse: flagella two, dissimilar, the one coiled round the base of the other or in a traverse groove; pellicle often dense, of numerous armour-like plates 10. Dinoflagellata
Ceratium Schrank; Gymnodinium Stein; Peridinium Ehrb. (Fig. 46); Pouchetia Schütt; Pyrocystis Murray (Fig. 47); Polykrikos Bütschli.

The Protomastigaceae and Volvocaceae are so extensive as to require further subdivision.

Protomastigaceae

I. Oral spots 2. Flagella distant in pairs. Distomatidae
II. Oral spot 1 or 0.
{111}

A. Flagellum 1.

(a) No anterior process: often parasitic Oikomonadidae
Oikomonas K. (Figs. 37, 2, 8); Trypanosoma Gruby (Fig. 39, a-f); Treponema Vuill. (Fig. 39, g-i).
(b) Anterior process unilateral or proboscidiform: cell often thecate Bicoecidae
Bicoeca Clark; Poteriodendron St.
(c) Anterior process a funnel, surrounding the base of the flagellum: cells often thecate.
(i.) Funnel free Craspedomonadidae
Codosiga Clark; Monosiga Cl.; Polyoeca Kent; Proterospongia Kent; Salpingoeca Cl.
(ii.) Funnel not emerging from the general gelatinous investment Phalansteridae
B. Flagella 2, unequal or dissimilar in function, the one sometimes short and thick.
(a) Both flagella directed forwards Monadidae
Monas St.; Anthophysa Bory (Fig. 37, 13).
(b) One flagellum, usually the longer, turned backwards Bodonidae
Bodo St. (Fig. 38).
C. Flagella 2, equal and similar Amphimonadidae
Amphimonas Duj.; Diplomita K. (Fig. 37, 10); Rhipidodendron St. (Fig. 37, 14).
D. Flagella 3 Trimastigidae
Dallingeria K. (Fig. 37, 6); Costia Leclercq.
E. Flagella 4 or more: mostly parasitic in Metazoa Polymastigidae
Trichomonas Donne; Tetramitus Perty (Fig. 37, 7); Hexamitus Duj.; Lamblia Blanchard.
F. Flagella numerous, sometimes constituting a complete ciliiform investment, and occasionally accompanied by an undulating membrane: parasitic in Metazoa.
(a) Flagella long: nucleus single: parasitic in insects Trichonymphidae
Dinenympha Leidy; Joenia Grassi; Pyrsonympha Leidy; Trichonympha Leidy; Lophomonas St.; Maupasia Schew.
(b) Flagella short, ciliiform, uniformly distributed: nuclei very numerous, all similar: parasitic in Amphibia Opalinidae
Opalina Purkinje and Valentin (Fig. 41).

Volvocaceae

A. Cells usually isolated, separating after fission or brood-formation. Usually green (sometimes red), more rarely colourless saprophytes Chlamydomonadidae
Chlamydomonas Ehrb.; Phacotus Perty; Polytoma Ehrb.; Sphaerella Sommerf. (Fig. 43); Zoochlorella.
B. Cells multiplying in the active state by radial divisions in the same plane and usually incurving to form a spherical colony, united in a gelatinous investment, sometimes traversed by plasmic threads Volvocidae
Gonium O.F.M.; Eudorina Ehrb.; Pandorina Bory (Fig. 45); Stephanosphaera Cohn; Volvox L. (Fig. 44).
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fig37

Fig. 37.—Various forms of Flagellata. 2, 6-8, 10, 13, 14, Protomastigaceae; 11, 12, Chrysomonadaceae; 9, Cryptomonadaceae; 1, 3, Euglenaceae; 4, Pantostomata: note branched stalk in 13; branched tubular theca in 14; distinct thecae in 11; stalk and theca in 10. In 2, flagellate (a) and amoeboid (b) phases are shown; in 5, flagellate (a) and Heliozoan (b) phases[116]; in 8 are shown two stages in the ingestion of a food particle (f); chr, plastoids; c.vac, contractile vacuole; f, food particle; g, gullet; l, theca; nu, nucleus; p, protoplasm; per, peristome; v.i, vacuole of ingestion. (From Parker and Haswell, mostly from Bütschli's Protozoa.)

{113}

The modes of nutrition are threefold: the simplest forms live in liquids containing decaying organic matter which they absorb through their surface ("saprophytic"): others take in food either Amoeba fashion, or into a vacuole formed for the purpose, or into a definite mouth ("holozoic"): others again have coloured plastids, green or brown or yellow ("holophytic"), having the plant's faculty of manufacturing their own food-supply. But we meet with species that show chromatophores at one time and lack them at another; or, again, the same individual (Euglena) may pass from holozoic life to saprophytic (Paramoeba, some Dinoflagellates) as conditions alter.

Many secrete a stalk at the hinder end: by "continuous" formation of this, without rupture at fission, a branching colony is formed (Polyoeca). This stalk may have a varying consistency. In Anthophysa (Fig. 37, 13) it appears to be due to the welding of excrementitious particles voided at the hinder end of the body with a gelatinous excretion; but the division of the stalk is here occasional or intermittent, so that the cells are found in tufts at the apex of the branches. A corresponding secretion, gelatinous or chitinous, around the body of the cell forms a cup or "theca," within which the cell lies quite free or sticking to it by its surface, or attached to it by a rigid or contractile thread. The theca, again, may assume the form of a mere gelatinous mass in which the cell-bodies may be completely plunged, so that only the flagella protrude, as in Volvocidae, Proterospongia (Fig. 75, p. 182), and Rhipidodendron (Fig. 37, 14). Often this jelly assumes the form of a fan (Phalansterium), the branching tubes of which it is composed lying for some way alongside, and ultimately diverging. In Hydrurus, the branching jelly assumes the form of a branching Confervoid.[117]

The cell-body may be bounded by an ill-defined plasmatic layer in Chrysomonadaceae and some Protomastigaceae,[118] or it may form a plasmatic membrane or "pellicle," sometimes very firm and tough, or striated as in Euglenaceae, or it may have a separate "cuticle" (in the holophytic species formed of cellulose), or even a bivalve or multivalve shell of distinct plates, hinged or overlapping (Cryptoglena, Phacotus, Dinoflagellates). The wall of the {114}Coccolithophoridae, a family of Chrysomonadaceae, is strengthened by embedded calcareous spicules ("coccoliths," "cyatholiths," "rhabdoliths"), which in the most complex forms (cyatholiths) are like a shirt-stud, traversed by a tube passing through the stem and opening at both ends. These organisms[119] constitute a large proportion of the plankton; the spicules isolated, or in their original state of aggregation ("coccospheres," "rhabdospheres"), enter largely into the composition of deep-sea calcareous oozes. They occur fossil from Cambrian times (Potsdam sandstone of Michigan and Canada), and are in some strata extremely abundant, 800,000 occurring to the mm. cube in an Eocene marl.

The Silicoflagellates have siliceous skeletons resembling that of many Radiolaria, to which they were referred until the living organism was described (see pp. 79, 86 f.).

The flagellum has been shown by Fischer to have one of two forms: either it is whip-like, the stick, alone visible in the fresh specimen, being seen when stained to be continued into a long lash, hitherto invisible; or the whole length is fringed with fine ciliiform lateral outgrowths. If single it is almost always protruded as a tugging organ ("tractellum");[120] the chief exceptions are the Craspedomonads, where it is posterior and acts as a scull ("pulsellum"), and some Dinoflagellates, where it is reversible in action or posterior. In addition to the anterior flagellum there may be one or more posterior ones, which trail behind as sense organs, or may anchor the cell by their tips. Dallingeria has two of these, and Bodo saltans a single anterior anchoring lash, by which they spring up and down against the organic débris among which they live, and disintegrate it. The numerous similar long flagella of the Trichonymphidae afford a transition in the genus Pyrsonympha to the short abundant cilia of Opalina, usually referred to the Ciliate Infusoria.

{115}

An undulating membrane occurs, sometimes passing into the flagellum in certain genera, all parasitic, such as Trypanosoma (incl. Herpetomonas), Trichomonas, Hexamitus, and Dinenympha.

In some cases the flagellum (or flagella) is inserted into a definite pit, which in allied forms is the mouth-opening. The contractile vacuole is present in the fresh-water forms, but not in all the marine ones, nor in the endoparasites. It may be single or surrounded by a ring of minute "formative" vacuoles or discharge into a permanently visible "reservoir." This again may discharge directly to the surface or through the pit or canal in which the flagellum takes origin (Euglena).

The "chromatophore" may be a single or double plate, or multiple.[121] In the peculiar form Paramoeba the chromatophore may degenerate and be reproduced anew. It often encloses rounded or polygonal granules of uncoloured plasma, very refractive, known as "pyrenoids." These, like the chromatophores, multiply by direct fission. The "reserves" may be (1) fat-globules; (2) granules of a possibly proteid substance termed "leucosin"; (3) a carbohydrate termed "paramylum," differing slightly from starch (see p. 95); (4) true starch, which is usually deposited in minute granules to form an investment for the pyrenoid when such is present.

A strongly staining granule is usually present in the plasma near the base of the flagellum. This we may term a "blepharoplast" or a "centrosome" in the wider sense.

Fission is usually longitudinal in the active state; a few exceptions are recorded. Encystment is not uncommon; and in the coloured forms the cyst-wall is of cellulose. Division in the cyst is usually multiple;[122] in the coloured forms, however, vegetative growth often alternates with division, giving rise to plant-like bodies. Polytoma and other Chlamydomonadidae multiply by "brood-formation" in the active state; the blepharoplast, as Dangeard suggests, persisting to continue the motion of the flagella of the parent, while the rest of the plasm divides to form the brood. Conjugation has been observed in many species. In some species of Chlamydomonas it takes place after one or both of the two {116}cells have come to rest, but in most cases it occurs between active cells. We find every transition between equal unions and differentiated sexual unions, as we shall see in discussing the Volvocaceae.[123] The "coupled-cell" differs in behaviour in the different groups, but almost always goes to rest and encysts at once, whatever it may do afterwards.

The life-history of many Flagellates has been successfully studied by various observers, and has shed a flood of light on many of the processes of living beings that were hitherto obscure. The first studies were carried through by the patient labours of Drysdale and Dallinger. A delicate mechanical stage enabled the observer to keep in the field of view a single Flagellate, and, when it divided into two, to follow up one of the products. A binocular eye-piece saved much fatigue, and enabled the observers to exchange places without losing sight of the special Flagellate under observation; for the one who came to relieve would put one eye to the instrument and recognise the individual Flagellate under view as he passed his hand round to the mechanism of the stage before the first watcher finally relinquished his place at the end of the spell of work. Spoon-feeding by Mrs. Dallinger enabled such shifts to be prolonged, the longest being one of nine hours by Dr. Dallinger.

{117}
fig38

Fig. 38.—Bodo saltans. A, the positions assumed in the springing movements of the anchored form; B, longitudinal fission of anchored forms; C, transverse fission of the same; D, fission of free-swimming form; E1-E4, conjugation of free-swimming with anchored form; E5, zygote; E6, emission of spores from zygote; F, growth of spores: c.vac, contractile vacuole; fl.1, anterior; fl.2, ventral flagellum; nu, nucleus. (From Parker's Biology, after Dallinger.)

The life-cycles varied considerably in length. It was in every case found that after a series of fissions the species ultimately underwent conjugation (more or less unequal or bisexual in character);[124] the zygote encysted; and within the cyst the protoplasmic body underwent brood-formation, the outcome of which was a mass of {118}spores discharged by the rupture of the cyst (Fig. 38). These spores grow from a size too minute for resolution by our microscopes into the ordinary flagellate form. They withstand the effects of drying, if this be effected immediately on their escape from the ruptured cyst; so that it is probable that each spore has itself a delicate cyst-wall and an aplanospore, from which a single zoospore escapes. The complex cycle, of course, comprises the whole course from spore-formation to spore-formation. Such complete and regular "life-histories," each characteristic of the species, were the final argument against those who held to the belief that spontaneous generation of living beings took place in infusions of decomposing organic matter.

Previous to the work of these observers it had been almost universally believed that the temperature of boiling water was adequate to kill all living germs, and that any life that appeared in a closed vessel after boiling must be due to spontaneous change in its contents. But they now showed that, while none of the species studied resisted exposure in the active condition to a temperature of 138°-140° F., the spores only succumbed, in liquid, to temperatures that might even reach 268° F., or when dry, even 300° F. or more. Such facts explain the constant occurrence of one or more such minute species in liquids putrefying under ordinary conditions, the spores doubtless being present in the dust of the air. Very often several species may co-exist in one infusion; but they separate themselves into different zones, according to their respective need for air, when a drop of the liquid is placed on the slide and covered for examination. Dallinger[125] has made a series of experiments on the resistance of these organisms in their successive cycles to a gradual rise of temperature. Starting with a liquid containing three distinct species, which grew and multiplied normally at 60° F., he placed it under conditions in which he could slowly raise the temperature. While all the original inmates would have perished at 142° F., he succeeded in finally producing races that throve at 158° F., a scalding heat, when an accident put an end to that series of experiments. In no instance was the temperature raised so much as to kill off the beings, so that the increased tolerance of their descendants was due not, as might have been anticipated, to selection of those that best resisted, but to the inheritance of {119}an increased toleration and resistance from one generation or cycle to another.

As we noted above (p. 40), the study of the Flagellates has been largely in the hands of botanists. After the work of Bütschli in Bronn's Thier-Reich, Klebs[126] took up their study; and the principal monographs during the last decade have appeared in Engler and Prantl's Pflanzenfamilien, where Senn[127] treats the Flagellates generally, Wille[128] the Volvocaceae, and Schütt the "Peridiniales" or Dinoflagellata;[129] while only the Cystoflagellata, with but two genera, have been left to the undisputed sway of the zoologists.[130]

Among this group the majority are saprophytes, found in water containing putrefying matter or bacteria. The forms so carefully studied by Dallinger and Drysdale belong to the genera Bodo, Cercomonas, Tetramitus, Monas, and Dallingeria. Many others are parasites in the blood or internal cavities of higher animals, some apparently harmless, such as Trichomonas vaginalis, parasitic in man, others of singular malignity. Costia necatrix, infesting the epithelial scales of fresh-water fish, often devastates hatcheries. The genus Trypanosoma, Gruby, contributes a number of parasites, giving rise to deadly disease in man and beast.[131] T. lewisii is common in Rodents, but is relatively harmless. T. evansii is the cause of the Surra disease of Ruminants in India, and is apparently communicated by the bites of "large brown flies" (almost certainly Breeze Flies or Tabanidae, Vol. VI. p. 481). T. brucei, transferred to cattle by the Tsetse Fly, Glossina morsitans (see Vol. VI. Fig. 244, p. 513) in Equatorial Africa, is the cause of the deadly Nagana disease, which renders whole tracts of country impassable to ox or horse. Other Trypanosomic diseases of animals are, in Algeria and the Punjab, "dourine," infecting horses and dogs; in South America, Mal de Caderas (falling-sickness), an epidemic paralysis of cattle. During the printing of this book, much additional knowledge has been gained on this genus and the diseases it engenders. The Trypanosomic {120}fever recently recognised on the West Coast has been found to be the early stage of the sleeping-sickness, that well-known and most deadly epidemic of Tropical Africa. Through the researches of Castellani, Nabarro, and especially Colonel and Mrs. Bruce, we know now that the parasite T. gambiense is transferred by an intermediate host, a kind of Tsetse Fly (Glossina palpalis). Schaudinn's full study of a parasite of the blood corpuscles of the Owl has shown that while in its intracorpuscular state it resembles closely the malarial parasites in behaviour, and in its schizogenic multiplication, so that it was considered an Acystosporidian, under the name of Halteridium, it is really a Trypanosoma;[132] for the accomplishment of successful sexual reproduction it requires transference to the gut of a gnat (Culex). The germs may infect the ovary, and give the offspring of the insect the innate power of infecting Owls. Thus a new light is shed on the origin of the Coccidiaceae, whose "blasts" in the insect host resemble Trypanosoma in their morphology.

fig39

Fig. 39.—Morphology of Trypanosoma. a-f, Stages in development of Trypanosoma noctuae from the active zygote ("ookinete"); b, first division of nucleus into larger (trophic) and smaller (kineto-) nucleus; c, d, division of smaller nucleus and its transformations to form "blepharoplast" and myonemes; f, adult Trypanosoma; g, h, i, Treponema zeemannii of Owl; g, Trypanosome form; h, Spirochaeta form; i, rosette aggregate. (After Schaudinn.)

The human Tick fever of the Western United States and the epizootic Texas fever are known to be due to blood parasites of the genus Piroplasma (Babesia), of which the free state is that of a Trypanosome. It appears certain that Texas fever, though due to Tick bites, is not transferred directly from one beast to another by the same Tick; but the offspring of a female Tick that has sucked an infected ox contains Trypanosome germs, and will by their bites infect other animals. {121}It would seem probable that the virulence of the Persian Tick (Argas persica) is due to similar causes. The Indian maladies known as "Kala Azar" and "Oriental Sore" are characterised by blood parasites, at first called after their discoverer the "Leishman bodies," which have proved to be the effects of a Piroplasma.

Trypanosoma is distinguished by the expansion of its flagellum into an undulating membrane, that runs down the edge of the body, and may project behind as a second lash. In this membrane run eight fine muscular filaments, or myonemes, four on either surface, within the undulating membrane; at their lower end they are all connected with a rounded body, the "blepharoplast," which is here in its origin, as well as in its behaviour in reproductive processes, a true modified nucleus, comparable in some respects, as was first noted by Plimmer and Rose Bradford,[133] with the micronucleus of the Infusoria. Part of the segmentation spindle persists in the form of a filament uniting the blepharoplast with the large true functional nucleus (Fig. 39, a-f).

The blood of patients suffering from relapsing fever contains a fine wriggling parasite, which was described as a Schizomycete, allied to the bacteria, and hitherto termed Spirochaeta obermeieri. Schaudinn has shown that this and other similar blood parasites are closely allied to Trypanosoma; and since the original genus was founded on organisms of putrefaction which are undoubtedly Schizomycetes, Vuillemin has suggested the name Treponema. T. pallidum is found in syphilitic patients, and appears to be responsible for their illness.[134]

The Craspedomonadidae (often called Choanoflagellates, Fig. 40) are a group whose true nature was elucidated some forty years ago by the American zoologist, H. James-Clark. They are attached either to a substratum, by a stalk produced by the base of the cell, or to other members of the same colony; they are distinguished by the protrusion of the cytoplasm around the base of the single flagellum into a pellucid funnel,[135] in which the plasma is in constant motion, though the funnel retains its shape and size, except when, as sometimes happens, it is retracted.

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fig40

Fig. 40.—Various forms of Craspedomonadidae. 2, a, Adult cell; 2, b, longitudinal fission; 2, c, the production of flagellulae by brood-formation; c, collar; c.vac, contractile vacuole; fl, flagellum; l, theca; nu, nucleus; s, stalk. (After Saville Kent.)

The agitation of the flagellum determines a stream of water upwards along the outer walls of the funnel; and the food-particles brought along adhere to the outside of the funnel, and are carried by its streaming movement to the basal constriction, where they are swallowed by the plasma, which appears to form a swallowing vacuole at that point. Longitudinal fission is the ordinary mode of reproduction, extending up through the funnel. If the two so formed continue to produce a stalk, the result is the formation of a tree-like stem, whose twigs bear at the ends the funnelled cells, or "collar-cells" as they are usually called. In Salpingoeca, as in so many other Flagellates, each cell forms a cup or theca, often of most graceful vase-like outline, the rim being elegantly turned back. Proterospongia (Fig. 75, p. 182) secretes a gelatinous investment for the colony, which is attached to solid bodies. In this species, according to Saville Kent, the central members of the colony retract their collar, lose their flagellum, become amoeboid, and finally undergo brood-formation to produce minute zoospores. This is the form which by its differentiation recalls the Sponges, and has been regarded as a {123}transition towards them; for the flagellate, nutritive cells of the Sponges are provided with a collar, which exists in no other group of Metazoa (see pp. 171, 181, and Fig. 70, p. 176). The most recent monographer of the family is Raoul Francé, but James-Clark and Saville Kent did the pioneering work.

fig41

Fig. 41.—Opalina ranarum. A, living specimen; B, stained specimen showing nuclei; C, stages in nuclear division; D-F, stages in fission; G, final product of fission; H, encysted form; I, young form liberated from cyst; K, the same after multiplication of the nucleus has begun. nu, Nucleus. (From Parker's Biology, after Saville Kent and Zeller.)

Of the life-history of the Trichonymphidae,[136] all of which are parasitic in the alimentary canal of Insects, especially Termites or White Ants (Vol. V. p. 356), nothing is known. Some of them have a complete investment of motile flagella, like enormously long cilia, which in Dinenympha appear to coalesce into four longitudinal undulating membranes. Lophomonas inhabits the gut of the Cockroach and Mole-cricket. The Opalinidae have also a complete investment of cilia, which are short, and give the aspect of a Ciliate to the animal, which is common in the rectum of Amphibia, and dies when transferred to water. But despite the outward resemblance, the nuclei, of which there may be as many as 200, are all similar, and consequently this group cannot be placed among the Infusoria at all. Opalina has no mouth nor contractile vacuole. It multiplies by dividing {124}irregularly and at intervals, resolving finally into 1-nucleate fragments, which encyst and pass into the water. When swallowed the cyst dissolves, its contents enlarge, and ultimately assume the adult form.[137]

Maupasia has a partial investment of cilia, a single long flagellum and mouth, a contractile vesicle, and a single simple nucleus. It seems to find an appropriate place near the two above groups, though it is free, and possesses a mouth.

fig42

Fig. 42.—Longitudinal Fission of Eutreptia viridis (Euglenaceae), showing chloroplasts, nucleus, and flagella arising from pharynx-tube. (After Steuer.)

Among the Euglenaceae, Euglena viridis is a very common form, giving the green colour to stagnant or slow-flowing ditches and puddles in light places, especially when contaminated by a fair amount of dung, as by the overflow of a pig-sty, in company with a few hardy Rotifers, such as Hydatina senta (Vol. II. Fig. 106, p. 199) and Brachionus. Euglena is about 0.1 mm. in length when fully extended, oval, pointed behind, obliquely truncate in front, with a flagellum arising from the pharyngeal pit. It shows a peculiar wriggling motion, waves of transverse constriction passing along the body from end to end, as well as flexures in different meridians. Such motions are termed "euglenoid." The front part is colourless, but under a low {125}power the rest of the cell is green, owing to the numerous chlorophyll bodies or chloroplasts. The outermost layer of the cytoplasm shows a somewhat spiral longitudinal striation, possibly due to muscular fibrils. The interior contains many laminated plates of paramylum, and a large single nucleus. At the front of the body at the base of the flagellum is a red "eye-spot" on the dorsal side of the pharynx-tube or pit, from which the flagellum protrudes. Wager has shown that this tube receives, also on its dorsal side, the opening of a large vacuole, sometimes called the reservoir, for into it discharges the contractile vacuole (or vacuoles). The eye-spot is composed of numerous granules, containing the vegetal colouring matter "haematochrome." It embraces the lower or posterior side of the communication between the tube and the reservoir. The flagellum has been traced by Wager through the tube into the reservoir, branching into two roots where it enters the aperture of communication, and these are inserted on the wall of the reservoir at the side opposite the eye-spot. But on one of the roots near the bifurcation is a dilatation which lies close against the eye-spot, so that it can receive the light reaction. Euglena is an extremely phototactic organism. It shows various wrigglings along the longitudinal axis, and transverse waves of contraction and expansion may pass from pole to pole.[138]

Among the Chrysomonadaceae the genus Zooxanthella, Brandt, has already been described under the Radiolaria (p. 86), in the jelly of which it is symbiotic. It also occurs in similar union in the marine Ciliates, Vorticella sertulariae and Scyphidia scorpaenae, and in Millepora (p. 261) and many Anthozoa (pp. 373 f., 396).

Of the Chlamydomonadidae, Sphaerella (Haematococcus, Ag.) pluvialis (Fig. 43), and S. nivalis, in which the green is masked by red pigment, give rise to the phenomena of "red snow" and "bloody rain." The type genus, Chlamydomonas, is remarkable for the variations from species to species in the character and behaviour of the gametes. Sometimes they are equal, at other times of two sizes. In some species they fuse immediately on approximation, in the naked active state; in others, they encyst on approaching, and unite by the emission of a fertilising tube, {126}as in the Algal Conjugatae. Zoochlorella is symbiotic in green Ciliata (pp. 153 f., 158), Sponges (p. 175), Hydra (p. 256), and Turbellaria (Vol. II. p. 43).

fig43

Fig. 43.—Sphaerella pluvialis. A, motile stage; B, resting stage; C, D, two modes of fission; E, Sphaerella lacustris, motile stage. chr, Chromatophores; c.vac, contractile vacuole; c.w, cell-wall; fl, flagella; nu, nucleus; nu', nucleolus; pyr, pyrenoids. (From Parker's Biology.)

Of the Volvocidae, Volvox (Fig. 44) is the largest and most conspicuous genus. Its colony forms a globe the size of a pin's head, floating on the surface of ponds, drains, or even puddles or water-barrels freely open to the light. It has what may be called a skeleton of gelatinous matter,[139] condensed towards the surface into a denser layer in which the minute cells are scattered. These have each an eye-spot, a contractile vacuole, and two flagella, by the combined action of which the colony is propelled. Delicate boundary lines in the colonial wall mark out the proper investment of each cell. The cells give off delicate plasmic threads which meet those of their neighbours, and form a bond between them. In that half of the hemisphere which is posterior in swimming, a few (five to eight) larger cells ("macrogonidia" of older writers) are evenly distributed, protruding as they increase in size into the central jelly. These as they grow segment to form a new colony.

{127}
fig44

Fig. 44.—Volvox globator. A, entire colony, enclosing several daughter-colonies; B, the same during sexual maturity; C, four zooids in optical section; D1-D5, development of parthenogonidium; E, ripe spermogonium; F, sperm; G, ovum; H, oosperm. a, Parthenogonidia; fl, flagellum; ov, ovum; ovy, ovaries; pg, pigment spot; sp, sperms; Spy, spermogonia dividing to form sperms. (From Parker's Biology, after Cohn and Kirchner.)

The divisions are only in two planes at right angles, so that the young colony is at first a plate, but as the cells multiply the plate bends up (as in the gastrulation of the double cellular plate of the Nematode Cucullanus, Vol. II. p. 136), and finally forms a hollow sphere bounded by a single layer of cells: the site of the original orifice may be traced even in the adult as a blank space larger than exists elsewhere. Among the cells of the young colony some cease to divide, but continue to grow at an early period, and these are destined to become in turn the mothers ("parthenogonidia") of a new colony; they begin segmenting before the colony of which they are cells is freed. The young colonies are ultimately liberated by the rupture of the sphere as small-sized spheres, which henceforth only grow by enlargement of the sphere as a whole, and the wider separation of the vegetative cells. Thus the vegetative cells soon cease to grow; all the supply of food material due to their living activities goes to the nourishment of the parthenogonidia, or the young colonies, as {128}the case may be. These vegetative cells have therefore surrendered the power of fission elsewhere inherent in the Protist cell. Moreover, when the sphere ruptures for the liberation of the young colonies, it sinks and is doomed to death, whether because its light-loving cells are submerged in the ooze of the bottom, or because they have no further capacity for life. When conjugation is about to take place, it is the cells that otherwise would be parthenogonidia that either act as oospheres or divide as "spermogonia" to form a flat brood of minute yellow male cells ("sperms"). These resemble vegetative cells, in the possession of an eye-spot and two contractile vacuoles, but differ in the enormously enlarged nucleus which determines a beaked process in front. After one of these has fused with the female cell ("oosphere") the product ("oosperm") encysts, passes into a stage of profound rest, and finally gives rise to a new colony. The oospheres and sperm-broods may arise in the same colony or in distinct ones, according to the species.

Before we consider the bearings of the syngamic processes of Volvox, we will study those presented by its nearer allies, which have the same habitat, but are much more minute. Three of these are well known, Stephanosphaera, Pandorina, and Eudorina, all of which have spherical colonies of from eight to thirty-two cells embedded at the surface of a sphere, and no differentiation into vegetative cells and parthenogonidia (or reproductive cells).

Stephanosphaera has its eight cells spindle shaped, and lying along equidistant meridians of its sphere; in vegetative reproduction each of these breaks up in its place to form a young colony, and the eight daughter-colonies are then freed. In conjugation, each cell of the colony breaks up into broods of 4, 8, 16, or 32 small gametes, which swim about within the general envelope, and pair and fuse two and two: this is "isogamous," "endogamous" conjugation. In Pandorina (Fig. 45) the cells are rounded, and are from 16 to 32 in each colony. The vegetative reproduction in this, as in Eudorina, is essentially the same as in Stephanosphaera. In conjugation the cells are set free, and are of three sizes in different colonies, small (S), medium (M), and large (L). The following fusions may occur: S × S, S × M, S × L, M × M, M × L. Thus the large are always female, as it were, the medium may play the part of male to the large, female to the small; the small are males to the medium and to the large. The medium {129}and small are capable, each with its like, of equal, undifferentiated conjugation; so that we have a differentiation of sex far other than that of ordinary, binary sex. Eudorina, however, has attained to "binary sex," for the female cells are the ordinary vegetative cells, at most a little enlarged, and the male cells are formed by ordinary cells producing a large flat colony of sixty-four minute males or sperms. In some cases four cells at the apex of a colony are spermogonia, producing each a brood of sperms, while the rest are the oospheres. The transition to Volvox must have arisen through the sterilisation of the majority of cells of a colony for the better nutrition of the few that are destined alone for reproduction.

fig45

Fig. 45.—Pandorina morum. A, entire colony; B, asexual reproduction, each zooid dividing into a daughter-colony; C, liberation of gametes; D-F, three stages in conjugation of gametes; G, zygote; H-K, development of zygote into a new colony. (From Parker's Biology, after Goebel.)

Volvox, as we have seen, has attained a specialisation entirely comparable to that of a Metazoon, where the segmentation of the fertilised ovum results in two classes of cells: those destined {130}to form tissues, and condemned to ultimate death with the body as a whole, and those that ultimately give rise to the reproductive cells, ova, and sperms. But this is a mere parallelism, not indicating any sort of relationship: the oospores of the Volvocaceae show that tendency to an encysted state, in which fission takes place, that is so characteristic of Algae, and these again show the way to Cryptogams of a higher status. Thus, Volvox, despite the fact that in its free life and cellular differentiation it is the most animal of all known Flagellates, is yet, with the rest of the Volvocaceae, inseparable from the Vegetable Kingdom, and is placed here only because of the impossibility of cleaving the Flagellates into two.

The Dinoflagellata (Figs. 46, 47) are often of exceptionally large dimensions in this class, attaining a maximum diameter of 150 µ (1⁄160") and even 375 µ (1⁄67") in Pyrocystis noctiluca. The special character of the group is the presence of two flagella; the one, filiform, arises in a longitudinal groove, and extending its whole length projects behind the animal, and is the conspicuous organ of motion: the other, band-like, arises also in the longitudinal groove, but extends along a somewhat spiral transverse groove,[140] and never protrudes from it in life, executing undulating movements that simulate those of a girdle of cilia, or a continuous undulating membrane (Fig. 46). This appearance led to the old name "Cilioflagellata," which had of course to be abandoned when Klebs discovered the true structure.[141] There is a distinct cellulose membrane, sometimes silicified, to the ectoplasm, only interrupted by a bare space in the longitudinal groove, whence the flagella take origin. This cuticle is usually hard, sculptured, and divided into plates of definite form, bevelled and overlapping at their junction; occasionally the cell has been seen to moult them.

A large vacuolar space, traversed by plasmic strings, separates the peripheral cytoplasm from the central, within which is the large nucleus. There are in most species one or more chromatophores, coloured by a yellowish or brownish pigment, which is a mixture of lipochromes, distinct from diatomin. In a few species the presence of these is not constant, and these species {131}show variability as to their nutrition, which is sometimes holozoic. Under these conditions the cell can take in food-particles as bulky as the eggs of Rotifers and Copepods, by the protrusion of a pseudopod at the junction of the two grooves. As in most coloured forms an eye-spot is often present, a cup-shaped aggregation of pigment, with a lenticular refractive body in its hollow. A contractile vacuole, here termed a "pusule," occurs in many species, communicating with the longitudinal groove by a canal. Nematocysts (see p. 246 f.) are present in Polykrikos, trichocysts (see p. 142) in several genera.

fig46

Fig. 46.—Peridinium divergens. a, Flagellum of longitudinal groove; b, flagellum of transverse groove; cr.v, contractile vacuole surrounded by formative vacuoles; n, nucleus. (After Schütt.)

Division is usually oblique, dividing the body into two dissimilar halves, each of which has to undergo a peculiar growth to reconstitute the missing portion, and complete the shell. The incomplete separation of the young cells leads to the formation of chains, notably in Ceratium and Polykrikos, the latter dividing transversely and occurring in chains of as many as eight. The process of division may take place when the cell is active, or in a cyst, as in Pyrocystis (Fig. 47). Again, encystment may precede multiple fission, resulting in the formation of a brood of minute swarmers. It has been suggested that these are capable of playing the part of gametes, and conjugating in pairs.[142]

The Dinoflagellates are for the most part pelagic in habit, floating at the surface, and when abundant tinge the water of fresh-water lakes or even ponds red or brown. Peridinium (Fig. 46) and Ceratium (the latter remarkable for the horn-like backward prolongations of the lower end) are common genera both in the sea {132}and fresh-waters. Gymnodinium pulvisculus is sometimes parasitic in Appendicularia (Vol. VII. p. 68). Polykrikos[143] has four transverse grooves, each with its flagellum, besides the terminal one. Many of the marine species are phosphorescent, and play a large part in the luminosity of the sea, and some give it a red colour.

Several fossil forms have been described. Peridinium is certainly found fossil in the firestone of Delitzet, belonging to the Cretaceous. A full monograph of the group under the name "Peridiniales" was published by Schütt.[144]

fig47

Fig. 47.—Pyrocystis fusiformis, Murray. × 100. From the surface in the Guinea Current. (From Wyville Thomson.)

The Cystoflagellates contain only two genera,[145] Noctiluca, common at the surface of tranquil seas, to which, as its name implies, it gives phosphorescence, and Leptodiscus, found by R. Hertwig in the Mediterranean. Noctiluca is enormous for a Flagellate, for with the form of a miniature melon it measures about 1 mm. (1⁄25") or more in diameter. In the depression is the "oral cleft," from one end of which rises, by a broad base, a large coarse flagellum, as long as the body or longer and transversely striated. In front of the base of the flagellum are two lip-like {133}prominences, of which one, a little firmer than the other, and transversely ridged, is called the tooth; at the junction of the two is a second, minute, flagellum, usually called the cilium. Behind these the oral groove has an oval space, the proper mouth; behind this, again, the oral groove is continued for some way, with a distinct rod-like ridge in its furrow. The whole body, including the big flagellum, is coated by a strong cuticular pellicle, except at the oblong mouth, and the lips and rod are mere thickenings of this. The cytoplasm has a reticulate arrangement: the mouth opens into a central aggregate, from which strands diverge branching as they recede to the periphery, where they pass into a continuous lining for the cuticular wall, liquid filling the interspaces. The whole arrangement is not unlike that found in many plant-cells, but the only other Protists in which it occurs are the Ciliata Trachelius (Fig. 56, p. 153) and Loxodes. The central mass contains the large nucleus. Noctiluca is an animal feeder, and expels its excreta through the mouth. The large flagellum is remarkable for the transverse striation of its plasma, especially on the ventral side. The cuticle may be moulted as in the Dinoflagellates. As a prelude to fission the external differentiations disappear, the nucleus divides in the plane of the oral groove, and a meridional constriction parts the two halves, the new external organs being regenerated. Conjugation occurs also, the two organisms fusing by their oral region; the locomotive organs and pharynx disappear; the conjoined cytoplasms unite to form a sphere, and the nuclei fuse to form a zygote or fertilisation nucleus. This conjugation is followed by sporulation or brood-formation.[146]

fig48

Fig. 48.—Noctiluca miliaris, a marine Cystoflagellate. (From Verworn.)

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The nucleus passes towards the surface, undergoes successive fissions, and as division goes on the numerous daughter-nuclei occupy little prominences formed by the upgrowth of the cytoplasm of the upper pole. The rest of the cytoplasm atrophies, and the hillocks formed by the plasmic outgrowths around the final daughter-nuclei become separate as so many zoospores (usually 256 or 512); each of these is oblong with a dorsal cap-like swelling, from the edge of which arises a flagellum pointing backwards; parallel to this the cap is prolonged on one side into a style also extending beyond the opposite pole of the animal.[147] In this state the zoospore is, to all outward view, a naked Dinoflagellate, whence it seems that the Cystoflagellates are to be regarded as closely allied to that group. Leptodiscus is concavo-convex, circular, with the mouth central on the convex face, 1-flagellate, and attains the enormous size of 1.5 mm. (1⁄16") in diameter.

The remarkable phosphorescence of Noctiluca is not constant. It glows with a bluish or greenish light on any agitation, but rarely when undisturbed. A persistent stimulus causes a continuous, but weak, light. This light is so weak that several teaspoonsful of the organism, collected on a filter and spread out, barely enable one to read the figures on a watch a foot away. As in other marine phosphorescence, no rise of temperature can be detected. The luminosity resides in minute points, mostly crowded in the central mass, but scattered all through the cytoplasm. A slight irritation only produces luminosity at the point touched, a strong one causes the whole to flash. Any form of irritation, whether of heat, touch, or agitation, electricity or magnetism, is stated to induce the glow. By day, it is said, Noctiluca, when present in abundance, may give the sea the appearance of tomato soup.

The earliest account of Noctiluca will be read with interest. Henry Baker writes in Employment for the Microscope:[148]—"A curious Enquirer into Nature, dwelling at Wells upon the Coast of Norfolk, affirms from his own Observations that the Sparkling of Sea Water is occasioned by Insects. His Answer to a Letter wrote to him on that Subject runs thus, 'In the Glass of Sea Water I send with this are some of the Animalcules which cause the Sparkling Light in Sea Water; they may be seen by holding {135}the Phial up against the Light, resembling very small Bladders or Air Bubbles, and are in all Places of it from Top to Bottom, but mostly towards the Top, where they assemble when the Water has stood still some Time, unless they have been killed by keeping them too long in the Phial. Placing one of these Animalcules before a good Microscope, an exceeding minute Worm may be discovered, hanging with its Tail fixed to an opake Spot in a Kind of Bladder, which it has certainly a Power of contracting or distending, and thereby of being suspended at the Surface, or at any Depth it pleases in the including Water.'"

"The above-mentioned Phial of Sea Water came safe, and some of the Animalcules were discovered in it, but they did not emit any Light, as my Friend says they do, upon the least Motion of the Phial when the Water is newly taken up. He likewise adds, that at certain Times, if a Stone be thrown into the Sea, near the Shore, the Water will become luminous as far as the Motion reacheth: this chiefly happens when the Sea hath been greatly agitated, or after a Storm." Obviously what Mr. Sparshall, Baker's correspondent, took for a worm was the large flagellum.

The chief investigators of this group have been Huxley, Cienkowski, Allman, Bütschli, and G. Pouchet, while Ischikawa and Doflein have elucidated the conjugation.

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CHAPTER VI

PROTOZOA (CONTINUED): INFUSORIA (CILIATA AND SUCTORIA)

IV. Infusoria.

Complex Protozoa, never holophytic save by symbiosis with plant commensals, never amoeboid, with at some period numerous short cilia, of definite outline, with a double nuclear apparatus consisting of a large meganucleus and a small micronucleus (or several),[149] the latter alone taking part in conjugation (karyogamy), and giving rise after conjugation to the new nuclear apparatus.

The name Infusoria was formerly applied to the majority of the Protozoa, and included even the Rotifers. For the word signifies organisms found in "infusions" of organic materials, including macerations. Such were made with the most varied ingredients, pepper and hay being perhaps the favourites. They were left for varying periods exposed to the air, to allow the organisms to develop therein, and were then examined under the microscope.[150] With the progress of our knowledge, group after group was split off from the old assemblage until only the ciliate or flagellate forms were left. The recognition of the claims of the Flagellates to independent treatment left the group more natural;[151] while it was enlarged by the admission of the Acinetans (Suctoria), which had for some time been regarded as a division of the Rhizopoda.

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I. Ciliata

Infusoria, with a mouth, and cilia by which they move and feed; usually with undulating membranes, membranellae, cirrhi, or some of these. Genera about 144: 27 exclusively marine, 50 common to both sea and fresh water, 27 parasitic on or in Metazoa, the rest fresh water. Species about 500.

We divide the Ciliata thus:[152]

(I.) Mouth habitually closed, opening by retraction of its circular or slit-like margin; cilia uniform Order 1. Gymnostomaceae.
Lacrymaria, Ehrb.; Loxodes, Ehrb.; Loxophyllum, Duj.; Lionotus, Wrez.; Trachelius, Schrank; Amphileptus, Ehrb.; Actinobolus, St.; Didinium, St.; Scaphiodon, St; Dysteria, Huxl.; Coleps, Nitzsch.; Dileptus, Duj.; Ileonema, Stokes; Mesodinium, St.
(II.) Mouth permanently open, usually equipped with one or more undulating membranes, receiving food by ciliary action (Trichostomata, Bütschli)
(a) Cilia nearly uniform, usually extending over the whole body, without any special adoral wreath of long cilia or membranellae; mouth with one or two undulating membranes at its margin or extending into the short pharynx. Order 2. Aspirotrichaceae.
Paramecium, Hill; Colpoda, O. F. Müll.; Colpidium, St.; Leucophrys, Ehrb.; Cyclidium, Cl. and L.; Lembadion, Perty; Cinetochilum, Perty; Pleuronema, Duj.; Ancistrum, Maup.; Glaucoma, Ehrb.; Uronema, Duj.; Lembus, Cohn; Urocentrum, Nitzsch; Icthyophtheirius, Fouquet.
(b) Strong cilia or membranellae forming an adoral wreath, and bounding a more or less enclosed area, the "peristome," at one point of which the mouth lies.
(i.) Body more or less equally covered with fine cilia; adoral wreath an open spiral Order 3. Heterotrichaceae
Spirostomum, Ehrb.; Bursaria, O. F. Müll.; Stentor, Oken; Folliculina, Lamk.; Conchophtheirus, St.; Balantidium, Cl. and L.; Nyctotherus, Leidy; Metopus, Cl. and L.; Caenomorpha, Perty; Discomorpha, Levander; Blepharisma, Perty.
(ii.) Body cilia limited in distribution or absent; peristome anterior, nearly circular, sinistrorse. Order 4. Oligotrichaceae.
Halteria, Duj.; Maryna, Gruber; Tintinnus, Schrank; Dictyocystis, Ehrb.; Strombidium, Cl. and L. (= Torquatella, Lank.).
(iii.) Peristome extending backwards along the ventral face, which alone is provided with motile cirrhi, etc.; dorsal cilia fine, motionless. Order 5. Hypotrichaceae.
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Stylonychia, Ehrb.; Kerona, O. F. Müll.; Oxytricha, Ehrb.; Euplotes, Ehrb.; Stichotricha, Perty; Schizotricha, Gruber.

(iv.) Body cilia reduced to a posterior girdle, or temporarily or permanently absent; peristome anterior, nearly circular, edged by the adoral wreath,[153] bounded by a gutter edged by an elevated rim or collar. Order 6. Peritrichaceae.
Lichnophora, Cl.; Trichodina, Ehrb.; Vorticella, L.; Zoothamnium, Bory; Carchesium, Ehrb.; Epistylis, Ehrb.; Opercularia, Lamk.; Vaginicola, Lamk.; Pyxicola, Kent; Cothurnia, Ehrb.; Scyphidia, Lachmann; Ophrydium, Bory; Spirochona, St.

The Ciliata have so complex an organisation that, as with the Metazoa, it is well to begin with the description of a definite type. For this purpose we select Stylonychia mytilus, Ehrb. (Fig. 49), a species common in water rich in organic matter, and relatively large (1⁄75" = ⅓ mm.). It is broadly oval in outline, with the wide end anterior, truncate, and sloping to the left side behind; the back is convex, thinning greatly in front; the belly flat. It moves through the water either by continuous swimming or by jerks, and can either crawl steadily over the surface of a solid or an air surface such as an air bubble, or advance by springs, which recall those of a hunting spider. The boundary is everywhere a thin plasmic pellicle, very tender, and readily undergoing diffluence like the rest of the cell. From the pellicle pass the cilia, which are organically connected with it, though they may be traced a little deeper; they are arranged in slanting longitudinal rows, and are much and variously modified, according to their place and function. On the edge of the dorsal surface they are fine and motionless, probably only sensory (s.h.); except three, which protrude well over the hinder end (c.p.), stout, pointed, and frayed out at the ends, and possibly serving as oars or rudders for the darting movements. These are distinguished from simple cilia as "cirrhi."

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fig49

Fig. 49.—Ventral view of Stylonychia mytilus. a.c, Abdominal cirrhi; an, anus discharging the shell of a Diatom; c.c, caudal cirrhi; c.p, dorsal cirrhi; cv, contractile vacuole; e, part of its replenishing canal; f.c, frontal cirrhi; f.v, food vacuoles; g, internal undulating membrane; l, lip; m, mouth or pharynx; mc, marginal cirrhi; N, N, lobes of meganucleus; n, n, micronuclei; o, anterior end; per, adoral membranellae; poc, preoral cilia; p.om, preoral undulating membrane; s.h, sense hairs. (Modified from Lang.)

At the right hand of the frontal area there begins, just within the dorsal edge, a row of strong cilium-like organs (Fig. 49, per); these, on careful examination, prove to be transverse triangular plates, which after death may fray into cilia.[154] They are the "adoral membranellae." This row passes to the left blunt angle, and there crosses over the edge of the body to the ventral aspect, and then curves inwards towards the median line, which it reaches about half-way back, where it passes into the pharynx (m). It forms the front and left-hand boundary of a wedge-shaped depression, the "peristomial area," the right-hand boundary being the "preoral ridge" or lip (l), which runs nearly on the median line, projecting downward and over the depression. This ridge bears on its inner and upper side a row of fine "preoral cilia" (poc) and a wide "preoral undulating membrane" (p.om), which extends horizontally across, below the peristomial area. The roof of this area bears along its right-hand edge an "internal undulating membrane" (g), and then, as we pass across to the left, first an "endoral membrane" and then an "endoral" row of cilia. In some allied genera (not in Stylonychia), at the base and on the inner side of each adoral membranella, is a "paroral" cilium. {140}All these motile organs, with the exception of the preoral cilia, pass into the pharynx; but the adoral membranellae soon stop short for want of room. There are some seventy membranellae in the adoral wreath.

The rest of the ventral surface is marked by longitudinal lines, along which the remaining appendages are disposed. On either side is a row of "marginal cirrhi" (mc.), which, like the membranellae, may fray out into cilia, but are habitually stiff spine-like, and straight in these rows; these are the chief swimming organs. Other cirrhi, also arranged along longitudinal rows, with so many blank spaces that the arrangement has to be carefully looked for, occur in groups along the ventral surface. On the right of the peristome are a group which are all curved—the "frontal cirrhi" (f.c.). Behind the mouth is a second group—the "abdominal cirrhi" (a.c.), also curved hooks; and behind these again the straight spine-like "caudal" or "anal" cirrhi (c.c), which point backwards. These three sets of ventral cirrhi are the organs by which the animal executes its crawling and darting movements. Besides the mouth there are two other openings, both indistinguishable save at the very moment of discharge; the anus (an) which is dorsal, and the pore of the contractile vacuole, which is ventral.

The protoplasm of the body is sharply marked off into a soft, semi-fluid "endoplasm" or "endosarc," and a firmer "ectoplasm" or "ectosarc." The former is rich in granules of various kinds, and in food-vacuoles wherein the food is digested. The mode of ingestion, etc., is described below (p. 145). The ectoplasm is honeycombed with alveoli of definite arrangement, the majority being radial to the surface or elongated channels running lengthwise; inside each of these lies a contractile plasmic streak or myoneme. The contractile vacuole (cv) lies in this layer, a little behind the mouth, and is in connexion with two canals, an anterior (e) and a posterior, from which it is replenished.

The nuclear apparatus lies on the inner boundary of the ectoplasm; it consists of (1) a large "meganucleus" formed of two ovoid lobes (N, N), united by a slender thread; and (2) two minute "micronuclei" (n, n), one against either lobe of the meganucleus.

Stylonychia multiplies by transverse fission, the details of which are considered on pp. 144, 147.

The protoplasm of Ciliata is the most differentiated that we {141}find in the Protista, and we can speak without exaggeration of the "organs" formed thereby.

The form of the body, determined by the firm pellicle or plasmic membrane, is fairly constant for each species, though it may be subject to temporary flexures and contractions. The pellicle varies in rigidity; where the cilia are abundant it is proportionately delicate, and scarcely differs from the ectoplasm proper, save for not being alveolate. In the Peritrichaceae it is especially resistant and proof against decay. In Coleps (Gymnostomaceae) it is hardened and sculptured into the semblance of plate-armour, and the prominent points of the plates around the mouth serve as teeth to lacerate other active Protista, its prey; but, like the rest of the protoplasm, this disappears by decay soon after the death of the Coleps. Where, as in certain Oligotrichaceae, cilia are absent over part of the body, the pellicle is hardened; and on the dorsal face and sides of Dysteria it even assumes the character of a bivalve shell, and forms a tooth-like armature about the mouth.

From the pellicle protrude the cilia, each of which is continued inwards by a slender basal filament to end in a "basal granule" or "blepharoplast." The body-cilia are fine, and often reversible in action, which is exceptional in the organic world. They may be modified or combined in various ways. We have seen that in Stylonychia some are motionless sensory hairs. The cirrhi and setae sometimes fray out during life, and often after death, into a brush at the tip, and have a number of blepharoplasts at their base. The same holds good for the membranellae and undulating membranes. They are thus comparable to the "vibratile styles" of Rotifers (Vol. II. p. 202) and the "combs" or "Ctenophoral plates" of the Ctenophora (p. 412 f.).[155]

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fig50

Fig. 50.—Ectosarc of Ciliata. a-f, from Stentor coeruleus; g, Holophrya discolor. a, Transverse section, showing cilia, pellicle, canals, and myonemes; b, surface view below pellicle, showing myonemes alternating with blue granular streaks; c, more superficial view, showing rows of cilia adjacent to myonemes; d, myoneme, highly magnified, showing longitudinal and transverse striation; e, two rows of cilia; f, g, optical sections of ectosarc, showing pellicle, alveolar layer (a), myonemes (m), and canals in ectosarc. (From Calkins, after Metschnikoff, Bütschli, and Johnson.)

The ectosarc has a very complex structure. Like other protoplasm it has a honeycombed or alveolate structure, but in this case the alveoli are permanent in their arrangement and position. Rows of these alveoli run under the surface; and the cilia are given off from their nodal points where the vertical walls of several unite, and wherein the basal granule or blepharoplast is contained. Longitudinal threads running along the inner walls of the alveoli of the superficial layer are differentiated into muscular fibrils or "myonemes," to which structures so many owe their marked longitudinal striation on the one hand, and their power of sudden contraction on the other. The appearance of transverse striation may be either due to transverse myonemes, or produced by the folds into which the contraction of longitudinal fibrils habitually wrinkles the pellicles, when it is fairly dense (Peritrichaceae); circular muscular fibrils, however, undoubtedly exist in the peristomial collar of this group. Embedded in the ectosarc are often found trichocysts,[156] analogous {143}to the nematocysts of the Coelenterata (p. 247), and doubtless fulfilling a similar purpose, offensive and defensive. A trichocyst is an oblong sac (4 µ long in Paramecium) at right angles to the surface, which on irritation, chemical (by tannin, acids, etc.) or mechanical, emits or is converted into a thread several times the length of the cilia (33 µ), often barbed at the tip. In the predaceous Gymnostomaceae, such as Didinium, the trichocysts around (or even within) the mouth are of exceptional size, and are ejected to paralyse, and ultimately to kill, the active Infusoria on which they feed. In most of the Peritrichaceae they are, when present, limited to the rim around the peristome, while in the majority of species of Ciliata they have not been described. Fibrils, possibly nervous,[157] have been described in the deepest layer of the ectosarc in Heterotrichaceae.

The innermost layer of the ectosarc is often channelled by a system of canals,[158] usually inconspicuous, as they discharge continuously into the contractile vacuole; but by inducing partial asphyxia (e.g. by not renewing the limited supply of air dissolved in the drop of water on the slide under the cover-glass), the action of the vacuole is slackened, and these canals may be more readily demonstrated. The vacuole, after disappearance, forms anew either by the coalescence of minute formative vacuoles, or by the enlargement of the severed end of the canal or canals. The pore of discharge to the surface is visible in several species, even in the intervals of contraction.[159] The pore is sometimes near that of the anus, but is only associated with it in Peritrichaceae, where it opens beside it into the vestibule or first part of the long pharynx, often through a rounded reservoir (Fig. 60, r) or elongated canal.

The endosarc, in most Ciliates well differentiated from the ectosarc, is very soft; though it is not in constant rotation like that of a Rhizopod, it is the seat of circulatory movements alternating with long periods of rest. Thus it is that the food-vacuoles, after describing a more or less erratic course, come to discharge their undigested products at the one point, the anus. {144}In a few genera (Didinium, for instance) the course from mouth to anus is a direct straight line, and one may almost speak of a digestive tract. In Loxodes and Trachelius (Fig. 56) the endosarc, as in the Flagellate Noctiluca (Fig. 48, p. 133), has a central mass into which the food is taken, and which sends out lobes, which branch as they approach and join the ectoplasm. The endosarc contains excretory granules, probably calcium phosphate, droplets of oil or dissolved glycogen, proteid spherules, paraglycogen grains, etc.

The nuclear apparatus lies at the inner boundary of the ectoplasm. The "meganucleus" may be ovoid, elongated, or composed of two or more rounded lobes connected by slender bridges (Stentor, Stylonychia). The "micronucleus" may be single; but even when the meganucleus is not lobed it may be accompanied by more than one micronucleus, and when it is lobed there is at least one micronucleus to each of its lobes.[160] The meganucleus often presents distinct granules of more deeply staining material, varying with the state of nutrition; these are especially visible in the band-like meganuclei of the Peritrichaceae (Figs. 51, 60). At the approach of fission it is in many cases distinctly fibrillated.[161] But all other internal differentiation, as well as any constriction, then disappears; and the ovoid or rounded figure becomes elongated and hour-glass shaped, and finally constricts into two ovoid daughter-meganuclei, which, during and after the fission of the cell, gradually assume the form characteristic of the species. The micronuclei (each and all when they are multiple) divide by modification of karyokinesis (or "mitosis") as a prelude to fission: in this process the chromatin is resolved into threads which divide longitudinally, but the nuclear wall {145}remains intact. If an Infusorian be divided into small parts, only such as possess a micronucleus and a fragment of the meganucleus are capable of survival. We shall see how important a part the micronuclei play in conjugation, a process in which the old meganuclei are completely disorganised and broken up and their débris expelled or digested.

The mouth of the Gymnostomaceae is habitually closed, opening only for the ingestion of the living Protista that form their prey. It usually opens into a funnel-shaped pharynx, strengthened with a circle of firm longitudinal bars, recalling the mouth of an eel-trap or lobster-pot ("Reusenapparat" of the Germans); and this is sometimes protrusible. In Dysteria the rods are replaced by a complicated arrangement of jaw- or tooth-like thickenings, which are not yet adequately described. We have above noted the strong adoral trichocysts in this group.

In all other Ciliates[162] the "mouth" is a permanent depression lined by a prolongation of the pellicle, and containing cilia and one or more undulating membranes, and when adoral membranellae are present, a continuation of these. In some species, such as Pleuronema (Fig. 57), one or two large membranes border the mouth right and left. In Peritrichaceae the first part of the pharynx is distinguished as the "vestibule," since it receives the openings of the contractile vacuole or its reservoir and the anus. The pharynx at its lower end (after a course exceptionally long and devious in the Peritrichaceae; Figs. 51, 60) ends against the soft endosarc, where the food-particles accumulate into a rounded pellet; this grows by accretion of fresh material until it passes into the endosarc, which closes up behind it with a sort of lurch. Around the pellet liquid is secreted to form the food-vacuole. If the material supplied be coloured and insoluble, like indigo or carmine, the vacuoles may be traced in a sort of irregular, discontinuous circulation through the endosarc until their remains are finally discharged as faeces through the anus. No prettier sight can be watched under the microscope than that of a colony of the social Bell-animalcule (Carchesium) in coloured water—all producing food-currents brilliantly shown up by the wild eddies of the pigment granules, and the vivid blue or crimson colour of {146}the food-vacuoles, the whole combining to present a most attractive picture. Ehrenberg fancied that a continuous tube joined up the vacuoles, and interpreted them as so many stomachs threaded, as it were, along a slender gut; whence he named the group "Polygastrica."

fig51

Fig. 51.—Carchesium polypinum. Scheme of the path taken by the ingested food in digestion and expulsion of the excreta. The food enters through the pharynx and is transported downward (small circles), where it is stored in the concavity of the sausage-shaped meganucleus (the latter is recognised by its containing darker bodies). It remains here for some time at rest (small crosses). Then it passes upward upon the other side (dots) and returns to the middle of the cell, where it undergoes solution. The excreta are removed to the outside, through the vestibule and cell mouth. The black line with arrows indicates the direction of the path. (From Verworn, after Greenwood.)

We owe to Miss Greenwood[163] a full account of the formation and changes of the food-vacuoles in Carchesium polypinum. The vacuole passes steadily along the endosarc for a certain time after its sudden admission into it, and then enters on a phase of quiescence. A little later the contents of the vacuole aggregate together in the centre of the vacuole, where they are surrounded by a zone of clear liquid; this takes place in the hollow of the meganucleus, in this species horseshoe-shaped. The vacuole then slowly passes on towards the peristome, lying deep in the endosarc, and the fluid peripheral zone is absorbed. {147}For some time no change is shown in the food-material itself: this is the stage of "storage." Eventually a fresh zone of liquid, the true digestive vacuole, forms again round the food-pellet, and this contains a peptic juice, of acid reaction. The contents, so far as they are capable of being digested, liquefy and disappear. Ultimately the solid particles in their vacuole reach the anal area of the vestibule, and pass into it, to be swept away by the overflow of the food-current. The anus is seated on a transverse ridge about a third down the tube, the remaining two-thirds being the true pharynx.

Fission is usually transverse; but is oblique in the conical Heterotrichaceae, and longitudinal in the Peritrichaceae. It involves the peristome, of which one of the two sisters receives the greater, the other the lesser part; each regenerates what is missing. When there are two contractile vacuoles, as in Paramecium, either sister receives one, and has to form another; where there is a canal or reservoir divided at fission, an extension of this serves to give rise to a new vacuole in that sister which does not retain the old one. In some cases the fission is so unequal as to have the character of budding (Spirochona). We have described above (p. 144) the relations of the nuclear apparatus in fission.

Several of the Ciliata divide only when encysted, and then the divisions are in close succession, forming a brood of four, rarely more. This is well seen in the common Colpoda cucullus. In the majority, however, encystment is resorted to only as a means of protection against drought, etc., or for quiet rest after a full meal (Lacrymaria).

Maupas[164] has made a very full study of the life-cycles of the Ciliata. He cultivated them under the usual conditions for microscopic study, i.e. on a slide under a thin glass cover supported by bristles to avoid pressure, preserved in a special moist chamber; and examined them at regular intervals.

{148}
fig52

Fig. 52.—Paramecium caudatum, stages in conjugation. gul, Gullet; mg.nu, meganucleus; Mg.nu, reconstructed meganucleus; mi.nu, micronucleus; Mi.nu, reconstructed micronucleus; o, mouth. (From Parker and Haswell, after Hertwig.)

The animals collect at that zone where the conditions of aeration are most suitable, usually just within the edge of the cover, and when well supplied with food are rather sluggish, not swimming far, so that they are easily studied and counted. When well supplied with appropriate food they undergo binary fission at frequent intervals, dividing as often as five times in the twenty-four hours at a temperature of 65-69° F. (Glaucoma scintillans), so that in this period a single individual has resolved itself into a posterity of 32; but such a rapid increase is exceptional. At a minimum and a maximum temperature multiplication is arrested, the optimum lying midway. If the food-supply is cut off, encystment occurs in those species capable of the process; but when there is a mixture of members of different broods of the same species, subject to the limitations that we shall learn, conjugation ensues. Under the conditions of Maupas' investigations he found a limit to the possibilities of continuous fissions, even when interrupted by occasional encystment. The individuals of a series ultimately dwindle in size, their ciliary apparatus is reduced, and their nuclear apparatus degenerates. Thus the ultimate members of a fission-cycle show a progressive decay, notably in the nuclear apparatus, which Maupas has aptly compared to "senility" or "old age" in the Metazoan. If by the timely mixture of broods conjugation be induced, these senile degenerations do not occur.[165] In Stylonychia {149}mytilus the produce of a being after conjugation died of senility after 336 fissions; in Leucophrys after 660.

Save in the Peritrichaceae (p. 151) conjugation takes place between similar mates, either of the general character and size of the species, or reduced by fissions, in rapid succession, induced by the same conditions as those of mating. The two mates approach, lying parallel and with their oral faces or their sides (Stentor) together, and partially fuse thereby; though no passage of cytoplasm is seen it is probable that there is some interchange or mixture.[166]

fig53

Fig. 53.—Diagram of conjugation in Colpidium colpoda. Horizontal line means degeneration; parallel vertical lines, separation of gametes; broken lines (above), boundary between pairing animals; (below), first fission; single vertical line, continuity or enlargement. M, Meganucleus; µ, micronucleus; Z, zygote-nucleus.

{150}
fig54

Fig. 54.—Four individuals of Coleps hirtus (Gymnostomaceae) swarming about and ingesting a Vorticella (?) (From Verworn.)

The meganucleus lengthens, becomes irregularly constricted, and breaks up into fragments, which are ultimately extruded or partially digested. The micronucleus enlarges (Fig. 52, A) and undergoes three successive divisions, or, strictly speaking, two fissions producing four nuclei, of which one only undergoes the third. The other three nuclei of the second fission degenerate like the meganucleus.[167] Of the two micronuclei of this last division one remains where it is as a "stationary" pairing nucleus, while its sister passes as a "migratory" pairing-nucleus into the other mate, and fuses with its stationary pairing-nucleus. Thus in either mate is formed a "zygote-nucleus," or "fusion-nucleus." All these processes are simultaneous in the two mates; and the migratory nuclei cross one another on the bridge of junction of the two mates (Fig. 52, C). Each mate now has its original cytoplasm (subject to the qualification above), {151}but its old nuclear apparatus is replaced by the fusion-nucleus. This new nucleus undergoes repeated fissions; its offspring enlarge unequally, the larger being differentiated as mega-, the smaller as micro-nuclei. The mates now separate (Fig. 52, F, G), and by the first (or subsequent) fission of each, the new mega- and micro-nuclei are distributed to the offspring. Colpidium colpoda offers the simplest case, on which we have founded our diagram showing the nuclear relations. During conjugation the oral apparatus often atrophies, and is regenerated; and in some cases the pellicle and ciliary apparatus are also "made over."

fig55

Fig. 55.—Paramecium caudatum (Aspirotrichaceae). A, The living animal from the ventral aspect; B, the same in optical section, the arrows show the course taken by food-particles. buc.gr, Buccal groove; cort, cortex; cu, cuticle; c.vac, contractile vacuole; f.vac, food vacuole; gul, gullet; med, medulla; mth, mouth; nu, meganucleus; pa.nu, micronucleus; trch, trichocysts discharged. (From Parker's Biology.)

In the Peritrichaceae the mates are unequal; the larger is the normal cell, and is fixed; the smaller, mobile, is derived from an ordinary individual by brood-divisions, which only occur under the conditions that induce conjugation (Fig. 60). Here, though the two pairs of nuclei are formed, it is only the migratory {152}nuclei that unite, the stationary ones aborting in both mates. During the final processes of conjugation the smaller mate is absorbed into the body of the larger, and so plays the part of male there. But this process, though one of true binary sex, is clearly derived from the peculiar type of equal reciprocal conjugation of the other Infusoria.

The Ciliata are almost all free-swimming animals with the exception of most of the Peritrichaceae, and of the genera we now cite. Folliculina forms a sessile tube open at either end; and Schizotricha socialis inhabits the open mouths of a branching gelatinous tubular stem, obviously secreted by the hinder end of the animal, and forking at each fission to receive the produce. A similar habit to the latter characterises Maryna socialis; all three species are marine, and were described by Gruber.[168] Stentor habitually attaches itself by processes recalling pseudopodia, and often forms a gelatinous sheath.

The majority of the Oligotrichaceous Tintinnidae inhabit free chitinous tests often beautifully fenestrated, as in Dictyocystis.

Many genera are parasitic in the alimentary canal of various Metazoa, but none appear to be seriously harmful except Ichthyophtheirius, which causes an epidemic in fresh-water fish. Quite a peculiar fauna inhabit the paunch of Ruminants. Nyctotherus and Balantidium are occasionally found in the alimentary canal of Man.[169]

The Gymnostomaceae are predaceous, feeding for the most part on smaller Ciliates. We have described the peculiar character of the mouth and pharynx in this group, and the mail-like pellicle of Coleps (Fig. 54). Loxophyllum is remarkable for the absence of cilia from one of the sides of its flattened body, and the tufts of trichocysts studding its dorsal edge at regular intervals. Actinobolus has numerous tentacles, exsertile and retractile, each bearing a terminal tuft of trichocysts, which serve to paralyse such active prey as Halteria. Ileonema has one tentacle overhanging the mouth; and Mesodinium has four short sucker-like projections around it.[170] It has only two girdles {153}of cilia, which are stout and resemble fine-pointed cirrhi. In Dysteria the cilia are exclusively ventral, and the naked dorsal surface has its pellicle condensed into a bivalve shell; a posterior motile process ("foot") and a complex pharyngeal armature add to the exceptional characters of the genus.

The Aspirotrichaceae are well known to every student of "Elementary Biology" by the "type" Paramecium (Fig. 55), so common in infusions, especially when containing a little animal matter. P. bursaria often contains in its endosarc the green symbiotic Flagellate Zoochlorella. Colpoda cucullus, very frequent in vegetable infusions, usually only divides during encystment, and forms a brood of four. Pleuronema chrysalis (Fig. 57) is remarkable for its habit of lying for long periods on its side and for its immense undulating membrane, forming a lip on the left of its mouth; Glaucoma has two, right and left.

fig56

Fig. 56.—Trachelius ovum. A, general view; B, section through sucker; C, section through contractile vacuole and its pore of discharge. al, Alveolar layer of ectoplasm; cil, cilia; c.v, contractile vacuole; m, mouth; N, meganucleus; s, sucker, from which pass inwards retractile myonemes. (After Clara Hamburger.)

{154}
fig57

Fig. 57.—Pleuronema chrysalis (Aspirotrichaceae). A, Unstimulated, lying quiet; B, stimulated, in the act of springing by the stroke of its cilia. (From Verworn.)

The Heterotrichaceae present very remarkable forms. Spirostomum is nearly cylindrical, and, a very giant, may attain a length of 4 mm. (1⁄6"). Stentor can attach itself by its hinder end, which is then finely tapered and prolonged into a few pseudopodia; its body is trumpet-shaped, with a spiral peristome forming a coil round its wide end, and leading on the left side into the mouth. Many species when attached secrete a gelatinous sheath or tube. S. polymorphus is often coloured green by Zoochlorella (p. 125); S. coeruleus[171] and S. igneus owe their names to the brilliant pigment, blue or scarlet, deposited in granules in lines between the conspicuous longitudinal myonemes. From their large size and elongated meganucleus accompanied by numerous micronuclei, these two genera have frequently been utilised for experiments on regeneration. In Metopus sigmoides the peristomial area forms a dome above its wreath of membranellae; and in M. pyriformis this is so great as to form the larger part of the cell, which is top-shaped, tapering behind to a point. Caenomorpha (Fig. 58) has the same general form, with a peg-like tail, and possesses a girdle of cirrhi.[172] The converse occurs in {155}Bursaria; the cell is a half ellipse, something like a common twin tobacco-pouch when closed: a deep depression thus occupies the whole ventral surface, and opens by a wide slit extending along the anterior end. The peristomial area occupies the dorsal side of the pocket so formed, and the mouth is in the hinder left-hand corner. Blepharisma sp. is parasitic in the Heliozoon Raphidiophrys viridis (Fig. 20, 1, p. 74).

fig58

Fig. 58.—Caenomorpha uniserialis. crh, Zone of cirrhi; c.t, cilia of tail; c.v, contractile vacuole; c.w, ciliary wreath; g, granular aggregate; m, zone of membranellae; N, meganucleus; n, micronucleus; oe, pharynx; t, tail-spine; t1, accessory spine; u.m, undulating membrane; v, vacuole; z, precaudal process. (After Levander.)

Among Oligotrichaceae, Halteria, common among the débris at the bottom of pools in woods containing dead leaves, is remarkable for an equatorial girdle of very long fine setae, and for its rapid erratic darting movements, alternating with a graceful bird-like hover. The Tintinnidae are mostly marine, pelagic, with the general look of a stalkless Vorticella; some have a latticed chitinous shell.[173]

 

{156}
fig59

Fig. 59.—Stentor polymorphus. I, Young individual attached, extended; II, adult in fission, contracted; cv in I, afferent canal of contractile vacuole; in II, contractile vacuole; N, moniliform meganucleus (micronuclei omitted); o, mouth; the fine lines are the myoneme fibrils. (From Verworn.)

Among Peritrichaceae, Vorticella (Fig. 60) and its allies have long been known as Bell-animalcules to every student of pond-life. The body has indeed the form of an inverted bell, closed at its mouth by the "peristome," or oral disc; this is a short, inverted truncate cone set obliquely so that its wide base hardly projects at one side, but is tilted high on the other; the edge of the bell is turned out into a rim or "collar," separated from the disc by a deep gutter. The collar, habitually everted, or even turned down, contracts over the retracted disc when the animal is retracted (E2), which is brought about by any sort of shock, or when it swims freely backwards. For the latter purpose a posterior ring of cilia (or rather membranellae) is developed round the hinder end of the bell (A, cr, E3). The cilia of the adoral wreath are very strong, united at the base into a continuous membrane, and indeed themselves partake of the composite nature of membranellae. The wreath forms more than one turn of a right-handed spiral, the innermost turn ending abruptly on the disc, the outer leading down into the mouth at the point where the disc is most tilted and the groove deepest.[174] The pharynx (p) is long, and contains an undulating membrane (u.m) on its inner side projecting out through the mouth, and numerous cilia; it leads deep into the body (p). The first part is distinguished as the "vestibule" (v), as into it opens the anus, and the contractile vacuole (c.v.), the latter sometimes opening by a reservoir (r). The body contains in the ectoplasm {157}myonema-fibrils which, by their contraction, withdraw the disc, and at the same time circular fibrils close the peristome over it. In the type-genus the pellicle is continued into a long, slender elastic stalk (s), of which the longitudinal myoneme fibrils of the ectoplasm converge to the stalk, and are prolonged into it as a spirally winding fibre, sometimes transversely striated.[175] The effect of the contraction of this is to pull the stalk into a helicoid spiral (like a coil-spring), with the line of insertion of the muscle along the inner side of the coils, which is, of course, the shortest path from one end to the other (Fig. 60, B).

fig60

Fig. 60.—Vorticella. A, expanded; B, stalk in contraction; c, eversible collar below peristome; cr, line of posterior ciliary ring; c.v, contractile vacuole; m, muscle of stalk; N, meganucleus; n, micronucleus; p, pharynx; r, reservoir of contractile vacuole; s, tubular stalk; u.m, undulating membrane in vestibule; v, hinder end of vestibule. E1, E2, two stages in binary fission; E3, free zooid, with posterior wreath; F1, F2 division into mega- and micro-zooids (m); G1, G2, conjugation; m, microzooid. (Modified from Bütschli, from Parker and Haswell.)

The members of the Vorticellidae are very commonly attached to weeds or to various aquatic Metazoa, each species being more or less restricted in its haunts. Vorticella, the type, is singly {158}attached to a contractile stalk; fission takes place in the vertical plane, and one of the two so formed retains the original stalk, while the other swims off (Fig. 60, E1-E3), often to settle close by, so that the individuals are found in large social aggregates, side by side, fringing water-weeds with a halo visible to the naked eye, which disappears on agitation by the sudden contraction of all the stalks. Carchesium and Zoothamnium differ from Vorticella in the fact that the one daughter-cell remains attached by a stalk coming off a little below the body of the other, so as to give rise to large branching colonies.

In Carchesium (Fig. 51) the muscular threads of each cell are separate, while in Zoothamnium they are continuous throughout the colony. Epistylis has a solid, rigid stalk, and may give rise to branching colonies, which often infest the body of the Water-Fleas (Copepoda) of the genus Cyclops. Opercularia is characterised by the depth of the gutter, the height of the collar, and the tapering downward of the elongated disc. Vaginicola, Pyxicola, Cothurnia, Scyphidia, all inhabit tubes, some of extreme elegance. Ophrydium is a colonial form, found in ponds and ditches, resembling Opercularia, but inhabiting tubes of jelly[176] that coalesce by their outer walls into a large floating sphere; it usually contains the green symbiotic Flagellate Zoochlorella. Trichodina is free, short, and cylindrical, with both wreaths permanently exposed, and is provided with a circlet of hooks within the aboral wreath. It is often parasitic, or perhaps rather epizoic, on the surface of Hydra (see p. 254), gliding over its body[177] with a graceful waltzing movement; it occurs also in the bladder and genito-urinary passages of Newts, and even in their body-cavity and kidneys.

II. Suctoria = Tentaculifera

Infusoria with cilia only in the young state,[178] without mouth or anus, but absorbing food (usually living Ciliates) by one or more tentacles, perforated at the apex; mostly attached, frequently epizoic, rarely parasitic in the interior of other Protozoa.

{159}

Acineta, Ehrb. (Fig. 61, 2); Amoebophrya, Koppen; Choanophrya, Hartog (Fig. 62); Dendrocometes, St. (Fig. 61, 4); Dendrosoma, Ehrb. (Fig. 61, 9); Endosphaera, Engelm.; Ephelota, Str. Wright (Fig. 61, 5, 8); Hypocoma, Gruber; Ophryodendron, Cl. and L. (Fig. 61, 7); Podophrya, Ehrb. (Fig. 61, 1); Rhyncheta, Zenker (Fig. 61, 3); Sphaerophrya, Cl. and L. (Fig. 61, 6), Suctorella, Frenzel; Tokophrya, Bütschli.

This group, despite a superficial resemblance to the Heliozoa, show a close affinity to the Ciliata; the nuclear apparatus is usually double though a micronucleus is not always seen; the young are always ciliated, and the mode of conjugation is identical in all cases hitherto studied. Most of the genera are attached by a chitinous stalk (Fig. 61), continued in Acineta into a cup or "theca" surrounding the cell. The pellicle is firm, often minutely shagreened or "milled" in optical section by fine radial processes, whether superficial rods or the expression of the meeting edges of radial alveoli is as yet uncertain. The pellicle closely invests the ectosarc, is continued down into a tubular sheath, from the base of which the tentacle rises, and upwards to invest the tentacle, and is even prolonged into its cavity in Choanophrya, the only genus where the tentacles are large enough for satisfactory demonstration. These organs may be one or more, and vary greatly in character. They may be (1) pointed for prehension, puncture, and suction (Ephelota, Fig. 61, 5); (2) nearly cylindrical, with a slightly "flared" truncate apex (Podophrya, Fig. 61, 1a); (3) filiform with a terminal knob; (4) "capitate" (Acineta, Fig. 61, 2); (5) bluntly truncate and capable of opening into a wide funnel for the suction of food[179] (Choanophrya, Fig. 62; Rhyncheta, Fig. 61, 3). Their movements, too, are varied, including retraction and protrusion, and a degree of flexion which reaches a maximum in Rhyncheta (Fig. 61, 3), whose tentacle is as freely motile as an elephant's trunk might be supposed to be were it as slender in proportion to its length. They are continued into the body, and in Choanophrya may extend right across it. In Podophrya trold the pellicle rises into a conical tube about the base of the tentacle, which is retracted through it completely with the prey in deglutition. In Dendrocometes, Dendrosoma, and Ophryodendron (Fig. 61, 4, 9, 7), the tentacles arise from outgrowths of the cell-body.

{160}
fig61

Fig. 61.—Various forms of Suctoria. 1, a and b, two species of Podophrya; c, a tentacle much enlarged; 2, a, Acineta jolyi; 2, b, A. tuberosa, with four ciliated buds; in 6 the animal has captured several small Ciliata; 8, a, a specimen multiplying by budding; 8, b, a free ciliated bud; 9, a, the entire colony; 9, b, a portion of the stem; 9, c, a liberated bud. a, Organism captured as food; b.c, brood-cavity; bd, bud; c.vac, contractile vacuole; l, test; mg.nu, meganucleus; mi.nu, micronucleus; nu, nucleus; t, tentacle. (From Parker and Haswell, after Bütschli and Saville Kent.)

{161}

The mechanism of suction is doubtful; but from the way particles from a little distance flow into the open funnels of Choanophrya, it may be the result of an increase of osmotic pressure. The external pellicle of the tentacles is marked by a spiral constriction,[180] which may be prolonged over the part included in the sheath. The endosarc is rich in oil-drops, often coloured, and in proteid granules which sometimes absorb stains so readily as to have been named "tinctin bodies." It usually contains at least one contractile vacuole.

In Dendrocometes (and perhaps others) the whole cell may become ciliated, detach itself and swim off; this it does when its host (Gammarus) moults its cuticle.

In fission or budding we have to distinguish many modes. (1) In the simplest, after the nuclear apparatus has divided, the cell divides transversely; the distal half acquires cilia and swims off to attach itself elsewhere, while the proximal remains attached. The tentacles have previously disappeared and have to be formed afresh in both. (2) More commonly fission passes into budding on the distal face; a sort of groove deepens around a central prominence which becomes the ciliated larva (Fig. 62, em); the tentacles of the "parent" are retained. This process passes into (3) "internal budding," where a minute pit leads into a bottle-shaped cavity.[181] (4) Again, the budding may be multiple, the meganucleus protruding a branch for each bud, while the micronucleus, by successive divisions, affords the supply requisite. Sphaerophrya (Fig. 61, 6) and Endosphaera multiply freely by fission within their Ciliate hosts, and were indeed described by Stein as stages in their life-cycle. Conjugation of the same type as in most Ciliates has been fully worked out in Dendrocometes alone, by Hickson,[182] who has found the meganuclei (though destined to disorganisation) conjugate for a short time by the bridge of communication before the reciprocal conjugation of the micronuclei.

We have referred to the endoparasitism of two genera. Amoebophrya lives in several Acanthometrids, and in the aberrant Radiolarian Sticholonche (see p. 86). The attached species are {162}some of them indifferent to their base; others are only found on Algae, or again only epizoic on special Metazoan hosts, or even on special parts of these. Thus Rhyncheta is only found on the couplers of the thoracic limbs of Cyclops, and Choanophrya on the ventral surface of its head and the adjoining appendages.

fig62

Fig. 62.—Choanophrya infundibulifera. A, adult; B-D, tentacles in action in various stages; E, tentacle at rest; F, young, just settled down, a, a, a, Tentacles in various stages of activity; c, central cavity; c.v, contractile vacuole; em, ciliated embryo showing contractile vacuole and nucleus; f, spiral ridge; m, muscular wall of funnel; n, nucleus; tr, opening of funnel. (A-D, F, modified after Zenker; E, original.)

We owe our knowledge of this group to the classical works of Ehrenberg, Claparède and Lachmann, Stein, R. Hertwig, and Bütschli. Plate has shed much light on Dendrocometes, and Hickson has studied its conjugation. Ischikawa[183] has utilised modern histological methods for the cytological study of Ephelota bütschliana. René Sand has written a useful, but unequal, and not always trustworthy monograph of the Order,[184] containing an elaborate bibliography.

 

PORIFERA (SPONGES)

BY

IGERNA B. J. SOLLAS, B.Sc. (Lond.)

Lecturer on Zoology at Newnham College, Cambridge.

{165}

CHAPTER VII

PORIFERA (SPONGES)[185]

INTRODUCTION—HISTORY—DESCRIPTION OF HALICHONDRIA PANICEA AS AN EXAMPLE OF BRITISH MARINE SPONGES AND OF EPHYDATIA FLUVIATILIS FROM FRESH WATER—DEFINITION—POSITION IN THE ANIMAL KINGDOM.

Sponges occupy, perhaps, a more isolated position than any other animal phylum. They are not only the lowest group of multicellular animals, but they are destitute of multicellular relatives. They are all aquatic and—with the exclusion of a few genera found in fresh water—marine, inhabiting all depths from between tide marks to the great abysses of the ocean. They depend for their existence on a current of water which is caused to circulate through their bodies by the activity of certain flagellated cells. This current contains their food, it is their means of respiration, and it carries away effete matters. Consequently sponges cannot endure deprivation of oxygenated water except for short periods, and only the hardiest inhabit regions where the supply is intermittent, as between tide marks. This also renders useless attempts to keep specimens in tanks, unless the water is frequently renewed.

The outward appearance of sponges has an exceptionally wide range, so that it is difficult to give a novice any very definite picture of what he is to expect when searching for these animals. This diversity is in part due to the absence of organs of sufficient size to determine the shape of the whole or limit its variation, {166}that is to say, the separate organs are of an order of size inferior to that of the entire body. The animals are fixed or lie loose on the sea bottom; there are in no case organs of locomotion, and again no sense-organs, no segregated organs of sex, and as a rule no distinction into axis and lateral members. It is by these negative characters that the collector may easily recognise a sponge.

History.—Sponges are, then, in many of their characters unique; and they present a variety of problems for solution, both of special and general interest, they are widely distributed in time and space, and they include a host of forms. It therefore causes no little surprise to learn that they have suffered from a long neglect, even their animal nature having been but recently established. Though known to naturalists from the time of Aristotle, sponges have been left for modern workers as a heritage of virgin soil: it has yielded to them a rich harvest, and is as yet far from exhausted.

The familiar bath sponge was naturally the earliest known member of the phylum. It is dignified by mention in the Iliad and in the Odyssey, and Homer, in his choice of the adjective "full of holes," πολύτρητος, shows at least as much observation as many a naturalist of the sixteenth and seventeenth centuries. Aristotle based his ideas of sponges entirely upon the characters of the bath sponge and its near allies, for these were the only kinds he knew. With his usual perspicuity he reached the conclusion that sponges are animals, though showing points of likeness to plants.

The accounts of sponges after Aristotle present little of scientific interest until the last century. Doubtless this is in part due to the absence of organs which would admit of dissection, and the consequent necessity of finer methods of study. Like other attached forms, sponges were plant or animal as it pleased the imagination of the writer, and sometimes they were "plant animals" or Zoophyta: those who thought them animal were frequently divided among themselves as to whether they were "polypous" or "apolypous." An opinion which it is somewhat difficult to classify was that of Dr. Nehemiah Grew,[186] who says: "No Sponge hath any Lignous Fibers, but is wholly composed of those which make the Pith and all the pithy parts {167}of a Plant, ... So that a Sponge, instead of being a Zoophyton, is but the one-half of a Plant."

Sponges figure in herbals beside seaweeds and mushrooms, and Gerarde says:[187] "There is found growing upon rockes near unto the sea a certaine matter wrought together of the foame or froth of the sea which we call Spunges ... whereof to speak at any length would little benefit the reader ... seeing the use thereof is so well known." About the middle of the eighteenth century, authors, especially Peyssonnel, suggested that sponges were but the houses of worms, which built them much as a bee or wasp builds nests and cells. This was confuted by Ellis in 1765,[188] when he pointed out that the sponge could not be a dead structure, as it gave proof of life by "sucking and throwing out water." To Ellis, then, is due the credit of first describing, though imperfectly, a current set up by sponges. He mentions that Count Marsigli[189] had already made somewhat similar observations.

It was not till 1825 that attention was again turned to the current, when Robert Grant approached the group in a truly scientific manner, and was ably supported by Lieberkühn. It would be impossible to do justice to Grant in the brief summary to which we must limit ourselves. The most important of his contributions was the discovery that water enters the sponge by small apertures scattered over the surface, and leaves it at certain larger holes, always pursuing a fixed course. He made a few rough experiments to estimate the approximate strength of the current, and, though he failed to detect its cause, he supposed that it was probably due to ciliary action. Grant's suggestion was afterwards substantiated by Dujardin (1838), Carter (1847), Dobie (1852), and Lieberkühn (1857). These five succeeded in establishing the claims of sponges to a place in the animal kingdom, claims which were still further confirmed when James-Clark[190] detected the presence of the protoplasmic collar of the flagellated cells (see pp. 171, 176). Data were now wanted on which to base an opinion as to the position of sponges within the animal kingdom. In 1878 Schulze[191] furnished valuable embryological facts, in a description agreeing with an earlier one of Metschnikoff's, of the amphiblastula larva (p. 226) and its metamorphosis. {168}Then Bütschli[192] (1884) and Sollas[193] on combined morphological and embryological evidence (1884) concluded that sponges were remote from all the Metazoa, showing bonds only with Choanoflagellate Protozoa (p. 121). This the exact embryological work of Maas, Minchin, and Delage has done much to prove, but it has to be admitted that unanimity on the exact position of the phylum has not yet been attained, some authorities, such as Haeckel, Schulze, and Maas still wishing to include sponges in the Metazoa.

In this short history we have been obliged to refer only to work helping directly to solve the problem of the nature of a sponge, hence many names are absent which we should have wished to mention.

Halichondria panicea.

One of the commonest of British sponges, which may be picked up on almost any of our beaches, and which has also a cosmopolitan distribution, is known by the clumsy popular name of the "crumb of bread sponge," alluding to its consistency; or by the above technical name, with which even more serious fault may be found.[194]

In its outward form H. panicea affords an excellent case of a peculiarity common among sponges. Its appearance varies according to the position in which it has lived. In fact, Bowerbank remarks that it has no specific form. It may grow in sheets of varying thickness closely attached to a rock, when it is "encrusting," or it is frequently massive and lying free on the sea bottom; again, it may be fistular, consisting of a single long tube, or it may be ridge-like, apparently in this case consisting of a row of long tubes fused laterally. In this last form it used to be called the "cockscomb sponge," having been taken for a distinct species.

Bidder has proposed to call the different forms of the same species "metamps" of the species. Figures of the metamps of H. panicea will be found in Bowerbank's useful Monograph.[195]

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The colour of the species is as inconstant as its form, ranging from green to light brown and orange. MacMunn concludes from spectroscopic work that H. panicea, contains at least three pigments, a chlorophyll, a lipochrome, and a histohaematin.[196] Lipochromes vary from red to yellow, chlorophyll is always associated with one or more of them. Histohaematin is a respiratory pigment. Proof has not yet been adduced that the chlorophyll is proper to the sponge and is not contained in symbiotic algae.

In spite of all this inconstancy H. panicea, is one of the most easily determined species. It is only necessary to dry a small fragment, including the upper surface; a beautiful honeycomb-like structure is then visible on this surface, and among British sponges this is a property peculiar to the species (Bowerbank). Whatever the form of the sponge, one or more large rounded apertures are always present on the exterior; these are the "oscula." In the encrusting metamp the oscula are flush with the general surface, while in the other cases they are raised on conical projections; fistular specimens carry the osculum at the distal end, and the cockscomb has a row of them along its upper edge. Much more numerous than the oscula are smaller apertures scattered over the general surface of the sponge, and known as "ostia."

fig63

Fig. 63.—Portion of the surface of H. panicea, from dried specimen. A, natural size; B, magnified. The large shaded patches are ostia.

If the sponge be placed in a shallow glass dish of sea water the function of the orifices can be made out with the naked eye, especially if a little powdered chalk or carmine be added to the water. If the specimen has been gathered after the retreating tide has left it exposed for some time, this addition is unnecessary, for as soon as it is plunged into water its current bursts vigorously forth, and is rendered visible by the particles of detritus that have accumulated in the interior during the period of {170}exposure and consequent suspended activity. The oscula then serve for the exit of currents of water carrying particles of solid matter, while the entrance of water is effected through the ostia.

Sections show that the ostia lead into spaces below the thin superficial layer or "dermal membrane"; these are continued down into the deeper parts of the sponge as the "incurrent canals," irregular winding passages of lumen continually diminishing as they descend. They all sooner or later open by numerous small pores—"prosopyles"—into certain subspherical sacs termed flagellated chambers. Each chamber discharges by one wide aperture—"apopyle"—into an "excurrent canal." This latter is only distinguishable from an incurrent canal by the difference in its mode of communication with the chambers.

fig64

Fig. 64.—H. panicea: the arrows indicate the direction of the current, which is made visible by coloured particles. (After Grant.)

The excurrent canals convey to the osculum the water which has passed through the ostia and chambers. All the peripheral parts of the sponge from which chambers are absent are termed the "ectosome," while the chamber-bearing regions are the "choanosome."

The peculiar crumb-of-bread consistency is due to the nature of the skeleton, which is formed of irregular bundles and strands of minute needles or spicules composed of silica hydrate, a substance familiar to us in another form as opal: they are clear and transparent like glass. They are scattered through the tissues in great abundance.

The classes of cellular elements in the sponge are as follows: Flattened cells termed "pinacocytes" cover all the free surfaces, that is to say, the external surface and the walls of the {171}excurrent and incurrent canals. The flagellated chambers are lined by "choanocytes" (cf. Fig. 70, p. 176); these are cells provided at their inner end with a flagellum and a collar surrounding it. They resemble individuals of the Protozoan sub-class Choanoflagellata, and the likeness is the more remarkable because no other organisms are known to possess such cells. Taken together the choanocytes constitute the "gastral layer," and they are the active elements in producing the current. The tissue surrounding the chambers thus lying between the excurrent and incurrent canals consists of a gelatinous matrix colonised by cells drawn from two distinct sources. In the first place, it contains cells which have a common origin with the pinacocytes, and which together with them make up the "dermal layer"; these are the "collencytes" and "scleroblasts"; secondly, it contains "archaeocytes," cells of independent origin.

Collencytes are cells with clear protoplasm and thread-like pseudopodial processes; they are distinguished as stellate or bipolar, according as these processes are many or only two. Scleroblasts or spicule cells are at first rounded, but become elongated with the growth of the spicule they secrete, and when fully grown are consequently fusiform.

fig65

Fig. 65.—Diagrammatic section of a siliceous Sponge. a.p, Apopyle; d.o, dermal ostia; ex.c, excurrent, or exhalant canal; in.c, incurrent canal; o, osculum. (Modified from Wilson.)

Each spicule consists of an organic filamentar axis or axial fibre around which sheaths of silica hydrate are deposited successively by the scleroblast. Over the greater length of the spicule the sheaths are cylindrical, but at each end they taper to a point. The axial canal in which the axial fibre lies is open at both ends, and the fibre is continuous at these two points with an organic sheath, which invests the entire spicule. From this structure we may conclude that the spicule grows at both ends—i.e. it grows in two opposite directions along one line—it has two rays lying in one axis, and is classed among uniaxial diactinal spicules. Being {172}pointed at both ends it receives the special name oxea. The lamination of the spicule is rendered much more distinct by heating or treatment with caustic potash.[197]

fig66

Fig. 66.—Cut end of a length of a siliceous spicule from Hyalonema sieboldii, with the lamellar structure revealed by solution. × 104. (After Sollas.)

The archaeocytes are rounded amoeboid cells early set apart in the larva; they are practically undifferentiated blastomeres. Some of them become reproductive elements, and thus afford a good instance of "continuity of germ plasm," others probably perform excretory functions.[198]

fig67

Fig. 67.—Free-swimming larva of Gellius varius, in optical section. a, Outer epithelium; pi, pigment; x, hinder pole. (After Maas.)

The reproductive elements are ova and spermatozoa, and are to be found in all stages in the dermal jelly. Dendy states that the eggs are fertilised in the inhalant canals, to which position they migrate by amoeboid movements, and there become suspended by a peduncle.

The larva has unfortunately not been described, but as the course of development among the near relatives of H. panicea is known to be fairly constant, it will be convenient to give a description of a "Halichondrine type" of larva based on Maas' account of the development of Gellius varius.[199] The free-swimming larvae escape by the osculum; they are minute oval bodies moving rapidly by means of a covering of cilia. The greater part of the body is a dazzling white, while the hinder pole is of a brown violet colour. This coloured patch is non-ciliate, the general covering of cilia ending at its edge in a ring of cilia twice the length of the others. Forward {173}movement takes place in a screw line; when this ceases the larva rests on its hinder pole, and the cilia cause it to turn round on its axis.

Sections show that the larva is built up of two layers:—

1. "The inner mass," consisting of various kinds of cells in a gelatinous matrix.

2. A high flagellated epithelium, which entirely covers the larva with the exception of the hinder pole.

fig68

Fig. 68.—Longitudinal section through the hinder pole of the larva of G. varius. a, Flagellated cells; ma1, undifferentiated cell; ma2, differentiated cell; pi, pigment; x, surface of hinder pole. (After Maas.)

The cells in the inner mass are classified into (1) undifferentiated cells, recognised by their nucleus, which possesses a nucleolus; these are the archaeocytes; (2) differentiated cells, of which the nucleus contains a chromatin net; these give rise to pinacocytes, collencytes, and scleroblasts. Some of them form a flat epithelium, which covers the hinder pole. Some of the scleroblasts already contain spicules. Fixation occurs very early. The front pole is used for attachment, the pigmented pole becoming the distal end (Fig. 69). The larva flattens out, the margin of the attached end is produced into radiating pseudopodial processes. The flagellated cells retreat to the interior, leaving the inner mass exposed, and some of its cells thereupon form a flat outer epithelium. This is the most important process of the metamorphosis; it is followed by a pause in the outward changes, coinciding in time with rearrangements of the internal cells to give rise to the canal system; that is to say, lacunae arise in the inner mass, pinacocytes pass to the surface of the lacunae, and form their lining; the flagellated cells, which have lain in confusion, become grouped in small clusters. These become flagellated chambers, communications are established between the various portions of the canal system, and its external apertures arise. There is at first only one osculum. The larvae may be obtained by keeping the parent sponge in a dish of sea water, shielded from too bright a light, and surrounded by a second dish of water to keep the temperature constant. They will undergo {174}metamorphosis in sea water which is constantly changed, and will live for some days.

We have said that the young sponge has only one osculum. This is the only organ which is present in unit number, and it is natural to ask whether perhaps the osculum may not be taken as a mark of the individual; whether the fistular specimens, for example, of H. panicea may not be solitary individuals, and the cockscomb and other forms colonies in which the individuals are merged to different degrees. Into the metaphysics of such a view we cannot enter here. We must be content to refer to the views of Huxley and of Spencer on Individuality.

But it is advisable to avoid speaking of a multi-osculate sponge as a colony of many individuals, even in the sense in which it is usual to speak of a colony of polyps as formed of individuals. The repetition of oscula is probably to be regarded as an example of the phenomenon of repetition of parts, the almost universal occurrence of which has been emphasised by Bateson.[200] Delage[201] has shown that when two sponge larvae fixed side by side fuse together, the resulting product has but one osculum. This, though seeming to bear out our point of view, loses weight in this connexion, when it is recalled that two Echinoderm larvae fused together give rise in a later stage to but one individual.

fig69

Fig. 69.—Larva of Gellius varius shortly after fixation. The pigmented pole, originally posterior, is turned towards the reader. R, Marginal membrane with pseudopodia; x, hinder pole. (After Maas.)

Ephydatia fluviatilis.

In the fresh water of our rivers, ponds, and lakes, sponges are represented very commonly by Ephydatia (Spongilla) fluviatilis, a cosmopolitan species. The search for specimens is most likely {175}to be successful if perpendicular timbers such as lock-gates are examined, or the underside of floating logs or barges, or overhanging branches of trees which dip beneath the surface of the water.

The sponge is sessile and massive, seldom forming branches, and is often to be found in great luxuriance of growth, masses of many pounds weight having been taken off barges in the Thames. The colour ranges from flesh-tint to green, according to the exposure to light. This fact is dealt with in a most interesting paper by Professor Lankester,[202] who has shown not only that the green colour is due to the presence of chlorophyll, but that the colouring matter is contained in corpuscles similar to the chlorophyll corpuscles of green plants, and, further, that the flesh-coloured specimens contain colourless corpuscles, which, though differing in shape from those which contain the green pigment, are in all probability converted into these latter under the influence of sufficient light. The corpuscles, both green and colourless, are contained in amoeboid cells of the dermal layer;[203] and in the same cells but not in the corpuscles are to be found amyloid substances.

The anatomy of Ephydatia fluviatilis is very similar to that of Halichondria panicea, differing only in one or two points of importance. The ectosome is an aspiculous membrane of dermal tissue covering the whole exterior of the sponge and forming the roof of a continuous subdermal space. This dermal membrane is perforated by innumerable ostia, and is supported above the subdermal cavity by means of skeletal strands, which traverse the subdermal cavity and raise the dermal membrane into tent-like elevations, termed conuli. The inhalant canals which arise from the floor of the subdermal cavity are as irregular as in H. panicea, and interdigitate with equally irregular exhalant canals; these latter communicate with the oscular tubes. Between the two sets of canals are the thin folds of the choanosome with its small subspherical chambers provided with widely open apopyles (Fig. 70). The soft parts are supported on a siliceous skeleton of oxeas, which may have a quite smooth surface or may {176}be covered in various degrees with minute conical spines (Fig. 72, a, b). These spicules are connected by means of a substance termed spongin deposited around their overlapping ends, so as to form an irregular network of strands, of which some may be distinguished as main strands or fibres, others as connecting fibres. In the main fibres several spicules lie side by side, while in the connecting fibres fewer or frequently single spicules form the thickness of the fibre. The fibres are continuous at the base with a plate or skin of spongin, which is secreted over the lower surface of the sponge and intervenes between it and the substratum. Of the chemical composition of spongin we shall speak later (see p. 237). It is a substance which reaches a great importance in some of the higher sponges, and forms the entire skeleton of certain kinds of bath sponge. Lying loose in the soft parts and hence termed flesh spicules, or microscleres, are minute spicules of peculiar form. These are the amphidiscs, consisting of a shaft with a many-rayed disc at each end (Fig. 72).

fig70

Fig. 70.—Ephydatia fluviatilis. Section of flagellated chamber, showing the choanocytes passing through the apopyle. (After Vosmaer and Pekelharing.)

In addition to its habitat the fresh-water sponge is worthy of attention on account of its methods of reproduction, which have arisen in adaptation to the habitat. A similar adaptation is widespread among fresh-water members of most aquatic invertebrates.[204]

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Ephydatia fluviatilis normally produces not only free-swimming larvae of sexual origin, but also internal gemmules arising asexually. These bodies appear in autumn, distributed throughout the sponge, often more densely in the deeper layers, and they come into activity only after the death of the parent, an event which happens in this climate at the approach of winter.

fig71

Fig. 71.—Portion of the skeletal framework of E. fluviatilis. a, Main fibres; b, connecting fibres. (After Weltner.)

fig72

Fig. 72.—Spicules of E. fluviatilis. a. b. c. Oxeas, spined and smooth; d. e, amphidiscs, side and end views. (After Potts.)

Weltner[205] has shown that on the death and disintegration of the mother sponge some of the gemmules remain attached to the old skeleton, some sink and some float. Those which remain attached are well known to reclothe the dead fibres with living tissue. They inherit, as it were, the advantages of position, which contributed to the survival of the parent, as one of the selected fittest. The gemmules which sink are doubtless rolled short distances along the bottom, while those which float have the opportunity of widely distributing the species with the risk of being washed out to sea. But even these floating gemmules are exposed to far less dangers than the delicate free-swimming larvae, for their soft parts are protected from shocks by a thick coat armed with amphidiscs.

The gemmules are likewise remarkable for their powers of {178}resistance to climatic conditions, powers which must contribute in no small way to the survival of a species exposed to the variable temperatures of fresh water. Thus, if the floating gemmules or the parent skeleton with its attached and dormant offspring should chance to be included in the surface layer of ice during the winter, so far from suffering any evil consequences they appear to benefit by these conditions. Both Potts and Weltner have confirmed the truth of this statement by experiments. Weltner succeeded in rearing young from gemmules which had suffered a total exposure of 17 days to a temperature "under 0° C."

Of important bearing on the question of the utility of the gemmules are certain instances in which E. fluviatilis has been recorded as existing in a perennial condition.[206] The perennial individuals may or may not bear gemmules, which makes it evident that, with the acquisition of the power to survive the winter cold, the prime necessity of forming these bodies vanishes.

The perennial specimens are described as exhibiting a diminished vegetative activity in winter, the flagellated chambers may be absent (Lieberkühn), or present in unusually small numbers (Weltner), the entire canal system may be absent (Metschnikoff), or, on the other hand, it may be complete except for the osculum.

fig73

Fig. 73.—Gemmule of E. fluviatilis. b, Amphidisc. (After Potts.)

In tropical countries gemmulation occurs as a defence against the ravages caused by the dry season when the waters recede down their banks, exposing all or most of their sponge inhabitants to the direct rays of the sun. The sponges are at once killed, but the contained gemmules being thoroughly dried, become efficient distributing agents of the species; they are light enough to be carried on the wind. It is probable that those individual sponges which escape desiccation survive the dry season without forming gemmules.

It has been shown experimentally that gemmules are not injured by drying—Zykoff found that gemmules kept dry for a period of two years had not lost the power of germination.

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The mature gemmules consist of a more or less spherical mass of cells, which we shall refer to as yolk cells, and of a complex coat. The latter is provided with a pore or pore tube (Fig. 74) which is closed in winter by an organic membrane.

There are three layers in the coat: an inner chitinous layer surrounded by an air-chamber layer, which is finely vesicular, showing a structure recalling plant tissue, and containing amphidiscs arranged along radii passing through the centre of the gemmule. One of the discs of each amphidisc lies in the inner chitinous coat, while the other lies in a similar membrane which envelopes the air-chamber layer and is termed the outer chitinous coat.

Marshall has suggested that one function of the amphidiscs is to weight the gemmules and thus protect them against the force of the river current; and no doubt the sinking or floating of individual gemmules depends on the relative degree of development of the air-chambers and of the amphidiscs.

A study of the development of Ephydatia gemmules vividly illustrates various characters of the inner processes of sponges. Specially noteworthy are the migrations of cells and the slight extent to which division of labour is carried: one and the same cell will be found to perform various functions.

fig74

Fig. 74.—Part of a longitudinal section of a gemmule of Ephydatia sp. passing through the pore (a). (After Potts.)

The beginning of a gemmule is first recognisable[207] as a small cluster of amoeboid archaeocytes in the dermal membrane. These move into the deeper parts of the sponge to form larger groups. They are the essential part of the gemmule, the yolk cells, which, when germination takes place, give rise to a new sponge. They are followed by two distinct troops of actively moving cells. Those forming the first troop arrange themselves round the yolk cells and ultimately assume a columnar form so that they make an epithelioid layer. They then secrete the inner chitinous coat. The cells of the second troop are entrusted with the nutrition of the gemmule. Consequently they pass in among the yolk cells, distribute their food supplies, and make their escape {180}by returning into the tissues of the mother sponge, before the columnar cells have completed the chitinous coat. Yet another migration now occurs, the cells—"scleroblasts"—which have been occupied in secreting amphidiscs at various stations in the sponge, carry the fully formed spicules to the gemmules and place them radially round the yolk cells between the radially lying cells of the columnar layer. The scleroblasts themselves remain with the amphidiscs, and becoming modified, contribute to the formation of the air-chamber layer. The columnar cells now creep out between the amphidiscs till their inner ends rest on the outer ends of these spicules. They then secrete the outer chitinous coat and return to the mother sponge.

Carter gives directions[208] for obtaining young sponges from the gemmules. The latter should be removed from the parent, cleaned by rolling in a handkerchief, and then placed in water in a watch-glass, protected with a glass cover and exposed to sunlight. In a few days the contents of the gemmule issue from the foramen and can be seen as a white speck. A few hours later the young sponge is already active and may be watched producing aqueous currents. At this age the sponge is an excellent object for studying in the living condition: being both small and transparent it affords us an opportunity of watching the movements of particles of carmine as they are carried by the current through the chambers.

Potts[209] describes how he has followed the transportal of spicules by dermal cells, the end of each spicule multiplying the motion, swaying like an oscillating rod.

In E. fluviatilis reproduction also occurs during the warmer months in this climate by means of sexual larvae. These are interesting for certain aberrant features in their metamorphosis.[210] While some of the flagellated chambers are formed in the normal way from the flagellated cells of the larva, others arise each by division of a single archaeocyte. This, it is suggested, is correlated with the acquisition of the method of reproduction by gemmules, the peculiarities (i.e. development of organs from archaeocytes) of which are appearing in the larvae.

Definition.—We may now define sponges as multicellular, {181}two-layered animals; with pores perforating the body-walls and admitting a current of water, which is set up by the collared cells of the "gastral" layer.

Position in the Animal Kingdom.—Sponges are the only multicellular animals which possess choanocytes, and their mode of feeding is unique. Since they are two-layered it has been sought to associate them with the Metazoan phylum Coelenterata, but they are destitute of nematocysts or any other form of stinging cell, and their generative cells arise from a class of embryonic cells set apart from the first, while the generative cells of Coelenterata are derived from the ectoderm, or in other cases from the endoderm. These weighty differences between sponges and that group of Metazoa to which they would, if of Metazoan nature at all, be most likely to show resemblance, suggest that we should seek a separate origin for sponges and Metazoa. We naturally turn to the Choanoflagellate Infusorian stock (see p. 121) as the source of Porifera, leaving the Ciliate stock as the progenitors of Metazoa.

That both Porifera and Metazoa are reproduced by ova and spermatozoa is no objection to this view, seeing that the occurrence of similar reproductive cells has been demonstrated in certain Protozoa (see pp. 100, 128).

Let us now see which view is borne out by facts of embryology. Suppose, for the moment, we regard sponges as Metazoa, then if the sponge larva be compared with the Metazoan larva we must assign the large granular cells to the endoderm; the flagellated cells to the ectoderm; and we are led to the anomalous statement that the digestive cells in the adult are ectodermal, the covering, outer cells endodermal; or conversely, if we start our comparisons with the adults, then it follows that the larval ectoderm has the characters of an endoderm, and the larval endoderm those of an ectoderm.

Thus both embryology and morphology lead us to the same point, they both show that in the absence of any fundamental agreement between Porifera and Metazoa it is necessary to regard the two stocks as independent from the very first, and hence the name Parazoa (Sollas) has been given to the group which contains the Porifera as its only known phylum.

Interesting in connexion with the phylogeny of Parazoa is the Choanoflagellate genus Proterospongia (Fig. 75), described by {182}Saville Kent, and since rediscovered both in England and abroad.[211] This is a colony of unicellular individuals embedded in a common jelly. The individuals at the surface are choanoflagellate, while in the interior the cells are rounded or amoeboid, and some of them undergo multiple fission to form reproductive cells. This is just such a creature as we might imagine that ancestral stage to have been of which the free-swimming sponge larva is a reminiscence: for we have seen that the flagellated cells of the larva are potential choanocytes.

fig75

Fig. 75.—Proterospongia haeckeli. a, Amoeboid cell; b, a cell dividing; c, cell with small collar; z, jelly. × 800. (After S. Kent.)

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CHAPTER VIII

PORIFERA (CONTINUED): FORMS OF SPICULES—CALCAREA—HOMOCOELA—HETEROCOELA—HEXACTINELLIDA—DEMOSPONGIAE—TETRACTINELLIDA—MONAXONIDA—CERATOSA—KEY TO BRITISH GENERA OF SPONGES

Sponges fall naturally into two branches differing in the size of their choanocytes: in the Megamastictora these cells are relatively large, varying from 5µ to 9µ in diameter; in Micromastictora they are about 3µ in diameter.[212] For further subdivision of the group the spicules are such important weapons in the hands of the systematist that it is convenient to name them according to a common scheme. This has been arrived at by considering first the number of axes along which the main branches of the spicules are distributed, and secondly whether growth has occurred in each of these axes in one or both directions from a point of origin.[213]

I. Monaxons.—Spicules of rod-like form, in which growth is directed from a single origin in one or both directions along a single axis. The axis of any spicule is not necessarily straight, it may be curved or undulating. The ray or rays are known as actines.

Biradiate monaxon spicules are termed "rhabdi" (Fig. 76, a). A rhabdus pointed at both ends is an "oxea," rounded at both ends a "strongyle," knobbed at both ends a "tylote." By branching a rhabdus may become a "triaene" (Fig. 110, k, l).

Uniradiate monaxon spicules are termed "styli."

II. Tetraxons.—Spicules in which growth proceeds from an {184}origin in one direction only, along four axes arranged as normals to the faces of a regular tetrahedron. Forms produced by growth from an origin in one direction along three axes lying in one plane are classed with tetraxons.

III. Triaxons.—Spicules in which growth is directed from an origin in both directions along three rectangular axes. One or more actines or one or two axes may be suppressed.

IV. Polyaxons.—Spicules in which radiate growth from a centre proceeds in several directions.

V. Spheres.—Spicules in which growth is concentric about the origin.

A distinction more fundamental than that of form is afforded by the chemical composition: all sponges having spicules composed of calcium carbonate belong to a single class, Calcarea, which stands alone in the branch Megamastictora.

fig76

Fig. 76.—Types of megascleres. a, Rhabdus (monaxon diactine); b, stylus (monaxon monactine); c, triod (tetraxon triactine); d, calthrop (tetraxon tetractine); e, triaxon hexactine; f, euaster.

BRANCH I. MEGAMASTICTORA

CLASS CALCAREA

Calcarea are marine shallow-water forms attached for the most part directly by the basal part of the body or occasionally by the intervention of a stalk formed of dermal tissue. They are almost all white or pale grey brown in colour. Their spicules are either monaxon or tetraxon or both. The tetraxons are either quadriradiate and then called "calthrops," or triradiate when the fourth actine is absent. The triradiates always lie more or less tangentially in the body-wall; similarly three rays of a calthrop are tangentially placed, the fourth lying across the thickness of the wall. It is convenient to include the triradiate and the three tangentially placed rays of a calthrop under the common {185}term "triradiate system" (Minchin). The three rays of one of these systems may all be equal in length and meet at equal angles: in this case the system is "regular." Or one ray or one angle may differ in size from the other rays or angles respectively, which are equal: in either of these two cases the system is bilaterally symmetrical and is termed "sagittal." A special name "alate" is given to those systems which are sagittal in consequence of the inequality in the angles. Thus all equiangular systems whether sagittal or not are opposed to those which are alate. This is the natural classification.[214]

Sub-Class I. Homocoela.

The Homocoela or Ascons possess the simplest known type of canal system, and by this they are defined. The body is a sac, branched in the adult, but simple in the young; its continuous cavity is everywhere lined with choanocytes, its wall is traversed by inhalant pores, and its cavity opens to the exterior at the distal end by an osculum. The simple sac-like young is the well-known Olynthus of Haeckel—the starting-point from which all sponges seem to have set out. Two processes are involved in the passage from the young to the adult, namely, multiplication of oscula and branching of the original Olynthus tube or sac. If the formation of a new osculum is accompanied by fission of the sac, and the branching of the latter is slight, there arises an adult formed of a number of erect, well separated main tubes, each with one osculum and lateral branches. Such is the case in the Leucosoleniidae. In the Clathrinidae, on the other hand, branching of the Olynthus is complicated, giving rise to what is termed reticulate body form, that is, a sponge body consisting of a network of tubules with several oscula, but with no external indication of the limits between the portions drained by each osculum. These outward characters form a safe basis for classification, because they are correlated with other fundamental differences in structure and development.[215]

As in Halichondria, and in fact all sponges, the body-wall is formed of two layers; the gastral layer, as we have said, forming a continuous lining to the Ascon tube and its branches. The {186}dermal layer includes a complete outer covering of pinacocytes, which is reflected over the oscular rim to meet the gastral layer at the distal end of the tube; a deeper gelatinous stratum in which lie scleroblasts and their secreted products—calcareous spicules; and finally porocytes.[216] These last are cells which traverse the whole thickness of the thin body-wall, and are perforated by a duct or pore. The porocytes are contractile, and so the pores may be opened or closed; they are a type of cell which is known only in Calcarea. It will be noticed that the fusiform or stellate "connective tissue cells" are absent. The layer of pinacocytes as a whole is highly contractile, and is capable of diminishing the size of the sponge to such an extent as quite to obliterate temporarily the gastral cavity.[217]

The choanocytes show certain constant differences in structure in the families Clathrinidae and Leucosoleniidae respectively. In the former, the nucleus of the choanocyte is basal; in the latter, it is apical, and the flagellum can be traced down to it (Fig. 77).

fig77

Fig. 77.—The two types of Asconid collar cells. A, of Clathrina, nucleus basal; B, of Leucosolenia, nucleus not basal, flagellum arising from the nuclear membrane. (A, after Minchin; B, after Bidder.)

The tetraxon spicules have "equiangular" triradiate systems in the Clathrinidae, while in Leucosoleniidae they are "alate." Finally, the larva of Clathrinidae is a "parenchymula" (see p. 226), that of Leucosoleniidae an "amphiblastula."

The fact that it is possible to classify the Calcarea Homocoela largely by means of histological characters is in accordance with the importance of the individual cell as opposed to the cell-layers generally throughout the Porifera, and is interesting in serving to emphasise the low grade of organisation of the Phylum. The organs of sponges are often unicellular (pores), or the products of the activity of a single cell (many skeletal elements); and even in the gastral layer, which approaches nearly to an epithelium, comparable with the epithelia of Metazoa, the component cells {187}still seem to assert their independence, the flagella not lashing in concert,[218] but each in its own time and direction.

Sub-Class II. Heterocoela.

fig78

Fig. 78.—Transverse section of the body-wall of Sycon carteri, showing articulate tubar skeleton, gastric ostia (a.p), tufts of oxeas at the distal ends of the chambers (fl.ch), and pores (p). (After Dendy.)

fig79

Fig. 79.—Sycon coronatum. At a a portion of the wall is removed, exposing the paragaster and the gastric ostia of the chambers opening into it.

The Heterocoela present a series of forms of successive grades of complexity, all derivable from the Ascons, from which they differ in having a discontinuous gastral layer. The simplest Heterocoela are included in the family Sycettidae, of which the British representative is Sycon (Fig. 79). In Sycon numerous tubular flagellated chambers are arranged radially round a central cavity, the "paragaster," into which they open (Figs. 78, 79). The chambers, which are here often called radial tubes, are close set, leaving more or less quadrangular tubular spaces, the {188}inhalant canals, between them; and where the walls of adjacent chambers come in contact, fusion may take place. Pores guarded by porocytes put the inhalant canals into communication with the flagellated chambers. The paragaster is lined by pinacocytes; choanocytes are confined to the flagellated chambers.

The skeleton is partly defensive, partly supporting; one set of spicules strengthens the walls of the radial tubes and forms collectively the "tubar skeleton." It is characteristic of Sycettidae that the tubar skeleton is of the type known as "articulate"—i.e. it is formed of a number of successive rings of spicules, instead of consisting of a single ring of large spicules which run the whole length of the tube.

fig80

Fig. 80.—Sycon setosum. Young Sponge. × 200. d, Dermal cell; g, gastral cell; o, osculum; p, pore cell; sp1, monaxon; sp3, triradiate spicule. (After Maas.)

The walls of the paragaster are known as the "gastral cortex"; they contain quadriradiate spicules, of which the triradiate systems lie tangentially in the gastral cortex, while the apical ray projects into the paragaster, and is no doubt defensive. The distal ends of the chambers bristle with tufts of oxeate spicules, and the separate chambers are distinguishable in surface view. It is interesting to notice that in some species of Sycon, the gaps between the distal ends of the chambers are covered over by a delicate perforated membrane, thus leading on, as we shall see presently, to the next stage of advance.[219] The larva of Sycon is an amphiblastula (see p. 227). Fig. 80 is a drawing of the young sponge soon after fixation; it would pass equally well for an ideally simple Ascon or, neglecting the arrangement of the spicules, for an isolated radial tube of Sycon. Figs. 81, 82 show the same sponge, somewhat older. From them it is seen that the Sycon type is produced from the young individual, in what {189}may be called its Ascon stage, by a process of outgrowth of tubes from its walls, followed by restriction of choanocytes to the flagellated chambers. Minute observation has shown[220] that this latter event is brought about by immigration of pinacocytes from the exterior. These cells creep through the jelly of the dermal layer and line the paragaster as fast as its original covering of choanocytes retreats into the newly formed chambers.

fig81

Fig. 81.—S. setosum. Young Sponge, with one whorl of radial tubes. o, Osculum; p, pore; sp1, monaxon; sp4, quadriradiate spicule. (After Maas.)

With a canal system precisely similar to that of Sycon, Ute (Fig. 83) shows an advance in structure in the thickening of the dermal layers over the distal ends of the chambers. The dermal thickenings above neighbouring chambers extend laterally and {190}meet; and there results a sheet of dermal tissue perforated by dermal ostia, which open into the inhalant canals, and strengthened by stout spicules running longitudinally. This layer is termed a cortex; it covers the whole sponge, compacting the radial tubes so that they form, together with the cortex, a secondary wall to the sponge, which is once more a simple sac, but with a complex wall. The cortex may be enormously developed, so as to form more than half the thickness of the wall (Fig. 84). The chambers taken together are spoken of as the chamber layer.

fig82

Fig. 82.—Sycon raphanus. A, Longitudinal section of young decalcified Sponge at a stage somewhat later than that shown in Fig. 81. B, Transverse section of the same through a whorl of tubes. d, Dermal membrane; g, gastral membrane; H, paragaster; sp4, tetraradiate spicule; T, radial tube. (After Maas.)

fig83

Fig. 83.—Transverse section of the body-wall of Ute, passing longitudinally through two chambers. a.p, Apopyle; d.o, dermal ostium; fl.ch, flagellated chamber or radial tube; i.c, inhalant canal; p, prosopyle. (After Dendy.)

We have already alluded to the resemblance between a young Ascon person and a radial tube of Sycon—a comparison which calls to mind the somewhat strange view of certain earlier authors, that the flagellated chambers are really the sponge individuals. If now we suppose each Ascon-like radial tube of Sycon to undergo that same process of growth by which the {191}Sycon itself was derived from the Ascon, we shall then have a sponge with a canal system of the type seen in Leucandra among British forms, but more diagrammatically shown in the foreign genus Leucilla (Fig. 85). The foregoing remarks do not pretend to give an account of the transition from Sycon to Leucilla as it occurred in phylogeny. For some indication of this we must await embryological research.

In Leucandra the fundamental structure is obscured by the irregularity of its canal system. It shows a further and most important difference from Leucilla in the smaller size and rounded form of its chambers. This change of form marks an advance in efficiency; for now the flagella converge to a centre, so that they all act on the same drop of water, while in the tubular chamber their action is more widely distributed and proportionately less intense (see p. 236).

fig84

Fig. 84.—Transverse section through the body-wall of Grantiopsis. d.o, Dermal ostium; fl.ch, flagellated chamber; i.c, long incurrent canal traversing the thick cortex to reach the chamber layer; p, apopyle. (After Dendy.)

fig85

Fig. 85.—Transverse section through the body-wall of Leucilla. d.o, Dermal ostium; ex.c, exhalant canal; fl.ch, chamber; i.c, inhalant canal. (After Dendy.)

Above are described three main types of canal system—that of Homocoela, of Sycon, and of Leucandra and Leucilla. These are conveniently termed the first, second, and third types respectively, and may be briefly described as related to one another somewhat in the same way as a scape, umbel, and compound umbel among {192}inflorescences. These types formed the basis of Haeckel's famous classification.[221] It has, however, been concluded[222] that the skeleton is a safer guide in taxonomy, at any rate for the smaller subdivisions; and in modern classifications genera with canal systems of the third type will be found distributed among various families; while in the Grantiidae, Ute and Leucandra stand side by side. This treatment implies a belief that the third type of canal system has been independently and repeatedly evolved within the Calcarea—an example of a phenomenon, homoplasy, strikingly displayed throughout the group. It is, remarkably enough, the case that all the canal systems found in the remainder of the Porifera are more or less modified forms of one or other of the second two types of canal system above described.

The families Grantiidae, Heteropidae, and Amphoriscidae, all possessing a dermal cortex, are distinguished as follows:—The Grantiidae by the absence of subdermal sagittal triradiate spicules and of conspicuous subgastral quadriradiates; the Heteropidae by the presence of sagittal triradiates; the Amphoriscidae by the presence of conspicuous subgastral quadriradiates.

Two families of Calcarea, possibly allied, remain for special mention—the Pharetronidae, a family rich in genera, and containing almost all the fossil forms of the group, and the Astroscleridae.

The Pharetronidae are with one, or perhaps two exceptions, fossil forms, having in common the arrangement of the spicules of their main skeletal framework in fibres. The family is divided into two sub-families:—

I. Dialytinae.—The spicules are not fused to one another; the exact mode of their union into fibres is unknown, but an organic cement may be present.

Lelapia australis, a recent species, should probably be placed here as the sole living representative. Dendy has shown[223] that this remarkable species has a skeleton of the same fibrous character as is found in typical Dialytinae, and that the triradiate spicules in the fibres undergo a modification into the "tuning-fork" type (Fig. 86, C), to enable them to be compacted into smooth fibres. {193}"Tuning-forks," though not exclusively confined to Pharetronids, are yet very characteristic of them.

fig86

Fig. 86.—Portions of the skeleton of Petrostroma schulzei. A, Framework with ensheathing pellicle; B, quadriradiate spicules with laterally fused rays; C, a "tuning-fork." (After Doederlein.)

II. Lithoninae.—The main skeletal framework is formed of spicules fused together, and is covered by a cortex containing free spicules.

fig87

Fig. 87.—A spicule from the skeleton framework of Plectroninia, showing the terminally expanded rays. (After Hinde.)

The sub-family contains only one living genus and a few recently described fossil forms. Petrostroma schulzei[224] lives in shallow water near Japan; Plectroninia halli[225] and Bactronella were found in Eocene beds of Victoria; Porosphaera[226] long known from the Chalk of England and of the Continent, has recently been shown by Hinde[226] to be nearly allied to Plectroninia; finally, Plectinia[227] is a genus erected by Počta for a sponge from Cenomanian beds of Bohemia. Doederlein, in 1896, expressed his opinion that fossil representatives of Lithoninae would most surely be discovered. The fused spicules are equiangular quadriradiates; they are united in Petrostroma by lateral fusion of the rays, in Plectroninia (Fig. 87) and Porosphaera by {194}fusion of apposed terminal flat expansions of the rays, and in some, possibly all, genera a continuous deposit of calcium carbonate ensheaths the spicular reticulum. Thus they recall the formation of the skeleton on the one hand of the Lithistida and on the other of the Dictyonine Hexactinellida (see pp. 202, 211). "Tuning-forks" may occur in the dermal membrane.

fig88

Fig. 88.—Astrosclera willeyana, Lister. A, the Sponge, × about 3. p, The ostia on its distal surface. B, a portion of the skeleton showing four polyhedra with radiating crystalline fibres. C, an ostium; the surrounding tissue contains young stages of polyhedra. (After Lister.)

The Astroscleridae, as known at present, contain a single genus and species, apparently the most isolated in the phylum. Astrosclera willeyana[228] was brought back from the Loyalty Islands, and from Funafuti of the Ellice group. Its skeleton is both chemically and structurally aberrant. In other Calcarea the calcium carbonate of the skeleton is present as calcite, in Astrosclera as aragonite, and the elements are solid polyhedra, {195}united by their surfaces to the total exclusion of soft parts (Fig. 88). Each element consists of crystalline fibres radially disposed around a few central granules, and terminating peripherally in contact with the fibres of adjacent elements. Young polyhedra are to be found free in the soft parts at the surface. The chambers are exceptionally minute, especially for a calcareous sponge, comparing with those of other sponges as follows:—

Astrosclera chambers, 10µ × 8µ to 18µ × 11µ.

Smallest chambers in Silicea, 15µ × 18µ to 24µ × 31µ.

Smallest chambers in Calcarea, 60µ × 40µ.

In its outward form Astrosclera resembles certain Pharetronids. The minute dimensions of the ciliated chambers relegate Astrosclera to the Micromastictora, and the fortunate fact that the calcium carbonate of its skeleton possesses the mineral characters not of calcite, but of aragonite, renders it less difficult to conceive that its relations may be rather with the non-calcareous than the calcareous sponges.

BRANCH II. MICROMASTICTORA

All sponges which do not possess calcareous skeletons are characterised by choanocytes, which, when compared with those of Calcarea, are conspicuous for their smaller size. The great majority (Silicispongiae) of the non-calcareous sponges either secrete siliceous skeletons or are connected with siliceous sponges by a nicely graded series of forms. The small remainder are entirely askeletal. All these non-calcareous sponges are included, under the title Micromastictora, in a natural group, opposed to the Megamastictora as of equal value.

The subdivision of the Micromastictora is a matter of some difficulty. The Hexactinellida alone are a well circumscribed group. After their separation there remains, besides the askeletal genera, an assemblage of forms, the Demospongiae, which fall into two main tribes. These betray their relationship by series of intermediate types, but a clue is wanting which shall determine decisively the direction in which the series are to be read. The askeletal genera are the crux of the systematist. It is perhaps safest, while recognising that many of them bear a likeness of {196}one kind or another to various Micromastictora, to retain them together in a temporary class, the Myxospongiae.

CLASS I. MYXOSPONGIAE

The class Myxospongiae is a purely artificial one, containing widely divergent forms, which possess a common negative character, namely, the absence of a skeleton. As a result of this absence they are all encrusting in habit.

One genus, Hexadella, has been regarded by its discoverer Topsent[229] as an Hexactinellid. The same authority places Oscarella with the Tetractinellida; it is more difficult to suggest the direction in which we are to seek the relations of the remaining type, Halisarca.

Hexadella, from the coast of France, is a remarkable little rose-coloured or bright yellow sponge, with large sac-like flagellated chambers and a very lacunar ectosome.

Oscarella is a brightly coloured sponge, with a characteristic velvety surface; it is a British genus, but by no means confined to our shores. Its canal system has been described by some authors as diplodal, by others as eurypylous. Topsent[230] has shown, and we can confirm his statement, that though the chambers have usually the narrow afferent and efferent ductules of a diplodal system, yet since each one may communicate with two or three canals, the canal system cannot be described as diplodal. The hypophare attains a great development, and in it the generative products mature. The pinacocytes, like those of Plakinidae, and perhaps of Aplysilla, are flagellated.

Halisarca, also British, is easily distinguished from Oscarella by the presence of a mucus-like secretion which oozes from it, and by the absence of the bright coloration characteristic of Oscarella. It naturally suggests itself that the coloration in the one case and the secretion in the other are protective, and in this respect perform one of the functions of the skeleton of other sponges. The chambers are long, tubular, and branched. There is no hypophare.

{197}

CLASS II. HEXACTINELLIDA[231]

Silicispongiae, defined by their spicules, of which the rays lie along three rectangular axes. The canal system is simple, with thimble-shaped chambers. The body-wall is divided into endosome, ectosome, and choanosome.

Some authors would elevate the Hexactinellida to the position of a third main sub-group of Porifera, thus separating them from other siliceous sponges. In considering this view it is important to realise at the outset that they are deep-water forms. They bear evident traces of the influence of their habitat, and like others of the colonists of the deep sea, are impressed with marked archaic features. Yet they are still bound to other Micromastictora, first by the small size of their choanocytes, and secondly by the presence of siliceous spicules. This second character is really a double link, for it involves not merely the presence of silica in the skeleton, but also the presence in each spicule of a well-marked axial filament. Now this axial filament is a structure which is gaining in importance, for purposes of classification, in proportion as its absence in Calcarea is becoming more probable. The Hexactinellida are the only sponges, other than the bath sponge, which are at all generally known. They have won recognition by their beauty, as the bath sponge by its utility, and, like it, one of their number—the Venus's Flower-Basket—forms an important article of commerce, the chief fishery being in the Philippine Islands. This wonderful beauty belongs to the skeleton, and is greatly concealed when the soft parts are present.

We have said that the Hexactinellids are deep-sea forms; they are either directly fixed to the bottom or more often moored in the ooze by long tufts of rooting spicules. In the "glass-rope sponge," the rooting tuft of long spicules, looking like a bundle of spun glass, is valued by the Japanese, who export it to us. In Monorhaphis the rooting tuft is replaced by a single giant spicule,[232] three metres in length, and described as "of the thickness of a little finger"! Probably it is as a result of their fixed life in the calm waters of the deep sea[233] that {198}Hexactinellids contrast with most other sponges by their symmetry. It should not, however, be forgotten that many of the Calcarea which inhabit shallow water exhibit almost as perfect a symmetry.

fig89

Fig. 89.—Longitudinal section of a young specimen of Lanuginella pupa O.S., with commencing formation of the oscular area. × 35. d.m, Dermal membrane; g.m, gastral membrane; pg, paragaster; sd.tr, subdermal trabeculae; Sg.tr, subgastral trabeculae. (After F. E. Schulze.)

The structure of the body-wall in Hexactinellida is so constant as to make it possible to give a general description applicable to all members of the group. It is of considerable thickness, but a large part is occupied by empty spaces, for the actual tissue is present in minimum quantity. In the wall the chamber-layer is suspended by trabeculae of soft tissue, between a dermal membrane on the outside and a similar gastral membrane on the inner side (Fig. 89). Thus the water entering the chambers through their numerous pores has first passed through the ostia in the dermal membrane and traversed the subdermal trabecular space; on leaving the chambers it flows through the subgastral trabecular space and the ostia in the gastral membrane, to enter the paragaster and leave the body at the osculum. The trabeculae and the dermal and gastral membranes together constitute the dermal layer. This conclusion is based on comparison with adults of the other groups, for in the absence of embryological knowledge no direct evidence is available. According to {199}the Japanese investigator, Isao Ijima,[234] the dermal and gastral membranes are but expansions of the trabeculae, and the trabeculae themselves are entirely cellular, containing none of the gelatinous basis met with in the dermal layer of all other sponges. There is no surface layer of pinacocytes, the cells forming the trabeculae being all of one type, namely, irregularly branching cells, connected with one another by their branches to form a syncytium. In the trabeculae are found scleroblasts and archaeocytes.

The chambers have a characteristic shape: they are variously described as "thimble-shaped," "tubular," or "Syconate," and they open by wide mouths into the subgastral trabecular space. Their walls have been named the membrana reticularis from the fact that, when preserved with only ordinary precautions, they are seen as a regular network of protoplasmic strands, with square meshes and nuclei at the nodes. This appearance recently found an explanation when Schulze, for the first time, succeeded in preserving the collared cells of Hexactinellids.[235] Schulze was then able to show that the choanocytes are not in contact with one another at their bases, where the nuclei are situated, but communicate with one another by stout protoplasmic strands. The form of the choanocyte can be seen in Fig. 91.

fig90

Fig. 90.—Portion of the body-wall of Walteria sp., showing the thimble-shaped flagellated chambers, above which is seen the dermal membrane. (After F. E. Schulze.)

To Schulze's description of the chamber, Ijima has added the important contributions that every mesh in the reticulum functions as a chamber pore or prosopyle; and that porocytes, such as are found in Calcarea, are wanting. This structure of the chamber-walls, the absence of gelatinous basis in the dermal layer, and the slight degree of histological differentiation in {200}the same layer, added to the more obvious character of thimble-shaped chambers, are the chief archaic features of Hexactinellid morphology.

fig91

Fig. 91.—Portion of a section of the membrana reticularis or chamber-wall of Schaudinnia arctica, × 1500. (After F. E. Schulze.)

The skeleton which supports the soft parts is, like them, simple and constant in its main features. It is secreted by scleroblasts, which lie in the trabeculae, and is made up of only one kind of spicule and its modifications. This is the hexactine, a spicule which possesses six rays disposed along three rectangular axes. Each ray contains an axial thread, which meets its fellow at the centre of the spicule, where they together form the axial cross. Modifications of the hexactine arise either by reduction or branching, by spinulation or expansion of one or more of the rays. The forms of spicule arising by reduction are termed pentactines, tetractines, and so on, according to the number of the remaining rays. Those rays which are suppressed leave the proximal portion of their axial thread as a remnant marking their former position (Fig. 94). Octactine spicules seem to form an exception to the above statements, but Schulze has shown that they too are but modifications of the hexactine arising by (1) branching of the rays of a hexactine, followed by (2) recombination of the secondary rays (Fig. 92).

fig92

Fig. 92.—A, discohexaster, in which the four cladi a, a', b, b', c of each ray start directly from a central nodule. B, disco-octaster, resulting from the redistribution of the twenty-four cladi of A into eight groups of three. (After Schulze, from Delage.)

The various spicules are named, irrespective of their form, according to their position and corresponding function. The {201}arrangement of the spicules is best realised by means of a diagram (Fig. 93).

fig93

Fig. 93.—Scheme to show the arrangement of spicules in the Hexactinellid skeleton. Canalaria, microscleres in the walls of the excurrent canals; Dermalia Autoderm[alia], microscleres in the dermal membrane; D. Hypoderm[alia], more deeply situated dermalia; Dictyonalia, parenchymalia which become fused to form the skeletal framework of Dictyonina; Gastralia Autogastr[alia], microscleres in the gastral membrane; Gastralia Hypogastr[alia], more deeply situated gastralia; Parenchymalia Principalia, main supporting spicules between the chambers; P. Comitalia, slender diactine or triactine spicules accompanying the last; P. Intermedia, microscleres between the P. principalia; Prostalia, projecting spicules; P. basalia, rooting spicules, from the base; P. marginalia, defensive spicules, round the oscular rim; P. pleuralia, defensive spicules, from the sides. (From Delage and Hérouard, after F. E. Schulze.)

The deviations from this ground-plan of Hexactinellid structure are few and simple. They are due to folding of the chamber-layer, or to variations in the shape of the chambers, and to increasing fusion of the spicules to form rigid skeletons. A simple condition of the chamber-layer, like that of the young sponge of Fig. 89, {202}occurs also in some adult Hexactinellids, e.g. in Walteria of the Pacific Ocean (Fig. 90). Thus is represented in this order the second type of canal system described among Calcarea. More frequently, however, instead of forming a smooth sheet, the chamber-layer grows out into a number of tubular diverticula, the cavities of which are excurrent canals; these determine a corresponding number of incurrent canals which lie between them. In this way there arises a canal system resembling the third type of Calcarea. By still further pouching so as to give secondary diverticula, opening into the first, a complicated canal system is formed, as, for example, in Euplectella suberea.

To return to the skeleton, the most complete fusion is attained by the deposit of a continuous sheath of silica round the apposed parallel rays of neighbouring spicules. This may be termed the dictyonine type of union, for it occurs in all those forms originally included under the term Dictyonina, in which the cement is deposited pari passu with the formation of the spicules. In other cases connecting bridges of silica unite the spicules, or there may be a connecting reticulum of siliceous threads, or, again, rays crossing obliquely may be soldered together at the point of contact. These more irregular methods occur in species where the spicules are free at their first formation. Spicules originally free may later be united in a true Dictyonine fashion. The terms Lyssacina and Dictyonina are useful to denote respectively: the former all those Hexactinellida in which the spicules are free at their first formation, and the latter those in which the deposit of the cementing layer goes hand in hand with the formation of the spicules. But the terms do not indicate separateness of origin of the groups denoted by them, for there is evidence that Dictyonine types have been derived repeatedly from Lyssacine types, and that in fact every Dictyonine was once a Lyssacine.

fig94

Fig. 94.—Amphidisc, at a are traces of the four missing rays.

The real or natural cleft in the class lies between those genera possessing amphidiscs (Figs. 94, 97) among their microscleres, and all the remainder of the Hexactinellida which bear hexasters (Fig. {203}96). The former set of genera constitute the sub-class Amphidiscophora, the latter the Hexasterophora.

fig95

Fig. 95.—Portion of body-wall of Hyalonema, in section, showing the irregular chambers.

Sub-Class 1. Amphidiscophora.Amphidiscs are present, hexasters absent. A tuft of rooting spicules or basalia is always present. The ciliated chambers deviate more or less from the typical thimble shape, and the membrana reticularis is continuous from chamber to chamber (Figs. 94, 95, 97).

fig96

Fig. 96.—Hexasters. A, Graphiohexaster; B, floricome; C, onychaster.

Sub-Class 2. Hexasterophora.Hexasters are present, amphidiscs absent. The chambers have the typical regular form, and are sharply marked off from one another (Figs. 90, 96).

All the Amphidiscophora have Lyssacine skeletons; in the Hexasterophora both types of skeleton occur. The subdivision of the Hexasterophora is determined by the presence or absence of uncinate spicules. An "uncinatum" is a diactine spicule, pointed at both ends and bearing barbs all directed towards one end. This method of classification gives us a wholly Dictyonine order, Uncinataria, and an order consisting partly of Dictyonine, partly of Lyssacine genera, which may be distinguished as the Anuncinataria. {204}Ova have rarely been found, and sexually produced larvae never; but Ijima has found archaeocyte clusters in abundance, and his evidence is in favour of the view that they give rise asexually to larvae, described by him in this class for the first time (see p. 231).

Both sub-classes are represented in British waters: the Amphidiscophora by Hyalonema thomsoni and Pheronema carpenteri; the Hexasterophora by Euplectella suberea and Asconema setubalense, and of course possibly by others.

Hyalonema thomsoni, one of the glass-rope sponges, was dredged by the Porcupine off the Shetland Islands in water of about 550 fathoms. The spindle-shaped body of the sponge is shown in Fig. 97. Its long rooting tuft is continued right up its axis, to end in a conical projection, which is surrounded by four apertures leading into corresponding compartments of the paragaster.

fig97

Fig. 97.—Hyalonema thomsoni. A, Whole specimen with rooting tuft and Epizoanthus crust; B, pinulus, a spicule characteristic of but not peculiar to the Amphidiscophora, occurring in the dermal and gastral membranes; C, amphidisc with axial cross; D, distal end of rooting spicule with grapnel. (After F. E. Schulze.)

The crust of Anthozoa of the genus Epizoanthus (p. 406) on the rooting tuft is a constant feature in this as in other species of Hyalonema. It contributed to make the sponge a puzzle, which long defied interpretation. The earliest diagnosis the genus received was the "Glass Plant." Then the root tuft was thought to be part of the Epizoanthus, which was termed a "most aberrant Alcyonarian with its base inserted in a sponge"; next we hear of the sponge as parasitic {205}on the Sea Anemone. Finally, the root tuft was shown to be proper to the sponge, which was, however, figured upside down, till some Japanese collectors described the natural position, or that in which they were accustomed to find it.

Pheronema carpenteri was found by the Lightning off the north of Scotland in 530 fathoms. The goblet shaped, thick walled body and broad, ill-defined root tuft are shown in Fig. 98, but no figure can do justice to the lustre of its luxuriant prostalia and delicate dermal network with stellate knots at regular intervals. The basalia are two-pronged and anchor-like.

fig98

Fig. 98.—Pheronema carpenteri. × ½. (From Wyville Thomson.)

Both the Hexasterophoran genera were dredged off the north of Scotland, and both conform to the Lyssacine type without uncinates. Euplectella suberea is a straight, erect tube, anchored by a tuft {206}of basalia. The upper end of the tube is closed by a sieve plate, the perforations in which are oscula, while the beams contain flagellated chambers, so that the sieve is simply a modified portion of the wall. It is a peculiarity of this as of one or two other allied genera that the lateral walls are perforated by oscula. They are termed parietal gaps, and are regularly arranged along spiral lines encircling the body.

fig99

Fig. 99.—Sieve plate of Euplectella imperialis. (After Ijima.)

Ijima, who has dredged Euplectellids from the waters near Tokyo, finds that in young specimens oscula are confined to the sieve plate; parietal gaps are secondary formations. The groundwork of the skeleton is a lattice similar to that shown in Fig. 100. The chamber-layer is much folded. Various foreign species of Euplectella afford interesting examples of association with a Decapod Crustacean, Spongicola venusta, of which a pair lives in the paragaster of each specimen. The Crustacean is light pink, the female distinguished by a green ovary, which can be seen through the transparent tissues. It is not altogether clear what the prisoner gains, nor what fee, if any, the host exacts.

Ijima relates that the skeleton of Euplectella is in great demand in Japan for marriage ceremonies. He also informs us that the Japanese name means "Together unto old age and unto the same grave," while by a slight alteration it becomes "Lobsters in the same cell," and remarks that the Japanese find this an amusing pun.

fig100

Fig. 100.—Skeletal lattice of Euplectella imperialis. (After Ijima.)

The same Spongicola lives in pairs in Hyalonema sieboldi. Another case of apparently constant association is that of the Hydroid stocks which inhabit Walteria. F. E. Schulze describes Stephanoscyphus mirabilis (see p. 318) in a specimen of Walteria flemmingi; the presence of the polyp causes the sponge to grow out into little dome-shaped elevations, each of which shelters one polyp; while in W. leuckarti Ijima finds a similar association in every specimen examined.

{207}

Fossil Hexactinellida.

This group has the distinction of including among its Lyssacine members the oldest known sponge, Protospongia fenestrata, of Cambrian age (Salter). As preserved it consists of a single layer of quadriradiate, or possibly quinqueradiate spicules, which, arranged as a square meshed lattice, supported the superficial layer of the sponge (Fig. 101). Whether or not the fossil represents the whole of the sponge-skeleton does not appear.[236]

fig101

Fig. 101.—Part of the specimen of Protospongia fenestrata in the Sedgwick Museum, Cambridge. Nat. size. (After Sollas.)

fig102

Fig. 102.—A portion of the outer surface of a Receptaculitid, Acanthoconia barrandei, in which the expanded outer rays of the spicules are partially destroyed, revealing the four tangential rays beneath, × 3. (After Hinde.)

The extraordinary Receptaculitidae are probably early Lyssacine forms: they are cup- or saucer-shaped fossils, abundant in Silurian and above all in Devonian strata, and have been "assigned in turn to pine cones, Foraminifera, Sponges, Corals, Cystideans," and Tunicata. Hinde[237] brings forward important arguments for retaining them among Hexactinellida. The only elements in the skeleton of the simpler genera, e.g. Ischadites, are structures comparable to Hexactinellid spicules. The surface of the fossil presents a series of lozenges forming a regular mosaic. Each lozenge is the expanded end of one of the rays of a spicule; it conceals four rays in one plane, tangential to the wall of the cup-shaped fossil, while the sixth ray projects vertically to the wall into the cavity of the cup. In the genus Receptaculites itself there is an inner layer of plates abutting against the inner {208}ends of the sixth rays, and at present problematic. An axial canal is present in each of the rays—the six canals meeting at the centre of the spicule. Special chinks between the spicules appear to have provided a passage for the water current.

The beautiful Ventriculites, so common in the Chalk and present in the Cambridge Greensand, are historically interesting, for the fact that they are fossil Hexactinellida of which the general and skeletal characters were very minutely described by Toulmin Smith long before recent representatives of the group were known. In common with a number of fossil Dictyonine species they are distinguished by the perforation of the nodes, a character due to the fact that the siliceous investment which unites the spicules together stops short before reaching the centre of each spicule, and bridges across the rays so as to form a skeleton octahedron. This character is rare in recent Hexactinellids, but, as first pointed out by Carter, it is presented by one or two forms, of which Aulocystis grayi Bwk is best known. The majority of the fossil Hexactinellida belong to the Dictyonine section, a fact attributable to the greater coherence of their skeleton. The "Dictyonina" are to be reckoned among the rock-builders of Jurassic and Cretaceous times.

fig103

Fig. 103.—A node of the skeleton of Ventriculites from the Cambridge Greensand. (After Sollas.)

The Octactinellida and Heteractinellida are two classes created by Hinde[238] to contain certain little-known Devonian and Carboniferous sponges, possessing in the one case 8-rayed spicules, of which 6 rays lie in one plane and 2 are perpendicular to this plane; in the other case, spicules with a number of rays varying from 6 to 30. Bearing in mind the manner in which octactine spicules are known to arise in recent Hexactinellida (p. 200), it is clearly possible to derive these 8-rayed spicules from hexactines by some similar method; while the typical {209}spicule of the Heteractinellida is a euaster. Hence we may refer the Octactinellid fossils to the class Hexactinellida, and the Heteractinellid forms either to the Monaxonida or Tetractinellida.

CLASS III. DEMOSPONGIAE

Silicispongiae in which triaxonid spicules are absent.

This class has attained the highest level of organisation known among Porifera; the most efficient current-producing apparatus is met with here, so, too, are protective coverings, stout coherent skeletons, and the highest degree of histological differentiation found in the phylum.

Correspondingly it is the most successful group, the majority of existing sponges coming within its boundaries. A few genera and species are exceedingly specialised, for example, Disyringa dissimilis (p. 215). These, however, contribute only a very small contingent to the Demosponge population, those species which are really prolific and abundant being, as we should expect, the less exaggerated types.

Canal System.—With a few exceptions the representatives of the Demospongiae may be said to have taken up the evolution of the canal system at the stage where it was left in Leucandra aspera—a stage which the ancestral Demosponges must have reached quite independently of the Calcarea. These commoner members are thus already gifted with the advantages pertaining to a spherical form of ciliated chamber, and so, too, is the Rhagon (Fig. 105), an immature stage noteworthy as the simplest form of Demosponge, and thus the starting-point for the higher types of canal system. The exceptions above alluded to are not without interest: they are the Dendroceratina, of doubtful affinities, (p. 220), which possess small tubular Syconate chambers. They may be regarded either as of independent origin from other Demospongiae, thus making the group polyphyletic, or more simply as representing the ancestral condition, and in this case we must look on the possession of spherical chambers by the Rhagon as a secondary feature. Occupying as it does the important position above indicated, the Rhagon merits a brief description. It is a small discoid or hemispherical body attached by a flat base. It contains a central paragaster, with a single osculum at the free end. Into the paragaster open directly a {210}few spherical flagellated chambers, which lie in the lateral walls of the body. The basal wall of the paragaster, the parts of its lateral walls between the openings of neighbouring chambers, and the entire outer surface of the body are covered with pinacocytes. It is convenient to call the basal part of the sponge from which chambers are absent the hypophare, the upper chamber-bearing part the spongophare. In some of the deeper dermal cells spicules may be already present. In the Rhagon, then, the canal system is of the second type, but all the adult Demosponges have advanced to the third type, and the further evolution in this system is in the direction of improving the mode of communication of the chambers with the canal system. The changes involved go hand in hand with increasing bulk of the dermal layer. A glance at the accompanying figures will show at once the connexion between the phenomena. The increase in the dermal layer (1) greatly reduces the extent of the lumen of the excurrent canals; and (2) results in the intervention of a narrow tube or aphodus between the mouth of each chamber and the excurrent canal. The chamber system is then converted from an "eurypylous" to an "aphodal" type. When the incurrent canal also opens into the chamber by way of narrow tubes, one proper to each chamber and termed "prosodus," the canal system is of the "diplodal" type.

fig104

Fig. 104.—Diagram of (A) eurypylous and (B) aphodal canal systems. a, Apopyle; a', aphodus; E, excurrent canal; I, incurrent canal; p, prosopyle; p', short prosodus. (After Sollas.)

Cortex.—All the stages in the formation of a cortex are to be seen among the adult members of the group. Certain species (e.g. Plakina monolopha, F.E.S.) are destitute even of an ectosome, {211}others have a simple dermal membrane (Halichondria panicea, Tetilla pedifera) and various others are provided with a cortex, either of simple structure or showing elaboration in one or more particulars. Thus a protective armature of special spicules may be present in the cortex, e.g. in Geodia, or to a less extent in Tethya, or there may be an abundance of contractile elements, and these may be arranged in very definite ways, forming valve-like apparatus that will respond to stimuli.

Everywhere among sponges the goal of the skeleton appears to have been coherence. We have seen how in Calcarea and in Hexactinellida this has been attained by the secretion around the separate elements of a continuous mineral sheath, calcareous in the one case and siliceous in the other. Here we had an excellent instance of the attainment of one end by similar means in two different groups, after their separation from the common stock, and therefore independently. In Demospongiae, on the other hand, the same end—coherence—has been secured by two new methods, each distinct from the former: first the spicules may be united in strands by an organic deposit, spongin; secondly, the spicules may assume irregular shapes and interlock closely with one another, forming dense and stout skeletons. The latter method is that characteristic of the Lithistid Tetractinellida.

Classification.—It is not of great moment which scheme of classification we maintain, seeing that all hitherto proposed are confessedly more or less artificial, and sufficient data for framing a natural one are not yet forthcoming. For convenience, we accept three subdivisions and define them thus:—

I. Tetractinellida.—Demospongiae possessing tetraxon or triaene spicules or Lithistid desmas.

II. Monaxonida.—Demospongiae possessing monaxon but not tetraxon spicules.

III. Ceratosa.—Demospongiae in which the main skeleton is formed of fibres of spongin. The fibres may have a core of sand-grains or of foreign spicules, but not of spicules proper to the sponge.

But at the same time we admit that some of the Ceratosa are probably descended from some of the families of Monaxonida, so that we should perhaps be justified in separating these families of Monaxonida from the rest, and associating them with the allied families of Ceratosa—a method of classification due to {212}Vosmaer. Again, some Monaxonida approximate to Tetractinellida, and we might, with Vosmaer, unite them under the title Spiculispongiae. This proceeding, though it has the advantage of being at least an attempt to secure a natural classification, involves too much assumption when carried out in detail to be wholly satisfactory.

Sub-Class I. Tetractinellida.[239]

Tetractinellida appear to flourish best in moderate depths from 50 to 200 fathoms, but they are found to be fairly abundant also in shallower water right up to the coast line, and in deep water up to and beyond the 1000 fathom line. Occasionally they lie free on the bottom, but are far more commonly attached; fixation may be direct or by means of rooting spicules; the occurrence of a stalk is rare. There is great variety in the root tuft, which may be a long loose wisp of grapnel-headed spicules, as in many species of Tetilla, or a massive tangle, as in Cinachyra barbata; in these cases the sponge is merely anchored, so that it rests at the level of the surface of the ooze; in other cases, e.g. Thenea wyvillei, the root tuft consists of a number of pillars of spicules which raise the sponge above the level of the ooze, into which they descend and there become continuous with a large dense and confused mass of spicules. The parachute-like base of Tetilla casula invites comparison with the "Crinorhiza" forms of some Monaxonids (p. 216).

Two Orders are distinguished thus:—

I. Choristida.—Tetractinellida with quadriradiate spicules, which are never articulated together into a rigid network.

II. Lithistida.—Tetractinellida with branching scleres (desmas), which may or may not be modified tetrad spicules, articulated together to form a rigid network. Triaene spicules may or may not be present in addition.

Order I. Choristida.

Plakina monolopha, from the Adriatic and Mediterranean, furnishes a connecting link between the Rhagon stage and other Tetractinellida. The choanosome is simply folded; there is no distinct ectosome; the chambers are eurypylous. The skeleton {213}consists of microcalthrops and their derivatives. The hypophare is well developed. Plakina thus shows a certain amount of resemblance to Oscarella (p. 196), with which it shares the very remarkable possession of flagellated pinacocytes.

One of the species of Tetilla, T. pedifera, continues the series. The folds of its choanosome are more complicated than in P. monolopha, and their outer ends are bridged together by a thin layer of ectosome (cf. species of Sycon among Calcarea); the chambers are still eurypylous.

The skeleton reaches a high level: it includes oxeas and triaenes radiately disposed and microscleres (sigmata) scattered throughout the dermal layer. The British Poecillastra compressa from the north of Scotland and Orkney and Shetland is at about the same stage of development, being without cortex and having eurypylous chambers, but it is not so good an example, as the folds of its choanosome are confused.

fig105

Fig. 105.—Diagrammatic vertical sections of A, Rhagon; B, Plakina; C, Tetilla pedifera.

From T. pedifera we pass to the other species of Tetilla and all the higher genera of Choristida; these possess a cortex not of homologous origin in the various cases, but probably to be classified under one of two heads, typified by Stelletta and Craniella respectively (Fig. 106).

fig106

Fig. 106.—A, Craniella type; B, Stellettid type. ch, Chone; co, collenchyma; d.o, dermal ostia; fb, fibrous tissue; i.c, intercortical cavity; sd, subdermal cavity; sp, sphincter. (After Sollas.)

{214}

In the Stellettids the cortex arises by the centrifugal growth of a dermal membrane such as that of Tetilla pedifera; in Craniella directly from the dermal tissue of the distal ends of the choanosomal folds.

In both cases the end result, after completion of cell differentiation, is a cortex either fibrous throughout or collenchymatous in its outer portion and fibrous in the deeper layers. In the Stellettid type the centrifugal growth of the dermal membrane involves the addition of secondary distal portions to the ends of the inhalant passages. These are the intercortical cavities or canals. Their most specialised form is the "chone." A chone is a passage through the cortex opening to the exterior by one or more ostia, and communicating with the deeper parts of the inhalant system by a single aperture provided with a sphincter (Fig. 106, B).

In the Craniella type the intercortical cavities are parts of the primary inhalant system. They communicate with its deeper parts by sphinctrate apertures. Without any knowledge of the development one would certainly have supposed that the subdermal cavity, pore-sieve and sphinctrate passages of Craniella represented a number of chones, of which the outer portions had become fused (Fig. 106, A).

fig107

Fig. 107.—Disyringa dissimilis. Diagrammatic longitudinal section of the Sponge. × ½. a, b, c, Transverse sections at the levels indicated to show subdivision of the lumina of the excurrent and incurrent tubes; e.t, excurrent tube; i.t, incurrent tube; o, osculum. (After Sollas.)

In both Craniella and Stelletta the chamber system is aphodal, and these genera may fairly be taken as representatives of the average level reached by Tetractinellida. The skeleton is of the radiate type: the type which prevails in the Choristida, but which has an erratic distribution, appearing in some genera of {215}each family but not in others. The genus Pachymatisma, of which we have the species P. johnstonia and P. normani in these islands, exemplifies this; it belongs to the highly differentiated family Geodiidae, possesses an elaborate cortex with chones, but its main skeleton is non-radiate.

Disyringa dissimilis is remarkable for the perfection of its symmetry, and for the absence of that multiplication of parts which is so common among sponges. It possesses a single inhalant tube and a single osculum (Fig. 107). Until quite recently it stood alone in the restriction of its inhalant apertures to a single area. Kirkpatrick, however, has now described a sponge—Spongocardium gilchristi[240]—from Cape Colony, in which the dermal ostia are concentrated in one sieve-like patch at the opposite pole to the single osculum. Disyringa is still without companions in the possession of an inhalant tube. The concentration of ostia into sieve areas occurs again in Cinachyra, each sponge possessing in this case several inhalant areas with or without scattered ostia also.

Order II. Lithistida.

The characteristic spicule of Lithistida—the desma—may be a modified calthrop (tetracrepid desma), or it may be produced by the growth of silica over a uniaxial spicule (rhabdocrepid desma) (Fig. 110, q), or it may be of the polyaxon type. It is probable that the group is polyphyletic,[241] and that some of its members should remain associated with Tetractinellida, while others should be removed to Monaxonida. Forms with tetracrepid desmas, and those forms with rhabdocrepid desmas which possess triaenes, have Tetractinellid affinities, while forms possessing rhabdocrepid desmas but lacking triaenes, and again those in which the desmas are polyaxon, are probably descendants of Monaxonida.

Owing to the consistency of the skeleton Lithistida are frequently found as fossils. The commonest known example is Siphonia.[242] As in the case of so many other fossil sponges the skeleton is often replaced by carbonate of lime, a fact which {216}misled some of the earlier investigators but was established by the researches of Sollas and Zittel.

Sub-Class II. Monaxonida.[243]

The Monaxonida inhabit for the most part shallow water, but they also extend through deep water into the abysses, thirteen species having been dredged from depths of over 2000 fathoms by the "Challenger" Expedition alone. In some cases, e.g. Cladorhiza, Chondrocladia, all the species of a genus may live in deep water, while in others the genus, or in others, again, the species, may have a wide bathymetrical range. Thus Axinella spp. occur in shallow water and in various depths down to 2385 fathoms, Axinella erecta ranges from 90 to 1600 fathoms, Stylocordyla stipitata from 7 to 1600, and so on. The symmetry of the deep-water forms contrasts strikingly with the more irregular shape of their shallow-water allies.[244] The shallow-water species are almost always directly attached, some few are stalked; those from deep water have either a long stalk or some special device to save them from sinking in the soft ooze or mud. Thus the deep-sea genus Trichostemma has the form of a low inverted cone, round the base of which a long marginal fringe of spicules projects, continuing the direction of the somal spicules, and so forming a supporting rim. The same form has been independently evolved in Halicnemia patera, and an approach to it in Xenospongia patelliformis. A similar and more striking case of homoplasy is afforded by the Crinorhiza form, which has been attained in certain species of the deep-sea genera Chondrocladia, Axoniderma, and Cladorhiza; here the sub-globular body is supported by a vertical axis or root, and by a whorl of stout processes radiating outwards and downwards from it, and formed of spicular bundles together with some soft tissue.

There is recognisable in the order Monaxonida a cleft between one set of genera, typically corticate, and suggesting by their structure a relationship, whether of descent or parentage, with the Tetractinellida, and a second set typically non-corticate: these latter are the Halichondrina, the former are the Spintharophora.

{217}

Order I. Halichondrina.

We have already seen typical examples of the Halichondrina in Halichondria panicea and Ephydatia fluviatilis. Within the Halichondrina the development of spongin reaches its maximum among spiculiferous sponges, and accordingly the Ceratosa take their multiple origin here (p. 220). Among Halichondrina spongin co-operates with spicules to form a skeleton in various ways, but always so as to leave some spicules bare or free in the flesh. It may bind the spicules end to end in delicate networks (as in Reniera or Gellius), or into strands, sometimes reaching a considerable thickness (as in Chalina and others). In a few cases there appears to be a kind of division of labour between the spicules and spongin, the latter forming the bulk of the fibre, i.e. fulfilling the functions of support, while the spicules merely beset its surface as defensive organs, rendering the sponge unfit for food. Fibres formed on this pattern are called plumose, and are typical of Axinellidae. The distinctive fibre of the Ectyoninae is as it were a combination of the Axinellid and Chalinine types: a horny fibre both cored with spicules and beset with them. Spicules besetting the surface of a fibre are termed "echinating." Whenever its origin has been investigated, spongin has proved to be the product of secretion of cells; in the great majority of cases it is poured out at the surface of the cell, and Evans showed,[245] at any rate in one species of Spongilla, that the spongin fibres are continuous with a delicate cuticle at the surface of the sponge. In Reniera spp. occurs a curious case of formation of spongin as an intracellular secretion. A number of spherical cells each secrete within themselves a short length of fibre; they then place themselves in rows, so orientated that their contained rods lie end to end in one line. The rods then fuse and make up continuous threads; the cells diminish in breadth, ultimately leaving the fibre free.[246]

Order II. Spintharophora.

These corticate forms are further characterised by the arrangement of their megascleres, which is usually, like that of most {218}Tetractinellida, radial, or approximating to radial. The microscleres are, when present, some form of aster. The cortex resembles that of Tetractinellida, and v. Lendenfeld has described chones in Tethya lyncurium.[247]

The existence of the above points of resemblance between Spintharophora and Tetractinellida suggests a relationship between the two groups as its cause. In judging this possibility the following reflections occur to us. A cortex exists in various independent branches of Tetractinellida. It has in all probability had a different phylogenetic history in each—why not then in these Monaxonida also? Within single genera of Tetractinellida some species are corticate, others not, witness Tetilla. The value of a cortex for purposes of classification may easily be overestimated. If we are to uphold the relationship between these two groups, we must base our argument on the conjunction of similar characters in each.

The genus Proteleia[248] is interesting for its slender grapnel-like spicules, which project beyond the radially disposed cortical spicules, and simulate true anatriaenes of minute proportions. That they are not anatriaenes is shown by the absence of an axial thread in their cladi. It is not surprising that a form of spicule of such obvious utility as the anatriaene should arise more than once.

Of exceptional interest, on account of their boring habit, are the Clionidae. How the process of boring is effected is not known; the presence of an acid in the tissues was suspected, but has been searched for in vain. The pieces of hard substance removed by the activity of the sponge take their exit through the osculum and have a fixed shape[249] (Fig. 108).

As borers into oyster shells, Clionidae may be reckoned as pests of practical importance, and in some coasts they even devastate the rocks, penetrating to a depth of some feet, and causing them to crumble away.[250]

Sponges, however, as agents in altering the face of the earth do not figure as destroyers merely. On the contrary, it has {219}been calculated[251] that sponge skeletons may give rise with considerable rapidity to beds of flint nodules; in fact, it appears that a period so short as fifty years is sufficient for the formation of a bed of flints out of the skeletons of sponges alone.

Suberites domuncula is well known for its constant symbiosis with the Hermit crab. The young sponge settles on a Whelk or other shell inhabited by a Pagurus, and gradually envelops it, becoming very massive, and completely concealing the shell, without however closing its mouth. The aperture of this always remains open to the exterior, however great the growth of the sponge, a tubular passage being left in front of it, which continues the lumen of the shell and maintains its spiral direction. When the crab has grown too big for the shell, it merely advances a little down this passage. The shell is never absorbed, as was once supposed.[252] The crab, besides being provided with a continually growing house, and being thus spared the great dangers attending a shift of lodgings, benefits continually by the concealment and protection afforded by the massive sponge; the latter in return is conveyed to new places by the crab.

fig108

Fig. 108.—A, calcareous corpuscle detached by Cliona; B, view of the galleries excavated by the Sponge. (After Topsent.)

Ficulina ficus is sometimes, like S. domuncula, found in symbiosis with Pagurus, but the constancy of the association is wanting in this case. The sponge has several metamps, one of which, from its fig-like shape, gives it its name.

{220}

Sub-Class III. Ceratosa.

The Ceratosa are an assemblage of ultimate twigs shorn from the branches of the Monaxonid tree. They are therefore related forms, but many of them are more closely connected with their Monaxonid relatives than with their associates in their own sub-class.

The genera Aulena and Phoriospongia, placed by v. Lendenfeld among Ceratosa, by Minchin among Monaxonida, show each in its own way how close is the link between these two sub-classes.

Aulena possesses in its deeper parts a skeleton of areniferous spongin fibres, in fact a typical Ceratose skeleton; but this is continuous with a skeleton in the more superficial parts, which is composed of spongin fibres echinated by spicules proper to the sponge, and precisely comparable to the ectyonine fibres of some Monaxonida.

Phoriospongia, as far as its main skeleton is concerned, is a typical Ceratose sponge, with fibres of the areniferous type, but it possesses sigmata free in the flesh.

The sub-class is confined to shallow water, no horny sponge having been dredged from depths greater than 410 fathoms.[253] The greatest number occur at depths between 10 and 26 fathoms.

In the majority of the Ceratosa the skeletal fibres are homogeneous, formed of concentric lamellae of spongin, deposited by a sheath of spongoblasts around a filiform axis. In others, however, the axis attains a considerable diameter, so as to form a kind of pith to the fibre, which is then distinguished as heterogeneous. In one or two cases some of the spongoblasts of a heterogeneous fibre are included in the fibre between the spongin lamellae. Ianthella is the best-known example in which this occurs.

Ceratosa are divided into Dictyoceratina and Dendroceratina, distinguished, as their names express, by the nature of the skeleton—net-like, with many anastomoses, in the one; tree-like, without anastomoses between its branches, in the other.

The Dictyoceratina comprise by far the larger number of Ceratosa. They fall into two main families, the Spongidae and Spongelidae, both represented in British waters. The Spongidae {221}are characterised by a granular ground substance and aphodal chamber system; the Spongelidae by a clear ground substance and sac-like eurypylous chambers.

The bath sponge, Euspongia officinalis, belongs to the Spongidae. The finest varieties come from the Adriatic, the coarser ones from the Dalmatian and North African coasts of the Mediterranean, from the Grecian Archipelago, from the West Indies, and from Australian seas. The softer species of the genus Hippospongia also form a source of somewhat inferior bath sponges.

Among Dendroceratina, Darwinella is unique and tempts to speculation, in that it possesses isolated spongin elements, resembling in their forms triaxon spicules.

Key to British Genera of Sponges.

1.

(a) Skeleton calcareous 2
(b) Skeleton siliceous 6
(c) Skeleton horny, or without free spicules 53
(d) Skeleton absent 55

2.

(a) Gastral layer continuous 3
(b) Gastral layer discontinuous, confined to chambers 4

3.

(a) Equiangular triradiate systems present Clathrina
(b) Triradiate systems all alate Leucosolenia

4.

(a) Chambers tubular, radially arranged 5
(b) Chambers spherical, irregularly scattered Leucandra

5.

(a) Tufts of oxeate spicules at the ends of the chambers Sycon
(b) Oxeate spicules lying longitudinally in the cortex Ute

6.

(a) All the spicules hexradiate or spicules easily derived from hexradiate type 7
(b) Some of the spicules calthrops or triaenes 10
(c) Megascleres uniaxial 15

7.

(a) Amphidiscs present 8
(b) Amphidiscs absent 9

8.

(a) Rooting spicules a well-defined wisp; four apertures lead into the gastric cavity Hyalonema thomsoni
(b) Rooting tuft diffuse; sponge oval; osculum single Pheronema carpenteri

9.

(a) Sponge tubular, dermal and gastral pinuli absent Euplectella suberea
(b) Sponge a widely open cup; dermal and gastral pinuli present Asconema setubalense

10.

(a) Tetractine spicule, a calthrop or triaene with short rhabdome; microsclere a spined microxea Dercitus bucklandi
(b) Triaenes with fully developed rhabdome 11
{222}
fig109

Fig. 109.—Microscleres of Demospongiae. a, b, Sigmaspires viewed in different directions; c, d, bipocilli viewed in different directions; e, toxaspire; f, f', spiraster; g, sanidaster; h, amphiaster; i, sigma; j, diaucistra; k, isochela; l, m, anisochelae viewed in different directions; n, cladotyle; o, toxa; p, forceps; q, oxyaster; r, spheraster; s, oxyaster with 6 actines; t, another with 4 actines; u, another with rays reduced to two (centrotylote microxea); v, tylote microrhabdus; w, trichodragmata; x, oxeate microrhabdus or microxea.

11.

(a) Microscleres sigmata Craniella cranium
(b) Sigmata absent, asters present 12

12.

(a) Microscleres include spirasters Poecillastra compressa
(b) Microscleres include sterrasters 14
(c) Microscleres include euasters: spirasters and sterrasters absent 13

13.

(a) Two kinds of euaster present Stelletta
(b) Microscleres include a euaster and a sanidaster or amphiaster Stryphnus ponderosus

14.

(a) Microscleres include microrhabdi Pachymatisma johnstonia
(b) Microscleres include many-rayed euasters Cydonium milleri

15.

(a) Some of the microscleres asters 16
(b) Microscleres absent, or not asters 17

16.

(a) Skeleton radiate; asters of more than one kind Tethya
(b) Sponge encrusting; asters of one kind only Hymedesmia
(c) Skeleton fibrous Axinella spp.

17.

(a) Megascleres all diactinal; chelae present Desmacidon
(b) Megascleres all diactinal; chelae absent 18
(c) Some or all of the megascleres monactinal 19
{223}

18.

(a) Habitat fresh water 56
(b) Habitat marine 22

19.

(a) Megascleres include cladotyles Acarnus
(b) Megascleres include dumb-bell or sausage-shaped spicules forming the main reticulum Plocamia
(c) Microscleres include bipocilli 20
(d) Microscleres include diancistra Hamacantha
(e) Megascleres include forceps Forcepia
(f) Skeleton formed of isolated monactines vertically placed Hymeraphia
(g) None of the above peculiarities present 21

20.

(a) Skeleton fibre not echinated Iophon
(b) Skeleton fibre echinated Pocillon

21.

(a) Skeleton with echinating spicules 28
(b) Skeleton without echinating spicules 30

22.

(a) Spongin abundant 23
(b) Spongin scanty 25

23.

(a) Fibre not echinated 24
(b) Fibre echinated Diplodemia

24.

(a) Fibre with a single axial series of spicules Chalina
(b) Fibres with numerous spicules arranged polyserially Pachychalina

25.

(a) Microscleres absent 26
(b) Microscleres sigmata and/or toxa 27

26.

(a) Skeleton confused Halichondria
(b) Skeleton reticulate Reniera

27.

(a) Rind and fistulous appendages present; microscleres sigmata Oceanapia
(b) No rind; skeleton reticulate; microscleres sigmata and/or toxa Gellius

28.

(a) Skeleton confused or formed of bundles of spicules with echinating spined styles 29
(b) Skeleton fibrous or reticulate, or formed of short columns 45
(c) Skeleton formed of a dense central axis, and columns radiating from it to the surface 52

29.

(a) Spicules of the ectosome styles Pytheas
(b) Spicules of the ectosome oxeas or absent Clathrissa
(c) Main skeleton confused. Special ectosomal skeleton absent Spanioplon

30.

(a) Megascleres of the choanosome not differing from those of the ectosome 31
(b) Megascleres of the choanosome differing from those of the ectosome 32

31.

(a) Chelae absent. 33
(b) Chelae present 44

32.

(a) Trichodragmata present Tedania
(b) Trichodragmata absent 42
{224}
fig110

Fig. 110.—Megascleres. a-l and q-s, Modifications of monaxon type. a, Strongyle; b, tylote; c, oxea; d, tylotoxea; e, tylostyle; f, style; g, spined tylostyle; h, sagittal triod (a triaxon form derived from monaxon); j, oxytylote; k, anatriaene; l, protriaene; m, sterraster (polyaxon); n, radial section through the outer part of m, showing two actines soldered together by intervening silica; o, desma of an Anomocladine Lithistid (polyaxon); q, crepidial strongyle, basis of rhabdocrepid Lithistid desma; r, young form of rhabdocrepid desma, showing crepidial strongyle coated with successive layers of silica; s, rhabdocrepid desma.

33.

(a) Skeleton reticulate or fibrous 34
(b) Skeleton radiate or diffuse 37
(c) Skeleton with radiating fibres forming a reticulum with others crossing them at right angles Quasillina

34.

(a) No microscleres 35
(b) Microscleres sigmata and/or toxa with or without trichodragmata Desmacella

35.

(a) Sponge fan- or funnel-shaped 36
(b) Sponge not fan- or funnel-shaped Hymeniacidon

36.

(a) Megascleres slender and twisted Phakellia
(b) Megascleres somewhat stout, not twisted Tragosia

37.

(a) Sigmata present, skeleton diffuse Biemma
(b) Sigmata absent 38

38.

(a) Skeleton more or less radiate 39
(b) Skeleton diffuse; sponge boring Cliona

39.

(a) Sponge discoid with marginal fringe Halicnemia
(b) Sponge massive or stipitate without marginal fringe 40

40.

(a) Sponge body prolonged into mammiform projections Polymastia
(b) Sponge body without mammiform projections 41

41.

(a) No microscleres. Megascleres tylostyles with or without styles Suberites
(b) Microscleres centrotylote. Megascleres styles or tylostyles Ficulina

42.

(a) Choanosomal megascleres smooth 43
(b) Choanosomal megascleres spined Dendoryx

43.

(a) Microscleres chelae and sigmata of about the same size Lissodendoryx
(b) Chelae, if present, smaller than the sigmata Yvesia
{225}

44.

(a) Isochelae Esperiopsis
(b) Anisochelae Esperella

45.

(a) Fibres or columns plumose 46
(b) Fibres or columns ectyonine 47

46.

(a) Microscleres toxa Ophlitaspongia
(b) Microscleres absent Axinella

47.

(a) Skeleton reticulate 48
(b) Skeleton not reticulate 49

48.

(a) Microscleres present. Spicules of the fibre core spined Myxilla
(b) Microscleres absent. Spicules of the fibre core smooth Lissomyxilla

49.

(a) Main skeleton formed of plume-like columns 50
(b) Main skeleton formed of horny fibres (ectyonine). Special dermal skeleton wanting Clathria

50.

(a) Dermal skeleton contains styles only Microciona
(b) Dermal skeleton contains diactine spicules with or without styli 51

51.

(a) Main skeleton columns with a core of smooth oxeas Plumohalichondria
(b) Main skeleton columns with a core of spined styles Stylostichon

52.

(a) Central axis contains much spongin. Echinating spined styli present Raspailia
(b) Central axis with little or no spongin. Spined styles absent. Pillars radiating from the axis support dermal skeleton Ciocalypta

53.

(a) Ground substance between chambers clear; chambers pear-shaped or oval; eurypylous Spongelia
(b) Ground substance granular. Chambers spherical with aphodi 54

54.

(a) Fibres not pithed; sponge fan-shaped Leiosella
(b) Fibres pithed; sponge massive Aplysina

55.

(a) Chambers long, tubular, branched Halisarca
(b) Chambers not much longer than broad; not branched Oscarella

56.

(a) Amphidiscs present Ephydatia
(b) Amphidiscs absent Spongilla
{226}

CHAPTER IX

PORIFERA (CONTINUED): REPRODUCTION, SEXUAL AND ASEXUAL—PHYSIOLOGY—DISTRIBUTION—FLINTS

The reproductive processes of Sponges are of such great importance in leading us to a true conception of the nature of a sponge that we propose to treat them here in a special section. Both sexual and asexual methods are common; the multiplication of oscula we do not regard as an act of reproduction (p. 174).

fig111

Fig. 111.—A, amphiblastula larva of Sycon raphanus; B, later stage, showing invagination of the flagellated cells. c.s, Segmentation cavity; ec, ectoderm; en, endoderm. (After F. E. Schulze, from Balfour.)

A cursory glance at a collection of sponge larvae from different groups would suggest the conclusion that they are divisible into two wholly distinct types. One of these is the amphiblastula, and the other the parenchymula. This was the conclusion accepted by zoologists not long ago. We are indebted to Delage, Maas, and Minchin for dispelling it, and showing that {227}these types are but the extreme terms of a continuous series of forms which have all the same essential constitution and undergo the same metamorphosis.

The amphiblastula of Sycon raphanus (Fig. 111) consists of an anterior half, formed of slender flagellated cells, and a posterior half, of which the cells are large, non-flagellate, and rounded. These two kinds of cell are arranged around a small internal cavity which is largely filled up with amoebocytes. The flagellated cells are invaginated into the dome of rounded cells during metamorphosis, in fact, become the choanocytes or gastral cells; the rounded cells, on the other hand, become the dermal cells—an astonishing fact to any one acquainted only with Metazoan larvae.

A typical parenchymula is that of Clathrina blanca (Fig. 112). When hatched it consists of a wall surrounding a large central cavity and built up of flagellated cells interrupted at the hinder pole by two cells (p.g.c)—the mother-cells of archaeocytes. Before the metamorphosis, certain of the flagellated cells leave the wall and sink into the central cavity, and undergoing certain changes establish an inner mass of future dermal cells. By subsequent metamorphosis the remaining flagellated cells become internal, not this time by invagination, but by the included dermal cells breaking through the wall of the larva, and forming themselves into a layer at the outside.

fig112

Fig. 112.—Median longitudinal section of parenchymula larva of Clathrina blanca. p.g.c, Posterior granular cells—archaeocyte mother-cells. (After Minchin.)

In the larva of C. blanca, after a period of free-swimming existence, the same three elements are thus recognisable as in that of Sycon at the time of hatching; in the newly hatched larva of C. blanca, however, one set of elements, the dermal cells, are not distinguishable. The difference, then, between the two newly hatched larvae is due to the earlier cell differentiation of the Sycon larva.[254]

Now consider the larva of Leucosolenia. It is hatched as a {228}completely flagellated larva; its archaeocytes are internal (as in Sycon); future dermal cells, recognisable as such, are absent. They arise, as in C. blanca, by transformation of flagellated cells; but (1) this process is confined to the posterior pole, and (2) the internal cavity is small and filled up with archaeocytes. Consequently the cells which have lost their flagella and become converted into dermal cells cannot sink in as in C. blanca: they accumulate at the hinder pole, and thus arises a larva half flagellated, half not; in fact, an amphiblastula. Or, briefly, in Leucosolenia the larva at hatching is a parenchymula, and when ready to fix is an amphiblastula; and, again, the difference between the newly hatched larva and that of Sycon is due to the earlier occurrence of cell differentiation in the latter. What completer transitional series could be desired?

Turning to the Micromastictora, the developmental history already sketched is fairly typical (p. 172). The differences between Mega- and Micro-mastictoran larvae are referable mainly to the fact that the dermal cells in the latter become at once differentiated among themselves to form the main types of dermal cell of the adult.[255] The metamorphosis is comparable to that of C. blanca. Among Tetractinellida and Hexactinellida sexually produced larvae have not been certainly identified.

Asexual reproduction takes place according to one of three types, which may be alluded to as (1) "budding," (2) "gemmulation," (3) formation of "asexual larvae."

By budding (Fig. 113) is meant the formation of reproductive bodies, each of which contains differentiated elements of the various classes found in the parent. A simple example of this is described by Miklucho Maclay in Ascons, where the bud is merely the end of one of the Ascon tubes which becomes pinched off and so set free.

In Leucosolenia botryoides[256] Vasseur describes a similar process; in this, however, a strikingly distinctive feature is present (Fig. 114), namely, the buds have an inverse orientation with respect to that of the parent, so that the budding sponge presents a contrast to a sponge in which multiplication of oscula has occurred. In fact, the free distal end of the bud becomes the base of the young sponge, and the osculum is formed at the opposite extremity, where the bud is constricted from the parent.

{229}
fig113

Fig. 113.—Lophocalyx philippensis. The specimen bears several buds attached to it by long tufts of spicules. (After F. E. Schulze.)

fig114

Fig. 114.—Leucosolenia botryoides. A, a piece of the Sponge laden with buds, a-f; i, the spicules of the buds directed away from their free ends; k, the spicules of the parent directed towards the osculum, j. B, a bud which has been set free and has become fixed by the extremity which was free or distal in A. (After Vasseur.)

Such a reversal of the position of the bud is noteworthy in view of its rarity, and the case is worth reinvestigating, for in other animal groups a bud or a regenerated part retains so constantly the same orientation as the parent that Loeb,[257] after experimenting on the {230}regeneration of Coelenterata and other forms, concluded that a kind of "polarity" existed in the tissues of certain animals.

In Oscarella lobularis[258] the buds are transparent floating bladders, derived from little prominences on the surface of the sponge. Scattered in the walls of the bladders are flagellated chambers, which open into the central cavity. The vesicular nature of the buds is doubtless an adaptation, lessening their specific gravity and so enabling them to float to a distance from the parent.

Gemmulation.Spongilla has already afforded us a typical example of this process. Gemmules very similar to those of Spongilla are known in a few marine sponges, especially in Suberites and in Ficulina. They form a layer attached to the surface of support of the sponge—a layer which may be single or double, or even three or four tiers deep. A micropyle is sometimes present in the spongin coat, sometimes absent; possibly its absence may be correlated with the piling of one layer of gemmules on another, as this, by covering up the micropyle, would of course render it useless. Presumably when a micropyle is present the living contents escape through it and leave the sponge by way of the canal system (Fig. 115).

fig115

Fig. 115.—Gemmules of Ficulina. A, vertical section of gemmules in situ; B, vertical section of upper portion of one gemmule. m, Micropyle.

The only case besides Spongilla in which the details of development from gemmules have been traced is that of Tethya.[259] Mere microscopic examination of a Tethya in active reproduction would suggest that the process was simple budding, but Maas has shown that the offspring arise from groups of archaeocytes in the cortex, that is to say, they are typical gemmules. As they develop they migrate outwards along the radial spicule-bundles {231}and are finally freed, like the buds of the Hexactinellid Lophocalyx (Fig. 113).

The comparison of the process of development on the one hand by gemmules, and on the other by larval development, is of some interest.[260] In both cases two cell layers—a dermal and a gastral—are established before the young sponge has reached a functional state. Differences of detail in the formation of the chambers occur in the gemmule; these find parallels in the differences in the same process exhibited by the larvae of various groups of sponges. On the other hand, the order of tissue differentiation is not the same in the gemmule as in the larva.

fig116

Fig. 116.—Development of the triradiate and quadriradiate spicules of Clathrina. (1) Three scleroblasts; (2) each has divided: the spicule is seen in their midst; (3) addition of the fourth ray by a porocyte. p, Dermal aperture of pore; r, fourth ray. (After Minchin.)

Of the reproduction of Tetractinellida extremely little is known. Spermatozoa occur in the tissues in profusion and are doubtless functional, but larvae have been seldom observed.

fig117

Fig. 117.—Three stages in the development of the triradiate spicules of Sycon setosum. × 1200. (After Maas.)

In Hexactinellida the place of sexually produced larvae is taken by bodies of similar origin to gemmules but with the appearance of parenchymulae. Ijima has indeed seen a few egg-cells in Hexactinellids.[261] He finds, however, that archaeocyte congeries occur in abundance, and there is good reason to believe with him that these are responsible for the numerous parenchymula-like asexually produced larvae he has observed. The discovery of "asexual larvae" was first made by Wilson in the Monaxonid Esperella; in this case the asexual larva is, as far as can be detected, identical with that developed from the fertilised egg. A similar phenomenon, the production {232}of apparently identical larvae by both sexual and asexual methods, has been observed in the Coelenterate Gonionema murbachii.[262]

Artificially, sponges may be reproduced with great advantage to commerce by means of cuttings. Cuttings of the bath sponge are fit to gather after a seven years' growth.

The development of the various forms of spicules is a subject about which little is yet known. Most spicules of which the development has been traced originate in a single dermal cell. The triradiate and quadriradiate spicules of Homocoela (Clathrinidae), as Minchin[263] has most beautifully shown, form an exception. Three cells co-operate to form the triradiate; these three divide to give six before the growth of the spicule is complete. A quadriradiate is formed from a triradiate spicule by addition of the fourth ray, which, again, has a separate origin in an independent cell, in fact a porocyte. The triradiate spicules of the Sycettidae, on the other hand, originate in a single cell,[264] but the quadriradiate spicules are formed from these by the addition of a fourth ray in a manner similar to that which has just been described for Clathrinidae.

fig118

Fig. 118.—Development of monaxon spicules. A, from Spongilla lacustris, showing the single scleroblast. (After Evans.) B, a very large monaxon, from Leucosolenia, on which many scleroblasts are at work. (After Maas.)

Monaxon spicules if not of large size undergo their entire development within a single scleroblast (Fig. 118, A). In some cases if their dimensions exceed certain limits, several cells take part in their completion; some of these are derived from the {233}division of the original scleroblast, others are drawn from the surrounding tissue. In Tethya, for example, and in Leucosolenia[265] the scleroblasts round the large monaxon spicules are so numerous as to have an almost epithelioid arrangement.

The large oxeas of Tetilla, Stelletta, and Geodia, however, are formed each within a single scleroblast.[266]

fig119

Fig. 119.—Development of spheraster. A, of Tethya, from union of two quadriradiate spicules. (After Maas.) B (a-e), of Chondrilla, from a spherical globule. (After Keller.)

Triaenes have been shown[267] to originate as monaxons with one swollen termination, from which later the cladi grow out. Information as to the scleroblasts in this case is needed.

The value of a knowledge of the ontogeny of microscleres might be great. Maas believes that he has shown that the spherasters of Tethya are formed by the union of minute tetractine calthrops (Fig. 119, A). If this view should be confirmed, it would afford a very strong argument for the Tetractinellid affinities of Tethya.

fig120

Fig. 120.—Stages in the development of the microscleres of Placospongia. (After Keller.)

Keller,[268] on the other hand, finds that the spherasters of the Tetractinellid Chondrilla {234}originate as spheres (Fig. 119, B); and spheres have been observed in the gemmule of a Tethya; no spherasters were as yet present in the gemmule, and spheres were absent in the adult.[269]

In the genus Placospongia certain spicules are present which outwardly closely resemble the sterrasters so characteristic of certain Tetractinellidae. Their development, however, as will be seen from Fig. 120, shows that they are not polyaxon but spiny monaxon spicules. Placospongia is consequently transferred to the Monaxonida Spintharaphora.

Sterrasters originate within an oval cell as a number of hairlike fibres[270] (trichites), which are united at their inner ends. The outer ends become thickened and further modified. The position occupied by the nucleus of the scleroblast is marked in the adult spicule by a hilum.

fig121

Fig. 121.—Three stages in the development of an anisochela. al, Ala; al', lower ala; f, falx; f', lower falx; r, rostrum; r', lower rostrum. (After Vosmaer and Pekelharing.)

The anisochela has been shown repeatedly to originate from a C-shaped spicule.[271]

What little is known of the development of Hexactinellid spicules we owe to Ijima.[272] Numerous cells are concerned in certain later developmental stages of the hexaster; a hexaster passes through a hexactin stage, and—a fact "possibly of importance for the phylogeny of spicules in Hexactinellida"—in two species the first formed spicules are a kind of hexactin, known as a "stauractin," and possessing only four rays all in one plane (cf. Protospongia, p. 207).

Physiology

Production of the Current.—It is not at first sight obvious that the lashing of flagella in chambers arranged as above {235}described, between an inhalant and an exhalant system of canals, will necessarily produce a current passing inwards at the ostia and outwards at the osculum. And the difficulty seems to be increased when it is found[273] that the flagella in any one chamber do not vibrate in concert, but that each keeps its own time. This, however, is of less consequence than might seem to be the case. Two conditions are essential to produce the observed results: (1) in order that the water should escape at the mouth of the chamber there must be a pressure within the chamber higher than that in the exhalant passages; (2) in order that water may enter the chamber there must be within it a pressure less than that in the inhalant passages. But the pressure in the inhalant and exhalant passages is presumably the same, at any rate before the current is started, therefore there must be a difference of pressure within the chamber itself, and the less pressure must be round the periphery. Such a distribution of pressures would be set up if each flagellum caused a flow of water directed away from its own cell and towards the centre of the chamber; and this would be true whether the flagellum beats synchronously with its fellows or not.

The comparative study of the canal systems of sponges[274] acquires a greater interest in proportion as the hope of correlating modifications with increase of efficiency seems to be realised. In a few main issues this hope may be said to have been realised. The points, so to speak, of a good canal system are (1) high oscular velocity, which ensures rapid removal of waste products to a wholesome distance; (2) a slow current without eddies in the flagellated chambers, to allow of the choanocytes picking up food particles (see below), and moreover to prevent injury to the delicate collars of those cells; (3) a small area of choanocytes, and consequent small expenditure of energy in current production.

It is then at once clear at what a disadvantage the Ascons are placed as compared with other sponges having canal systems of the second or third types. Their chamber and oscular currents can differ but slightly, the difference being obtained merely by narrowing the lumen of the distal extremity of the body to form the oscular rim. Further, the choanocytes are {236}acting on a volume of water which they can only imperfectly control, and it is no doubt due to the necessity of limiting the volume of water which the choanocytes have to set in motion that the members of the Ascon family are so restricted in size. The oscular rim is only a special case of a device adopted by sponges at the very outset of their career, and retained and perfected when they have reached their greatest heights; the volume of water passing per second over every cross-section of the path of the current is of course the same, therefore by narrowing the cross-sectional area of the path at any point, the velocity of the current is proportionally increased at that point. The lining of the oscular rim is of pinacocytes; they determine a smooth surface, offering little frictional resistance to the current, while choanocytes in the same position would have been a hindrance, not only by setting up friction, but by causing irregularities in the motion.

Canal systems of the second type show a double advance upon that of the Ascons, namely, subdivision of the gastral cavity and much greater length of the smooth walled exhalant passage. The choanocytes have now a task more equal to their strength, and, further, there is now a very great inequality between the total sectional areas of the flagellated chambers and that of the oscular tube.

Canal systems of the third type with tubular chambers are an improvement on those of the second, in that the area of choanocytes is increased by the pouching of the chamber-layer without corresponding increase in the size of the sponge. However, the area of choanocytes represents expenditure of energy, and the next problem to be solved is how to retain the improved current and at the same time to cut down expense. The first step is to change the form of the chamber from tubular to spherical. Now the energy of all the choanocytes is concentrated on the same small volume of water. The area of choanocytes is less, but the end result is as good as before. At the wide mouth of the spherical chamber there is nevertheless still a cause of loss of energy in the form of eddies, and it is as an obviation of these that one must regard the aphodi and prosodi with which higher members of the Demospongiae are provided. The correctness of this view receives support, apart from mechanical principles, from the fact that the mass of the body of any one of these sponges is greater relatively to the total flagellated area than in those sponges with eurypylous chambers; that is to say, a few {237}aphodal and diplodal chambers are as efficient as many of the eurypylous type.

It is manifest that the current is the bearer of the supply of food; but it requires more care to discover (1) what is the nature of the food; (2) by which of the cells bathed by the current the food is captured and by which digested. The answer to the latter question has long been sought by experimenters,[275] who supplied the living sponge with finely powdered coloured matters, such as carmine, indigo, charcoal, suspended in water. The results received conflicting interpretations until it became recognised that it was essential to take into account the length of time during which the sponge had been fed before its tissues were subjected to microscopic examination. Vosmaer and Pekelharing obtained the following facts: Spongilla lacustris and Sycon ciliatum, when killed after feeding for from half an hour to two hours with milk or carmine, contain these substances in abundance in the bodies of the choanocytes and to a slight degree in the deeper cells of the dermal tissue; after feeding for twenty-four hours the proportions are reversed, and if a period of existence in water uncharged with carmine intervenes between the long feed and death then the chambers are completely free from carmine. These are perhaps the most conclusive experiments yet described, and they show that the choanocytes ingest solid particles and that the amoeboid cells of the dermal layer receive the ingested matter from them. In all probability it is fair to argue from these facts that solid particles of matter suitable to form food for the sponge are similarly dealt with by it and undergo digestion in the dermal cells.

Choanocytes are the feeding organs par excellence; but the pinacocytes perform a small share of the function of ingestion, and in the higher sponges where the dermal tissue has acquired a great bulk the share is perhaps increased.

In the above experiments is implied the tacit assumption that sponges take their food in the form of finely divided solids. Haeckel[276] states his opinion that they feed on solid particles derived from decaying organisms, but that possibly decaying substances in solution may eke out their diet. Loisel, in 1898,[277] {238}made a new departure in the field of experiment by feeding sponges with coloured solutions, and obtained valuable results. Thus solutions, if presented to the sponge in a state of extreme dilution, are subjected to choice, some being absorbed, some rejected. When absorbed they are accumulated in vacuoles within both dermal and gastral cells, mixed solutions are separated into their constituents and collected into separate vacuoles. In the vacuoles the solutions may undergo change; Congo red becomes violet, the colour which it assumes when treated with acid, and similarly blue litmus turns red. The contents of the vacuoles, sometimes modified, sometimes not, are poured out into the intercellular gelatinous matrix of the dermal layer, whence they are removed partly by amoeboid cells, partly, so Loisel thinks, by the action of the matrix itself. It adds to the value of these observations to learn that Loisel kept a Spongilla supplied with filtered spring-water, to which was added the filtered juice obtained from another crushed sponge. This Spongilla lived and budded, and was in good health at the end of ten days.

Movement.—Sponges are capable of locomotion only in the young stage; in the adult the only signs of movement are the exhalant current, and in some cases movements of contraction sufficiently marked to be visible to the naked eye. Meresjkowsky was one of the early observers of these movements. He mentions that he stimulated a certain corticate Monaxonid sponge by means of a needle point: a definite response to each prick inside the oscular rim was given by the speedy contraction of the osculum.[278]

Pigments and Spicules.—Various reasons lead one to conclude that the spicules have some function other than that of support and defence, probably connected with metabolism. For the spicules are cast off, sometimes in large numbers, to be replaced rapidly by new ones, a process for which it is difficult to find an adequate explanation if the spicules are regarded as merely skeletal and defensive.[279] Potts remarks upon the striking profusion with which spicules are secreted by developing Spongillids from water in which the percentage of silica present must have been exceedingly small. The young sponges climbed {239}up the strands of spicules as they formed them, leaving the lower parts behind and adding to the upper ends.

Of the physiology of the pigments of sponges not much is yet known: a useful summary of facts will be found in Von Fürth's text-book.[280]

Spongin.—Von Fürth[280] points out that this term is really a collective one, seeing that the identity of the organic skeletal substance of all sponge species is hardly to be assumed. Spongin is remarkable for containing iodine. The amount of iodine present in different sponges varies widely, reaching in certain tropical species of the Aplysinidae and Spongidae the high figure 8 to 14 per cent. Seaweeds which are specially rich in iodine contain only 1.5 to 1.6 per cent.

In view of the fact that iodine is a specific for croup, it is of interest to observe that the old herb doctors for many centuries recognised the bath sponge as a cure for that disease.

fig122

Fig. 122.—The ordinals measure (i.) the number of species, a-f, and (ii.) the number of stations, a'-f', at which successful hauls were made. The abscissae measure the depth: thus at I. the depth is from 0 to 50 fathoms; at II. from 51 to 200; at III. from 201 to 1000; at IV. from 1001 upwards. a, a', are the curves for Sponges generally; b, b', for Monaxonida; c, c', for Hexactinellida; d, d', for Tetractinellida; e, e', for Calcarea; f, f', for Ceratosa.

Distribution in Space.—All the larger groups of Sponges are cosmopolitan. Each group has, however, its characteristic bathymetrical range: the facts are best displayed by means of curves, as in Fig. 122, which is based wholly on the results obtained by the "Challenger" Expedition. The information as to littoral species is consequently inadequate, and we have not the data requisite for their discussion.

Sponges generally (a) and Monaxonida in particular (b) are more generally distributed in water of depths of 51 to 200 fathoms than in depths of less than 50 fathoms; but localities in shallow water are {240}richer, for the station curve (a') rises abruptly from I. to II., while the species curve (a) in the same region is almost horizontal.

The Hexactinellid curve (c) culminates on III., showing that the group is characteristically deep water. That for Tetractinellida (d) reaches its greatest height on II., i.e. between 51 and 200 fathoms. Even here, in their characteristic depths, the Tetractinellida fall below the Hexactinellida, and far below the Monaxonida in numbers. Again, the Monaxonida are commoner than Hexactinellida in deep water of 201 to 1000 fathoms, and it is not till depths of 1000 fathoms are passed that Hexactinellida prevail, finally preponderating over the Monaxonida in the ratio of 2:1.

The Calcarea and Ceratosa are strictly shallow-water forms. It is a fact well worth consideration that the stations at which sponges have been found are situated, quite irrespective of depth, more or less in the neighbourhood of land. In the case of Calcarea and Ceratosa this is to be expected, seeing that shallow water is commonest near land, but it is surprising that it should be true also of the Hexactinellida and of the deep-water species of Tetractinellida and of Monaxonida.

While the family groups are cosmopolitan, this is not true of genera and species. The distribution of genera and species makes it possible to define certain geographical provinces for sponges as for other animals. That this is so, is due to the existence of ocean tracts bare of islands; for ocean currents, can act as distributing agents with success only if they flow along a coast or across an ocean studded with islands. It is, of course, the larval forms which will be transported; whether they will ever develop to the adult condition depends on whether the current carrying them passes over a bottom suitable to their species before metamorphosis occurs and the young sponge sinks. If such a bottom is passed over, and if the depth is one which can be supported by the particular species in question, then a new station may thus be established for that species.

The distance over which a larva may be carried depends on the speed of the current by which it is borne, and on the length of time occupied by its metamorphosis. Certain of the ocean currents accomplish 500 miles in six days; this gives some idea of the distance which may intervene between the birthplace and {241}the final station of a sponge; for six days is not an excessive interval to allow for the larval period of at any rate some species.

Distribution in Time.—All that space permits us to say on the palaeontology of sponges has been said under the headings of the respective classes. We can here merely refer to the chronological table shown in Fig. 123:[281]

fig123

Fig. 123.—Table to indicate distribution of Sponges in time.

Flints.—The ultimate source of all the silica in the sea and fresh-water areas is to be found in the decomposition of igneous rocks such as granite. The quantity of silica present in solution in sea water is exceedingly small, amounting to about one-and-a-half parts in 100,000; it certainly is not much more in average fresh water. This is no doubt due to its extraction by diatoms, which begin to extract it almost as soon as it is set free from the parent rock. It is from this small quantity that the siliceous sponges derive the supply from which they form their spicules. Hence it would appear that for the formation of one {242}ounce of spicules at least one ton of sea water must pass through the body of the sponge. Obviously from such a weak solution the deposition of silica will not occur by ordinary physical agencies; it requires the unexplained action of living organisms. This may account for the fact that deposits of flint and chert are always associated with organic remains, such as Sponges and Radiolaria. By some process, the details of which are not yet understood, the silica of the skeleton passes into solution. In Calcareous deposits, a replacement of the carbonate of lime by the silica takes place, so that in the case of chalk the shells of Foraminifera, such as Globigerina and Textularia and those of Coccoliths, are converted into a siliceous chalk. Thus a siliceous chalk is the first stage in the formation of a flint.

A further deposition of silica then follows, cementing this pulverulent material into a hard white porous flint. It is white for the same reason that snow is white. The deposition of silica continues, and the flint becomes at first grey and at last apparently black (black as ice is black on a pond). Frequently flints are found in all stages of formation: siliceous chalk with the corroded remains of sponge spicules may be found in the interior, black flint blotched with grey forming the mass of the nodule, while the exterior is completed by a thin layer of white porous flint. This layer must not be confused with the white layer which is frequently met with on the surface of weathered flints, which is due to a subsequent solution of some of the silica, so that by a process of unbuilding, the flint is brought back to the incompleted flint in its second stage. In the chalk adjacent to the flints, hollow casts of large sponge spicules may sometimes be observed, proving the fact, which is however unexplained, of the solution of the spicular silica. The formation of the flints appears to have taken place, to some extent at least, long after the death of the sponge, and even subsequent to the elevation of the chalk far above the sea-level, as is shown by the occurrence of layers of flints in the joints of the solid chalk.[282]

 

COELENTERATA AND CTENOPHORA

BY

S. J. HICKSON, M.A., F.R.S.

Formerly Fellow and now Honorary Fellow of Downing College, Beyer Professor of Zoology in the Victoria University of Manchester.

{245}

CHAPTER X

COELENTERATA

INTRODUCTION—CLASSIFICATION—HYDROZOA—ELEUTHEROBLASTEA—MILLEPORINA—GYMNOBLASTEA—CALYPTOBLASTEA—GRAPTOLITOIDEA—STYLASTERINA

The great division of the animal kingdom called Coelenterata was constituted in 1847 by E. Leuckart for those animals which are commonly known as polyps and jelly-fishes. Cuvier had previously included these forms in his division Radiata or Zoophyta, when they were associated with the Starfishes, Brittle-stars, and the other Echinodermata.

The splitting up of the Cuvierian division was rendered necessary by the progress of anatomical discovery, for whereas the Echinodermata possess an alimentary canal distinct from the other cavities of the body, in the polyps and jelly-fishes there is only one cavity to serve the purposes of digestion and the circulation of fluids. The name Coelenterata (κοῖλος = hollow, ἔντερον = the alimentary canal) was therefore introduced, and it may be taken to signify the important anatomical feature that the body-cavity (or coelom) and the cavity of the alimentary canal (or enteron) of these animals are not separate and distinct as they are in Echinoderms and most other animals.

Many Coelenterata have a pronounced radial symmetry, the body being star-like, with the organs arranged symmetrically on lines radiating from a common centre. In this respect they have a superficial resemblance to many of the Echinodermata, which are also radially symmetrical in the adult stage. But it cannot be insisted upon too strongly that this superficial resemblance of the Coelenterata and Echinodermata has no genetic significance. {246}The radial symmetry has been acquired in the two divisions along different lines of descent, and has no further significance than the adaptation of different animals to somewhat similar conditions of life. It is not only in the animals formerly classed by Cuvier as Radiata, but in sedentary worms, Polyzoa, Brachiopoda, and even Cephalopoda among the Mollusca, that we find a radial arrangement of some of the organs. It is interesting in this connexion to note that the word "polyp," so frequently applied to the individual Coelenterate animal or zooid, was originally introduced on a fancied resemblance of a Hydra to a small Cuttle-fish (Fr. Poulpe, Lat. Polypus).

The body of the Coelenterate, then, consists of a body-wall enclosing a single cavity ("coelenteron"). The body-wall consists of an inner and an outer layer of cells, originally called by Allman the "endoderm" and "ectoderm" respectively. Between the two layers there is a substance chemically allied to mucin and usually of a jelly-like consistency, for which the convenient term "mesogloea," introduced by G. C. Bourne, is used (Fig. 125).

The mesogloea may be very thin and inconspicuous, as it is in Hydra and many other sedentary forms, or it may become very thick, as in the jelly-fishes and some of the sedentary Alcyonaria. When it is very thick it is penetrated by wandering isolated cells from the ectoderm or endoderm, by strings of cells or by cell-lined canals; but even when it is cellular it must not be confounded with the third germinal layer or mesoblast which characterises the higher groups of animals, from which it differs essentially in origin and other characters. The Coelenterata are two-layered animals (Diploblastica), in contrast to the Metazoa with three layers of cells (Triploblastica). The growth of the mesogloea in many Coelenterata leads to modifications of the shape of the coelenteric cavity in various directions. In the Anthozoa, for example, the growth of vertical bands of mesogloea covered by endoderm divides the peripheral parts of the cavity into a series of intermesenterial compartments in open communication with the axial part of the cavity; and in the jelly-fishes the growth of the mesogloea reduces the cavity of the outer regions of the disc to a series of vessel-like canals.

Another character, of great importance, possessed by all Coelenterata is the "nematocyst" or "thread-cell" (Fig. 124). {247}This is an organ produced within the body of a cell called the "cnidoblast," and it consists of a vesicular wall or capsule, surrounding a cavity filled with fluid containing a long and usually spirally coiled thread continuous with the wall of the vesicle. When the nematocyst is fully developed and receives a stimulus of a certain character, the thread is shot out with great velocity and causes a sting on any part of an animal that is sufficiently delicate to be wounded by it.

The morphology and physiology of the nematocysts are subjects of very great difficulty and complication, and cannot be discussed in these pages. It may, however, be said that by some authorities the cnidoblast is supposed to be an extremely modified form of mucous or gland cell, and that the discharge of the nematocyst is subject to the control of a primitive nervous system that is continuous through the body of the zooid.

There is a considerable range of structure in the nematocysts of the Coelenterata. In Alcyonium and in many other Alcyonaria they are very small (in Alcyonium the nematocyst is 0.0075 mm. in length previous to discharge), and when discharged exhibit a simple oval capsule with a plain thread attached to it. In Hydra (Fig. 124) there are at least two kinds of nematocysts, and in the larger kind (0.02 mm. in length previous to discharge) the base of the thread is beset with a series of recurved hooks, which during the act of discharge probably assist in making a wound in the organism attacked for the injection of the irritant fluid, and possibly hold the structure in position while the thread is being discharged. In the large kind of nematocyst of Millepora and of Cerianthus there is a band of spirally arranged but very minute thorns in the middle of the thread, but none at the base. In some of the Siphonophora the undischarged nematocysts reach their maximum size, nearly 0.05 mm. in length.

fig124

Fig. 124.—Nematocyst (Nem) of Hydra grisea, enclosed within the cnidoblast. CNC, Cnidocil; f, thread of nematocyst; Mf, myophan threads in cnidoblast; N, nucleus of cnidoblast. (After Schneider.)

When a nematocyst has once been discharged it is usually {248}rejected from the body, and its place in the tissue is taken by a new nematocyst formed by a new cnidoblast; but in the thread of the large kind of nematocyst of Millepora there is a very delicate band, which appears to be similar to the myophan thread in the stalk of a Vorticella. Dr. Willey[283] has made the important observation that in this coral the nematocyst threads can be withdrawn after discharge, the retraction being effected with great rapidity. The "cnidoblast" is a specially modified cell. It sometimes bears at its free extremity a delicate process, the "cnidocil," which is supposed to be adapted to the reception of the special stimuli that determine the discharge of the nematocyst. In many species delicate contractile fibres (Fig. 124, Mf) can be seen in the substance of the cnidoblast, and in others its basal part is drawn out into a long and probably contractile stalk ("cnidopod"), attached to the mesogloea below.

There can be little doubt that new nematocysts are constantly formed during life to replace those that have been discharged and lost. Each nematocyst is developed within the cell-substance of a cnidoblast which is derived from the undifferentiated interstitial cell-groups. During this process the cnidoblast does not necessarily remain stationary, but may wander some considerable distance from its place of origin.[284] This habit of migration of the cnidoblast renders it difficult to determine whether the ectoderm alone, or both ectoderm and endoderm, can give rise to nematocysts. In the majority of Coelenterates the nematocysts are confined to the ectoderm, but in many Anthozoa, Scyphozoa, and Siphonophora they are found in tissues that are certainly or probably endodermic in origin. It has not been definitely proved in any case that the cnidoblast cells that form these nematocysts have originally been formed in the endoderm, and it is possible that they are always derived from ectoderm cells which migrate into the endoderm.

It is probably true that all Coelenterata have nematocysts, and that, in the few cases in which it has been stated that they are absent (e.g. Sarcophytum), they have been overlooked. It cannot, however, be definitely stated that similar structures do not occur in other animals. The nematocysts of the Mollusc Aeolis are not the product of its own tissues, but are introduced {249}into the body with its food.[285] The nematocysts that occur in the Infusorian Epistylis umbellaria and in the Dinoflagellate Polykrikos (p. 131) require reinvestigation, but if it should prove that they are the product of the Protozoa they cannot be regarded as strictly homologous with those of Coelenterata. In many of the Turbellaria, however, and in some of the Nemertine worms, nematocysts occur in the epidermis which appear to be undoubtedly the products of these animals.

 

The Coelenterata are divided into three classes:—

1. Hydrozoa.—Without stomodaeum and mesenteries. Sexual cells discharged directly to the exterior.

2. Scyphozoa.—Without stomodaeum and mesenteries. Sexual cells discharged into the coelenteric cavity.

3. Anthozoa = Actinozoa.—With stomodaeum and mesenteries. Sexual cells discharged into the coelenteric cavity.

The full meaning of the brief statements concerning the structure of the three classes given above cannot be explained until the general anatomy of the classes has been described. It may be stated, however, in this place that many authors believe that structures corresponding with the stomodaeum and mesenteries of Anthozoa do occur in the Scyphozoa, which they therefore include in the class Anthozoa.

Among the more familiar animals included in the class Hydrozoa may be mentioned the fresh-water polyp Hydra, the Hydroid zoophytes, many of the smaller Medusae or jelly-fish, the Portuguese Man-of-war (Physalia), and a few of the corals.

Included in the Scyphozoa are the large jelly-fish found floating on the sea or cast up on the beach on the British shores.

The Anthozoa include the Sea-anemones, nearly all the Stony Corals, the Sea-fans, the Black Corals, the Dead-men's fingers (Alcyonium), the Sea-pens, and the Precious Coral of commerce.

CLASS I. HYDROZOA

In this Class of Coelenterata two types of body-form may be found. In such a genus as Obelia there is a fixed branching colony of zooids, and each zooid consists of a simple tubular body-wall composed of the two layers of cells, the ectoderm and the {250}endoderm (Fig. 125), terminating distally in a conical mound—the "hypostome"—which is perforated by the mouth and surrounded by a crown of tentacles. This fixed colony, the "hydrosome," feeds and increases in size by gemmation, but does not produce sexual cells. The hydrosome produces at a certain season of the year a number of buds, which develop into small bell-like jelly-fish called the "Medusae," which swim away from the parent stock and produce the sexual cells. The Medusa (Fig. 126) consists of a delicate dome-shaped contractile bell, perforated by radial canals and fringed with tentacles; and from its centre there depends, like the clapper of a bell, a tubular process, the manubrium, which bears the mouth at its extremity. This free-swimming sexual stage in the life-history of Obelia is called the "medusome."

It is difficult to determine whether, in the evolution of the Hydrozoa, the hydrosome preceded the medusome or vice versâ. By some authors the medusome is regarded as a specially modified sexual individual of the hydrosome colony. By others the medusome is regarded as the typical adult Hydrozoon form, and the zooids of the hydrosome as nutritive individuals arrested in their development to give support to it. Whatever may be the right interpretation of the facts, however, it is found that in some forms the medusome stage is more or less degenerate and the hydrosome is predominant, whereas in others the hydrosome is degenerate or inconspicuous and the medusome is predominant. Finally, in some cases there are no traces, even in development, of a medusome stage, and the life-history is completed in the hydrosome, while in others the hydrosome stages are lost and the life-history is completed in the medusome.

If a conspicuous hydrosome stage is represented by H, a conspicuous medusome stage by M, an inconspicuous or degenerate hydrosome stage by h, an inconspicuous or degenerate medusome stage by m, and the fertilised ovum by O, the life-histories of the Hydrozoa may be represented by the following formulæ:—

1. O   —   H   —   O (Hydra)
2. O — H — m — O (Sertularia)
3. O — H — M — O (Obelia)
4. O — h — M — O (Liriope)
5. O   —   M   —   O (Geryonia)

The structure of the hydrosome is usually very simple. It {251}consists of a branched tube opening by mouths at the ends of the branches and closed at the base. The body-wall is built up of ectoderm and endoderm. Between these layers there is a thin non-cellular lamella, the mesogloea.

In a great many Hydrozoa the ectoderm secretes a chitinous protective tube called the "perisarc." The mouth is usually a small round aperture situated on the summit of the hypostome, and at the base of the hypostome there may be one or two crowns of tentacles or an area bearing irregularly scattered tentacles. The tentacles may be hollow, containing a cavity continuous with the coelenteric cavity of the body; or solid, the endoderm cells arranged in a single row forming an axial support for the ectoderm. The ectoderm of the tentacles is provided with numerous nematocysts, usually arranged in groups or clusters on the distal two-thirds of their length, but sometimes confined to a cap-like swelling at the extremity (capitate tentacles). The hydrosome may be a single zooid producing others asexually by gemmation (or more rarely by fission), which become free from the parent, or it may be a colony of zooids in organic connexion with one another formed by the continuous gemmation of the original zooid derived from the fertilised ovum and its asexually produced offspring. When the hydrosome is a colony of zooids, specialisation of certain individuals for particular functions may occur, and the colony becomes dimorphic or polymorphic.

fig125

Fig. 125.—Diagram of a vertical section through a hydrosome. Coel, Coelenteron; Ect, ectoderm; End, endoderm. Between the ectoderm and the endoderm there is a thin mesogloea not represented in the diagram. M, mouth; T, tentacle.

The medusome is more complicated in structure than the hydrosome, as it is adapted to the more varied conditions of a free-swimming existence. The body is expanded to form a disc, "umbrella," or bell, which bears at the edge or margin a number of tentacles. The mouth is situated on the end of a hypostome, called the "manubrium," situated in the centre of the radially symmetrical body. The surface that bears the manubrium is {252}called oral, and the opposite surface is called aboral. The cavity partly enclosed by the oral aspect of the body when it is cup- or bell-shaped is called the "sub-umbrellar cavity."

In the medusome of nearly all Hydrozoa there is a narrow shelf projecting inwards from the margin of the disc and guarding the opening of the sub-umbrellar cavity, called the "velum."

The mouth leads through the manubrium into a flattened part of the coelenteric cavity, which is usually called the gastric cavity, and from this a number of canals pass radially through the mesogloea to join a circular canal or ring-canal at the margin of the umbrella.

A special and important feature of the medusome is the presence of sense-organs called the "ocelli" and "statocysts," situated at the margin of the umbrella or at the base of the tentacles.

fig126

Fig. 126.—Diagram of a vertical section through a medusome. coel, Coelenteron; M, mouth; Man, manubrium; R, radial canal; r, ring or circular canal; T, tentacle; v, velum.

The ocelli may usually be recognised as opaque red or blue spots on the bases of the tentacles, in marked contrast to their transparent surroundings. The ocellus may consist simply of a cluster of pigmented cells, or may be further differentiated as a cup of pigmented cells filled with a spherical thickening of the cuticle to form a lens. The exact function of the ocelli may not be fully understood, but there can be little doubt that they are light-perceiving organs.

The function of the sense-organs known as statocysts, however, has not yet been so satisfactorily determined. They were formerly thought to be auditory organs, and were called "otocysts," but it appears now that it is impossible on physical grounds for these organs to be used for the perception of the waves of sound in water. It is more probable that they are organs of the static function, that is, the function of the perception of the position of the body in space, and they are consequently called statocysts. In the Leptomedusae each statocyst consists of a small vesicle in the mesogloea at the margin of the umbrella, containing a hard, stony body called the "statolith." In Geryonia and some other Trachomedusae the statolith is carried by a short tentacular process, the "statorhab," {253}projecting into the vesicle; in other Trachomedusae, however, the vesicle is open, but forms a hood for the protection of the statorhab; and in others, but especially in the younger stages of development, the statorhab is not sunk into the margin of the umbrella, and resembles a short but loaded tentacle. Recent researches have shown that there is a complete series of connecting links between the vesiculate statocyst of the Leptomedusae and the free tentaculate statorhab of the Trachomedusae, and there can be little doubt of their general homology.

In the free-swimming or "Phanerocodonic" medusome the sexual cells are borne by the ectoderm of the sub-umbrellar cavity either on the walls of the manubrium or subjacent to the course of the radial canals.

Order I. Eleutheroblastea.

This order is constituted mainly for the well-known genus Hydra. By some authors Hydra is regarded as an aberrant member of the order Gymnoblastea, to which it is undoubtedly in many respects allied, but it presents so many features of special interest that it is better to keep it in a distinct group.

Hydra is one of the few examples of exclusively fresh-water Coelenterates, and like so many of the smaller fresh-water animals its distribution is almost cosmopolitan. It occurs not only in Europe and North America, but in New Zealand, Australia, tropical central Africa, and tropical central America.

Hydra is found in this country in clear, still fresh water attached to the stalks or leaves of weeds. When fully expanded it may be 25 mm. in length, but when completely retracted the same individual may be not more than 3 mm. long. The tubular body-wall is built up of ectoderm and endoderm, enclosing a simple undivided coelenteric cavity. The mouth is situated on the summit of the conical hypostome, and at the base of this there is a crown of long, delicate, but hollow tentacles. The number of tentacles is usually six in H. vulgaris and H. oligactis,[286] and eight in H. viridis, but it is variable in all species.

During the greater part of the summer the number of individuals is rapidly increased by gemmation. The young Hydras produced by gemmation are usually detached from their parents {254}before they themselves produce buds, but in H. oligactis the buds often remain attached to the parent after they themselves have formed buds, and thus a small colony is produced. Sexual reproduction usually commences in this country in the summer and autumn, but as the statements of trustworthy authors are conflicting, it is probable that the time of appearance of the sexual organs varies according to the conditions of the environment.

Individual specimens may be male, female, or hermaphrodite. Nussbaum[287] has published the interesting observation that when the Hydras have been well fed the majority become female, when the food supply has been greatly restricted the majority become male, and when the food-supply is moderate in amount the majority become hermaphrodite. The gonads are simply clusters of sexual cells situated in the ectoderm. There is no evidence, derived from either their structure or their development, to show that they represent reduced medusiform gonophores. The testis produces a number of minute spermatozoa. In the ovary, however, only one large yolk-laden egg-cell reaches maturity by the absorption of the other eggs. The ovum is fertilised while still within the gonad, and undergoes the early stages of its development in that position. With the differentiation of an outer layer of cells a chitinous protecting membrane is formed, and the escape from the parent takes place.[288] It seems probable that at this stage, namely, that of a protected embryo, there is often a prolonged period of rest, during which it may be carried by wind and other agencies for long distances without injury.

The remarkable power that Hydra possesses of recovery from injury and of regenerating lost parts was first pointed out by Trembley in his classical memoir.[289]

A Hydra can be cut into a considerable number of pieces, and each piece, provided both ectoderm and endoderm are represented in it, will give rise by growth and regeneration to a complete zooid. There is, however, a limit of size below which fragments of Hydra will not regenerate, even if they contain {255}cells of both layers. The statement made by Trembley, that when a Hydra is turned inside out it will continue to live in the introverted condition has not been confirmed, and it seems probable that after the experiment has been made the polyp remains in a paralysed condition for some time, and later reverts, somewhat suddenly, to the normal condition by a reversal of the process. There is certainly no substantial reason to believe that under any circumstances the ectoderm can undertake the function of the endoderm or the endoderm the functions of the ectoderm.

fig127

Fig. 127.—A series of drawings of Hydra, showing the attitudes it assumes during one of the more rapid movements from place to place. 1, The Hydra bending over to one side; 2, attaching itself to the support by the mouth and tentacles; 3, drawing the sucker up to the mouth; 4, inverted; 5, refixing the sucker; 6, reassuming the erect posture. (After Trembley.)

One of the characteristic features of Hydra is the slightly expanded, disc-shaped aboral extremity usually called the "foot," an unfortunate term for which the word "sucker" should be substituted. There are no root-like tendrils or processes for attachment to the support such as are found in most of the solitary Gymnoblastea. The attachment of the body to the stem or weed or surface-film by this sucker enables the animal to change its position at will. It may either progress slowly by gliding along its support without the assistance of the tentacles, in a manner similar to that observed in many Sea-anemones; or more rapidly by a series of somersaults, as originally described by Trembley. The latter mode of locomotion has been recently described as follows:—"The body, expanded and with expanded tentacles, bends over to one side. As soon as the tentacles touch the bottom they attach themselves and contract. Now one of two things happens. The foot may loosen its hold on the bottom and the body contract. In this manner the animal comes to stand on its tentacles with the foot pointing upward. The body now bends over again until the foot attaches itself close to the attached tentacles. These loosen in their turn, and so the Hydra is again {256}in its normal position. In the other case the foot is not detached, but glides along the support until it stands close to the tentacles, which now loosen their hold."[290]

Hydra appears to be purely carnivorous. It will seize and swallow Entomostraca of relatively great size, so that the body-wall bulges to more than twice its normal diameter. But smaller Crustacea, Annelid worms, and pieces of flesh are readily seized and swallowed by a hungry Hydra. In H. viridis the chlorophyll corpuscles[291] of the endoderm may possibly assist in the nourishment of the body by the formation of starch in direct sunlight.

Three species of Hydra are usually recognised, but others which may be merely local varieties or are comparatively rare have been named.[292]

H. viridis.—Colour, grass-green. Average number of tentacles, eight. Tentacles shorter than the body. Embryonic chitinous membrane spherical and almost smooth.

H. vulgaris, Pallas (H. grisea, Linn.).—Colour, orange-brown. Tentacles rather longer than the body, average number, six. Embryonic chitinous membrane spherical, and covered with numerous pointed branched spines.

H. oligactis, Pallas (H. fusca, Linn.).—Colour, brown. Tentacles capable of great extension; sometimes, when fully expanded, several times the length of the body. Average number, six. Embryonic chitinous membrane plano-convex, its convex side only covered with spines.

The genera Microhydra (Ryder) and Protohydra (Greeff) are probably allied to Hydra, but as their sexual organs have not been observed their real affinities are not yet determined. Microhydra resembles Hydra in its general form and habits, and in its method of reproduction by gemmation, but it has no tentacles. It was found in fresh water in North America.

Protohydra[293] was found in the oyster-beds off Ostend, and resembles Microhydra in the absence of tentacles. It multiplies by transverse fission, but neither gemmation nor sexual reproduction has been observed.

Haleremita is a minute hydriform zooid which is also marine. {257}It was found by Schaudinn[294] in the marine aquarium at Berlin in water from Rovigno, on the Adriatic. It reproduces by gemmation, but sexual organs have not been found.

Another very remarkable genus usually associated with the Eleutheroblastea is Polypodium. At one stage of its life-history it has the form of a spiral ribbon or stolon which is parasitic on the eggs of the sturgeon (Acipenser ruthenus) in the river Volga.[295] This stolon gives rise to a number of small Hydra-like zooids with twenty tentacles, of which sixteen are filamentous and eight club-shaped. These zooids multiply by longitudinal fission, and feed independently on Infusoria, Rotifers, and other minute organisms. The stages between these hydriform individuals and the parasitic stolon have not been discovered.

Order II. Milleporina.

Millepora was formerly united with the Stylasterina to form the order Hydrocorallina; but the increase of our knowledge of these Hydroid corals tends rather to emphasise than to minimise the distinction of Millepora from the Stylasterina.

Millepora resembles the Stylasterina in the production of a massive calcareous skeleton and in the dimorphism of the zooids, but in the characters of the sexual reproduction and in many minor anatomical and histological peculiarities it is distinct. As there is only one genus, Millepora, the account of its anatomy will serve as a description of the order.

The skeleton (Fig. 128) consists of large lobate, plicate, ramified, or encrusting masses of calcium carbonate, reaching a size of one or two or more feet in height and breadth. The surface is perforated by numerous pores of two distinct sizes; the larger—"gastropores"—are about 0.25 mm. in diameter, and the smaller and more numerous "dactylopores" about 0.15 mm. in diameter. In many specimens the pores are arranged in definite cycles, each gastropore being surrounded by a circle of 5-7 dactylopores; but more generally the two kinds appear to be irregularly scattered on the surface.

When a branch or lobe of a Millepore is broken across and examined in section, it is found that each pore is continued as a {258}vertical tube divided into sections by horizontal calcareous plates (Fig. 129, Tab). These plates are the "tabulae," and constitute the character upon which Millepora was formerly placed in the now discarded group of Tabulate corals.

The coral skeleton is also perforated by a very fine reticulum of canals, by which the pore-tubes are brought into communication with one another. In the axis of the larger branches and in the centre of the larger plates a considerable quantity of the skeleton is of an irregular spongy character, caused by the disintegrating influence of a boring filamentous Alga.[296]

fig128

Fig. 128.—A portion of a dried colony of Millepora, showing the larger pores (gastropores) surrounded by cycles of smaller pores (dactylopores). At the edges the cycles are not well defined.

The discovery that Millepora belongs to the Hydrozoa was made by Agassiz[297] in 1859, but Moseley[298] was the first to give {259}an adequate account of the general anatomy. The colony consists of two kinds of zooids—the short, thick gastrozooids (Fig. 129, G) provided with a mouth and digestive endoderm, and the longer and more slender mouthless dactylozooids (D)—united together by a network of canals running in the porous channels of the superficial layer of the corallum. The living tissues of the zooids extend down the pore-tubes as far as the first tabulae, and below this level the canal-system is degenerate and functionless. It is only a very thin superficial stratum of the coral, therefore, that contains living tissues.

The zooids of Millepora are very contractile, and can be withdrawn below the general surface of the coral into the shelter of the pore-tubes. When a specimen is examined in its natural position on the reef, the zooids are usually found to be thus contracted; but several observers have seen the zooids expanded in the living condition. It is probable that, as is the case with other corals, the expansion occurs principally during the night.

The colony is provided with two kinds of nematocysts—the small kind and the large. In some colonies they are powerful enough to penetrate the human skin, and Millepora has therefore received locally the name of "stinging coral." On each of the dactylozooids there are six or seven short capitate tentacles (Fig. 129, t), each head being packed with nematocysts of the small kind; similar batteries of these nematocysts are found in the four short capitate tentacles of the gastrozooids. The nematocysts of the larger kind are found in the superficial ectoderm, some distributed irregularly on the surface, others in clusters round the pores. The small nematocysts are about 0.013 mm. in length before they are exploded, and exhibit four spines at the base of the thread; the large kind are oval in outline, 0.02 × 0.025 mm. in size, and exhibit no spines at the base, but a spiral band of minute spines in the middle of the filament. There is some reason to believe that the filament of the large kind of nematocysts can be retracted.[299]

At certain seasons the colonies of Millepora produce a great number of male or female Medusae. The genus is probably dioecious, no instances of hermaphrodite colonies having yet been found. Each Medusa is formed in a cavity situated above the last-formed tabula in a pore-tube, and this cavity, the "ampulla," having a greater diameter than that of the gastrozooid tubes, can be recognised even in the dried skeleton.

{260}
fig129

Fig. 129.—Diagrammatic sketch to show the structure of Millepora. Amp, an ampulla containing a medusa; Can.1, canal system at the surface; Can.2, canal system degenerating in the lower layers of the corallum; Cor, corallum; D, an expanded dactylozooid with its capitate tentacles; Ect, the continuous sheet of ectoderm covering the corallum (Cor); G, a gastrozooid, seen in vertical section; Med, free-swimming Medusae; t, tentacle; Tab, tabula in the pore-tubes. (Partly after Moseley.)

It is not known how frequently the sexual seasons occur, but from the rarity in the {261}collections of our museums of Millepore skeletons which exhibit the ampullae, it may be inferred that the intervals between successive seasons are of considerable duration.

The Medusae of Millepora are extremely simple in character. There is a short mouthless manubrium bearing the sexual cells, an umbrella without radial canals, while four or five knobs at the margin, each supporting a battery of nematocysts, represent all that there is of the marginal tentacles. The male Medusae have not yet been observed to escape from the parent, but from the fact that the spermatozoa are not ripe while they are in the ampullae, it may be assumed that the Medusae are set free. Duerden, however, has observed the escape of the female Medusae, and it seems probable from his observations that their independent life is a short one, the ova being discharged very soon after liberation.

Millepora appears to be essentially a shallow-water reef coral. It may be found on the coral reefs of the Western Atlantic extending as far north as Bermuda, in the Red Sea, the Indian and Pacific Oceans. The greatest depth at which it has hitherto been found is 15 fathoms on the Macclesfield Bank, and it flourishes at a depth of 7 fathoms off Funafuti in the Pacific Ocean.

Millepora, like many other corals, bears in its canals and zooids a great number of the symbiotic unicellular "Algae" (Chrysomonadaceae, see pp. 86, 125) known as Zooxanthellae. All specimens that have been examined contain these organisms in abundance, and it has been suggested that the coral is largely dependent upon the activity of the "Algae" for its supply of nourishment. There can be no doubt that the dactylozooids do paralyse and catch living animals, which are ingested and digested by the gastrozooids, but this normal food-supply may require to be supplemented by the carbohydrates formed by the plant-cells. But as the carbohydrates can only be formed by the "Algae" in sunlight, this supplementary food-supply can only be provided in corals that live in shallow water. It must not be supposed that this is the only cause that limits the distribution of Millepora in depth, but it may be an important one.

The generic name Millepora has been applied to a great many fossils from different strata, but a critical examination of their structure fails to show any sufficient reason for including many of them in the genus or even in the order. Fossils that are {262}undoubtedly Millepora occur in the raised coral reefs of relatively recent date, but do not extend back into Tertiary times. There seems to be no doubt, therefore, that the genus is of comparatively recent origin. Among the extinct fossils the genus that comes nearest to it is Axopora from the Eocene of France, but this genus differs from Millepora in having monomorphic, not dimorphic, pores, and in the presence of a minute spine or columella in the centre of each tube. The resemblances are to be observed in the general disposition of the canal system and of the tabulation. Whether Axopora is or is not a true Milleporine, however, cannot at present be determined, but it is the only extinct coral that merits consideration in this place.

Order III. Gymnoblastea—Anthomedusae.

This order was formerly united with the Calyptoblastea to form the order Hydromedusae, but the differences between the two are sufficiently pronounced to merit their treatment as distinct orders.

In many of the Gymnoblastea the sexual cells are borne by free Medusae, which may be recognised as the Medusae of Gymnoblastea by the possession of certain distinct characters. The name given to such Medusae, whether their hydrosome stage is known or not, is Anthomedusae. The Gymnoblastea are solitary or colonial Hydrozoa, in which the free (oral) extremity of the zooids, including the crown of tentacles, is not protected by a skeletal cup. The sexual cells may be borne by free Anthomedusae, or by more or less degenerate Anthomedusae that are never detached from the parent hydrosome. The Anthomedusae are small or minute Medusae provided with a velum, with the ovaries or sperm-sacs borne by the manubrium and with sense-organs in the form of ocelli or pigment-spots situated on the margin of the umbrella.

The solitary Gymnoblastea present so many important differences in anatomical structure that they cannot be united in a single family. They are usually fixed to some solid object by root-like processes from the aboral extremity, the "hydrorhiza," or are partly embedded in the sand (Corymorpha), into which long filamentous processes project for the support of the zooid. The remarkable species Hypolytus peregrinus[300] from Wood's Holl, {263}however, has no aboral processes, and appears to be only temporarily attached to foreign objects by the secretion of the perisarc. Among the solitary Gymnoblastea several species reach a gigantic size. Corymorpha is 50-75 mm. in length, but Monocaulus from deep water in the Pacific and Atlantic Oceans is nearly 8 feet in length. Among the solitary forms attention must be called to the interesting pelagic Pelagohydra (see p. 274).

The method of colony formation in the Gymnoblastea is very varied. In some cases (Clava squamata) a number of zooids arise from a plexus of canals which corresponds with the system of root-like processes of the solitary forms. In Hydractinia this plexus is very dense, and the ectoderm forms a continuous sheet of tissue both above and below. The colony is increased in size in these cases by the gemmation of zooids from the hydrorhiza. In other forms, such as Tubularia larynx, new zooids arise not only from the canals of the hydrorhiza, but also from the body-walls of the upstanding zooids, and thus a bushy or shrubby colony is formed.

In another group the first-formed zooid produces a hydrorhiza of considerable proportions, which fixes the colony firmly to a stone or shell and increases in size with the growth of the colony. This zooid itself by considerable growth in length forms the axis of the colony, and by gemmation gives rise to lateral zooids, which in their turn grow to form the lateral branches and give rise to the secondary branches, and these to the tertiary branches, and so one; each branch terminating in a mouth, hypostome and crown of tentacles. Such a method of colony formation is seen in Bougainvillia (Fig. 130). A still more complicated form of colony formation is seen in Ceratella, in which not a single but a considerable number of zooids form the axis of the colony and of its branches. As each axis is covered with a continuous coat of ectoderm, and each zooid of such an axis secretes a chitinous fenestrated tube, the whole colony is far more rigid and compact than is usual in the Gymnoblastea, and has a certain superficial resemblance to a Gorgoniid Alcyonarian (Fig. 133, p. 271).

The branches of the colony and a considerable portion of the body-wall of each zooid in the Gymnoblastea are usually protected by a thin, unjointed "perisarc" of chitin secreted by the ectoderm; but this skeletal structure does not expand distally to {264}form a cup-like receptacle in which the oral extremity of the zooid can be retracted for protection.

The zooids of the Gymnoblastea present considerable diversity of form and structure. The tentacles may be reduced to one (in Monobrachium) or two (in Lar sabellarum), but usually the number is variable in each individual colony. In many cases, such as Cordylophora, Clava, and many others, the tentacles are irregularly scattered on the sides of the zooids. In others there may be a single circlet of about ten or twelve tentacles round the base of the hypostome. In some genera the tentacles are arranged in two series (Tubularia, Corymorpha, Monocaulus), a distal series round the margin of the mouth which may be arranged in a single circlet or scattered irregularly on the hypostome, and a proximal series arranged in a single circlet some little distance from the mouth. In Branchiocerianthus imperator the number of tentacles is very great, each of the two circlets consisting of about two hundred tentacles.

fig130

Fig. 130.—Diagrammatic sketch to show the method of branching of Bougainvillia. gon, Gonophores; Hr, hydrorhiza; t.z, terminal zooid.

The zooids of the hydrosome are usually monomorphic, but there are cases in which different forms of zooid occur in the same colony. In Hydractinia, for example, no less than four different kinds of zooids have been described. These are called gastrozooids, dactylozooids, tentaculozooids, and blastostyles respectively. The "gastrozooids" are provided with a conical hypostome bearing the mouth and two closely-set circlets of some ten to thirty tentacles. The "dactylozooids" are longer than the gastrozooids and have the habit of actively coiling and {265}uncoiling themselves; they have a small mouth and a single circlet of rudimentary tentacles. The "tentaculozooids" are situated at the outskirts of the colony, and are very long and slender, with rudimentary tentacles and no mouth. The "blastostyles," usually shorter than the gastrozooids, have two circlets of rudimentary tentacles and a mouth. They bear on their sides the spherical or oval gonophores.

The medusome stage in the life-history of these Hydrozoa is produced by gemmation from the hydrosome, or, in some cases, by gemmation from the medusome as well as from the hydrosome. In many genera and species the medusome is set free as a minute jelly-fish or Medusa, which grows and develops as an independent organism until the time when the sexual cells are ripe, and then apparently it dies. In other Gymnoblastea the medusome either in the female or the male or in both sexes does not become detached from the parent hydrosome, but bears the ripe sexual cells, discharges them into the water, and degenerates without leading an independent life at all. In these cases the principal organs of the medusome are almost or entirely functionless, and they exhibit more or less imperfect development, or they may be so rudimentary that the medusoid characters are no longer obvious. Both the free and the undetached medusomes are gonophores, that is to say, the bearers of the sexual cells, but the former were described by Allman as the "phanerocodonic" gonophores, i.e. "with manifest bells," and the latter as the "adelocodonic" gonophores. The gonophores may arise either from an ordinary zooid of the colony (Syncoryne), from a specially modified zooid—the blastostyle—as in Hydractinia, or from the hydrorhiza as in certain species of Perigonimus. The free-swimming Medusa may itself produce Medusae by gemmation from the manubrium (Sarsia, Lizzia, Rathkea, and others), from the base of the tentacles (Sarsia, Corymorpha, Hybocodon), or from the margin of the umbrella (Eleutheria).

The free-swimming Medusae or phanerocodonic gonophores of the Gymnoblastea are usually of small size (1 or 2 mm. in diameter) when first liberated, and rarely attain a great size even when fully mature. They consist of a circular, bell-shaped or flattened disc—the umbrella—provided at its margin with a few or numerous tentacles, and a tubular manubrium bearing the mouth depending from the exact centre of the under (oral) {266}side of the umbrella (Fig. 132, A). The mouth leads into a shallow digestive cavity, from which radial canals pass through the substance of the umbrella to join a ring-canal at the margin (Fig. 131).

The sense-organs of the Medusae of the Gymnoblastea are in the form of pigment-spots or very simple eyes (ocelli), situated at the bases of the tentacles. The orifice of the umbrella is guarded by a thin shelf or membrane, as in the Calyptoblastea, called the velum. The sexual cells are borne by the manubrium (Figs. 131 and 132, A).

There are many modifications observed in the different genera as regards the number of tentacles, the number and character of the radial canals, the minute structure of the sense-organs, and some other characters, but they agree in having a velum, ocellar sense-organs, and manubrial sexual organs. The tentacles are rudimentary in Amalthea; in Corymorpha there is only one tentacle; in Perigonimus there are two; and in Bougainvillia they are numerous; but the usual number is four or six. The radial canals are usually simple and four in number, but there are six in Lar sabellarum, which branch twice or three times before reaching the margin of the umbrella (Fig. 132, B).

fig131

Fig. 131.—Medusa of Cladonema, from the Bahamas, showing peculiar tentacular processes on the tentacles, the ocelli at the base of the tentacles, the swellings on the manubrium that mark the position of the gonads, and the radial and ring-canals of the umbrella. (After Perkins.)

There can be no doubt that the Medusae of many Gymnoblastea undergo several important changes in their anatomical features during the period of the ripening of the sexual cells. Thus in Lar sabellarum the six radial canals are simple in the first stage of development (A); but in the second stage (B) each radial canal bifurcates before reaching the margin, and in the adult stage shows a double bifurcation. The life-history has, however, been worked out in very few of the Anthomedusae, and there can be little doubt that as our knowledge grows several forms which are now known as distinct species {267}will be found to be different stages of growth of the same species.

fig132

Fig. 132.—Two stages in the development of the Medusa of Lar sabellarum (Willsia stellata). A, first stage with six canals without branches; B, third stage with six canals each with two lateral branches. The developing gonads may be seen on the manubrium in A. (After Browne.)

The movements of the Medusae are well described by Allman[301] in his account of Cladonema radiatum:—"It is impossible to grow tired of watching this beautiful medusa; sometimes while dashing through the water with vigorous diastole and systole, it will all at once attach its grapples to the side of the vessel, and become suddenly arrested in its career, and then after a period of repose, during which its branched tentacles are thrown back over its umbrella and extended into long filaments which float, like some microscopic sea-weed in the water, it will once more free itself from its moorings and start off with renewed energy." The Medusa of Clavatella, "in its movements and mode of life, presents a marked contrast to the medusiform zooid of other Hydrozoa. The latter is active and mercurial, dancing gaily through the water by means of the vigorous strokes of its crystalline swimming-bell. The former strides leisurely along, or, using the adhesive discs as hands, climbs amongst the branches of the weed. In the latter stage of its existence it becomes stationary, fixing itself by means of its suckers; and {268}thus it remains, the capitate arms standing out rigidly, like the rays of a starfish, until the embryos are ready to escape."[302]

Among the Gymnoblastea there are many examples of a curious association of the Hydroid with some other living animals. Thus Hydractinia is very often found on the shells carried by living Hermit crabs, Dicoryne on the shells of various Molluscs, Tubularia has been found on a Cephalopod, and Ectopleura (a Corymorphid) on the carapace of a crab. There is but little evidence, however, that in these cases the association is anything more than accidental. The occurrence of the curious species, Lar sabellarum, on the tubes of Sabella, of Campaniclava cleodorae on the living shells of the pelagic Mollusc Cleodora cuspidata, and of a Gorgonia on the tubes of Tubularia parasitica, appear to be cases in which there is some mutual relationship between the two comrades. The genus Stylactis, however, affords some of the most interesting examples of mutualism. Thus Stylactis vermicola is found only on the back of an Aphrodite that lives at the great depth of 2900 fathoms. S. spongicola and S. abyssicola are found associated with certain deep-sea Horny Sponges. S. minoi is spread over the skin of the little rock perch Minous inermis, which is found at depths of from 45 to 150 fathoms in the Indian seas.

In many cases it is difficult to understand what is the advantage of the Hydroid to the animal that carries it, but in this last case Alcock[303] suggests that the Stylactis assists in giving the fish a deceitful resemblance to the incrusted rocks of its environment, in order to allure, or at any rate not to scare, its prey. Whether this is the real explanation or not, the fact that in the Bay of Bengal and in the Laccadive and Malabar seas the fish is never found without this Hydroid, nor the Hydroid without this species of fish, suggests very strongly that there is a mutual advantage in the association.

Cases of undoubted parasitism are very rare in this order. The remarkable form Hydrichthys mirus,[304] supposed to be a Gymnoblastic Hydroid, but of very uncertain position in the system, appears to be somewhat modified in its structure by its parasitic habits on the fish Seriola zonata. Corydendrium {269}parasiticum is said to be a parasite living at the expense of Eudendrium racemosum. Mnestra is a little Medusa which attaches itself by its manubrium to the Mollusc Phyllirhoe, and may possibly feed upon the skin or secretions of its host.

Nearly all the species of the order are found in shallow sea water. Stylactis vermicola and the "Challenger" specimen of Monocaulus imperator occur at a depth of 2900 fathoms, and some species of the genera Eudendrium and Myriothela descend in some localities to a depth of a few hundred fathoms. Cordylophora is the only genus known to occur in fresh water. From its habit of attaching itself to wooden piers and probably to the bottom of barges, and from its occurrence in navigable rivers and canals, it has been suggested that Cordylophora is but a recent immigrant into our fresh-water system. It has been found in England in the Victoria docks of London, in the Norfolk Broads, and in the Bridgewater Canal. It has ascended the Seine in France, and may now be found in the ponds of the Jardin des Plantes at Paris. It also occurs in the Elbe and in some of the rivers of Denmark.

The classification of the Gymnoblastea is not yet on a satisfactory basis. At present the hydrosome stage of some genera alone has been described, of others the free-swimming Medusa only is known. Until the full life-history of any one genus has been ascertained its position in the families mentioned below may be regarded as only provisional. The principal families are:—

Fam. Bougainvilliidae.—The zooids of the hydrosome have a single circlet of filiform tentacles at the base of the hypostome. In Bougainvillia belonging to this family the gonophores are liberated in the form of free-swimming Medusae formerly known by the generic name Hippocrene. In the fully grown Medusa there are numerous tentacles arranged in clusters opposite the terminations of the four radial canals. There are usually in addition tentacular processes (labial tentacles) on the lips of the manubrium. Bougainvillia is a common British zoophyte of branching habit, found in shallow water all round the coast. The medusome of Bougainvillia ramosa is said to be the common little medusa Margelis ramosa.[305] Like most of the Hydroids it has a wide geographical distribution. Other genera are Perigonimus, which has a Medusa with only two tentacles; and {270}Dicoryne, which forms spreading colonies on Gasteropod shells and has free gonophores provided with two simple tentacles, while the other organs of the medusome are remarkably degenerate. In Garveia and Eudendrium the gonophores are adelocodonic, in the former genus arising from the body-wall of the axial zooids of the colony, and in the latter from the hydrorhiza. Stylactis is sometimes epizoic (p. 268). Among the genera that are usually placed in this family, of which the medusome stage only is known, are Lizzia (a very common British Medusa) and Rathkea. In Margelopsis the hydrosome stage consists of a single free-swimming zooid which produces Medusae by gemmation.

Fam. Podocorynidae.—The zooids have the same general features as those of the Bougainvilliidae, but the perisarc does not extend beyond the hydrorhiza.

In Podocoryne and Hydractinia belonging to this family the hydrorhiza forms an encrusting stolon which is usually found on Gasteropod shells containing a living Hermit crab. In Podocoryne the gonophores are free-swimming Medusae with a short manubrium provided with labial tentacles. Hydractinia differs from Podocoryne in having polymorphic zooids and adelocodonic gonophores.

A fossil encrusting a Nassa shell from the Pliocene deposit of Italy has been placed in the genus Hydractinia, and four species of the same genus have been described from the Miocene and Upper Greensand deposits of this country.[306] These are the only fossils known at present that can be regarded as Gymnoblastic Hydroids.

The Medusa Thamnostylus, which has only two marginal tentacles and four very long and profusely ramified labial tentacles, is placed in this family. Its hydrosome stage is not known.

Fam. Clavatellidae.—This family contains the genus Clavatella, in which the zooids of the hydrosome have a single circlet of capitate tentacles. The gonophore is a free Medusa provided with six bifurcated capitate tentacles.

Fam. Cladonemidae.—This family contains the genus Cladonema, in which the zooids have two circlets of four tentacles, the labial tentacles being capitate and the aboral filiform. The gonophore is a free Medusa with eight tentacles, each provided with a number of curious capitate tentacular processes (Fig. 131).

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Fam. Tubulariidae.—This important and cosmopolitan family is represented in the British seas by several common species. The zooids of the hydrosome of Tubularia have two circlets of numerous filiform tentacles. The gonophores are adelocodonic, and are situated on long peduncles attached to the zooid on the upper side of the aboral circlet of tentacles. The larva escapes from the gonophore and acquires two tentacles, with which it beats the water and, assisted by the cilia, keeps itself afloat for some time. In this stage it is known as an "Actinula."[307]

fig133

Fig. 133.—Ceratella fusca. About nat. size. (After Baldwin Spencer.[308])

Fam. Ceratellidae.—The colony of Ceratella may be five inches in height. The stem and main branches are substantial, and consist of a network of branching anastomosing tubes supported by a thick and fenestrated chitinous perisarc. The {272}whole branch is enclosed in a common layer of ectoderm. The zooids have scattered capitate tentacles. The Ceratellidae occur in shallow water off the coast of New South Wales, extend up the coast of East Africa as far as Zanzibar, and have also been described from Japan.

Fam. Pennariidae.—In the hydrosome stage the zooids have numerous oral capitate tentacles scattered on the hypostome, and a single circlet of basilar filiform tentacles. The medusa of Pennaria, a common genus of wide distribution, is known under the name Globiceps.

Fam. Corynidae.—In the hydrosome stage the zooids of this family possess numerous capitate tentacles arranged in several circlets or scattered.

In Cladocoryne the tentacles are branched. Syncoryne is a common and widely distributed genus with numerous unbranched capitate tentacles irregularly distributed over a considerable length of the body-wall of the zooid. In many of the species the gonophores are liberated as Medusae, known by the name Sarsia, provided with four filiform tentacles and a very long manubrium. In some species (S. prolifera and S. siphonophora) the Medusae are reproduced asexually by gemmation from the long manubrium. A common British Anthomedusa of this family is Dipurena, but its hydrosome stage is not known. In the closely related genus Coryne the gonophores are adelocodonic, and exhibit very rudimentary medusoid characters.

Fam. Clavidae.—This is a large family containing many genera, some with free-swimming Medusae, others with adelocodonic gonophores. In the former group are included a number of oceanic Medusae of which the hydrosome stage has not yet been discovered. The zooids of the hydrosome have numerous scattered filiform tentacles. The free-swimming Medusae have hollow tentacles.

Clava contains a common British species with a creeping hydrorhiza frequently attached to shells, and with adelocodonic gonophores. Cordylophora is the genus which has migrated into fresh water in certain European localities (see p. 269). It forms well-developed branching colonies attached to wooden gates and piers or to the brickwork banks of canals. Several Anthomedusae, of which the hydrosome stage is not known, appear to be related to the Medusae of this family, but are sometimes separated as {273}the family Tiaridae. Of these Tiara, a very brightly coloured jelly-fish sometimes attaining a height of 40 mm., is found on the British coasts, and Amphinema is found in considerable numbers at Plymouth in September. Turritopsis is a Medusa with a hydrosome stage like Dendroclava. For Stomatoca, see p. 415.

Fam. Corymorphidae.—This family contains the interesting British species Corymorpha nutans. The hydrosome stage consists of a solitary zooid of great size, 50-75 mm. in length, provided with two circlets of numerous long filiform tentacles. The free-swimming Medusae are produced in great numbers on the region between the two circlets of tentacles. These Medusae were formerly known by the name Steenstrupia, and are noteworthy in having only one long moniliform tentacle, opposite to one of the radial canals.

The gigantic Monocaulus imperator of Allman was obtained by the "Challenger" at the great depth of 2900 fathoms off the coast of Japan. It was nearly eight feet in length. More recently Miyajima[309] has described a specimen from 250 fathoms in the same seas which was 700 mm. (27.5 in.) in length. Miyajima's specimen resembles those described by Mark from 300 fathoms off the Pacific coast of North America as Branchiocerianthus urceolus in the remarkable feature of a distinct bilateral arrangement of the circlets of tentacles. Owing to the imperfect state of preservation of the only specimen of Allman's species it is difficult to determine whether it is also bilaterally symmetrical and belongs to the same species as the specimens described by Mark and Miyajima. These deep-sea giant species, however, appear to differ from Corymorpha in having adelocodonic gonophores.

Fam. Hydrolaridae.—This family contains the remarkable genus Lar, which was discovered by Gosse attached to the margin of the tubes of the marine Polychaete worm Sabella. The zooids have only two tentacles, and exhibit during life curious bowing and bending movements which have been compared with the exercises of a gymnast. The Medusae (Fig. 132, A and B) have been known for a long time by the name Willsia, but their life-history has only recently been worked out by Browne.[310]

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Fam. Monobrachiidae.Monobrachium, found in the White Sea by Mereschkowsky, forms a creeping stolon on the shells of Tellina. The zooids of the hydrosome have only one tentacle.

Fam. Myriothelidae.—This family contains the single genus Myriothela. The zooid of the hydrosome stage is solitary and is provided, as in the Corynidae, with numerous scattered capitate tentacles. The gonophores are borne by blastostyles situated above the region of the tentacles. In addition to these blastostyles producing gonophores there are, in M. phrygia, supplementary blastostyles which capture the eggs as they escape from the gonophores and hold them until the time when the larva is ready to escape. They were called "claspers" by Allman. In some of the Arctic species Frl. Bonnevie[311] has shown that they are absent. Each zooid of M. phrygia is hermaphrodite.

fig134

Fig. 134.—Pelagohydra mirabilis. Fl, The float; M, position of the mouth; Ten.Fl, filamentous tentacles of the float. (After Dendy.)

Fam. Pelagohydridae.—This family was constituted by Dendy[312] for the reception of Pelagohydra mirabilis, a remarkable new species discovered by him on the east coast of the South Island of New Zealand. The hydrosome is solitary and free-swimming, the proximal portion of the body being modified to form a float, the distal portion forming a flexible proboscis terminated by the mouth and a group of scattered manubrial tentacles. The tentacles are filiform and scattered over the surface of the float. Medusae are developed on stolons between the tentacles of the float. They have tentacles arranged in four radial groups of five each, at the margin of the umbrella.

As pointed out by Hartlaub,[313] Pelagohydra is not the only genus in which the hydrosome floats. Three species of the genus Margelopsis have been found that have pelagic habits, and two {275}of them have been shown to produce numerous free-swimming Medusae by gemmation; but at present there is no reason to suppose that in these forms there is any extensive modification of the aboral extremity of the zooid to form such a highly specialised organ as the float of Pelagohydra.

The affinities of Pelagohydra are not clear, as our knowledge of the characters of the Medusa is imperfect; but according to Dendy it is most closely related to the Corymorphidae. Margelopsis belongs to the Bougainvilliidae.

Order IV. Calyptoblastea—Leptomedusae.

The hydrosome stage is characterised by the perisarc, which not only envelops the stem and branches, as in many of the Gymnoblastea, but is continued into a trumpet-shaped or tubular cup or collar called the "hydrotheca," that usually affords an efficient protection for the zooids when retracted. No solitary Calyptoblastea have been discovered. In the simpler forms the colony consists of a creeping hydrorhiza, from which the zooids arise singly (Clytia johnstoni), but these zooids may give rise to a lateral bud which grows longer than the parent zooid.

fig135

Fig. 135.—Part of a hydrocladium of a dried specimen of Plumularia profunda. Gt, Gonotheca; Hc, the stem of the hydrocladium with joints (j); Ht, a single hydrotheca; N, nematophores. Greatly enlarged. (After Nutting.)

The larger colonies are usually formed by alternate right and left budding from the last-formed zooid, so that in contrast to the Gymnoblast colony the apical zooid of the stem is the youngest, and not the oldest, zooid of the colony. In the branching colonies the axis is frequently composed of a single tube of perisarc, which may be lined internally by the ectoderm and endoderm tissues formed by the succession of zooids that have given rise to the branches by gemmation. Such a stem is said to be monosiphonic.

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In some of the more complicated colonies, however, the stem is composed of several tubes, which may or may not be surrounded by a common sheath of ectoderm and perisarc, as they are in Ceratella among the Gymnoblastea. Such stems are said to be "polysiphonic" or "fascicled." The polysiphonic stem may arise in more than one way, and in some cases it is not quite clear in what manner it has arisen.[314]

In many colonies the zooids are only borne by the terminal monosiphonic branches, which receive the special name "hydrocladia." The gonophores of the Calyptoblastea are usually borne by rudimentary zooids, devoid of mouth and tentacles (the "blastostyles"), protected by a specially dilated cup of perisarc known as the "gonotheca" or "gonangium." The shape and size of the gonothecae vary a good deal in the order. They may be simply oval in shape, or globular (Schizotricha dichotoma), or greatly elongated, with the distal ends produced into slender necks (Plumularia setacea). They are spinulose in P. echinulata, and annulated in P. halecioides, Clytia, etc.

In some genera there are special modifications of the branches and hydrocladia, for the protection of the gonothecae. The name "Phylactocarp" is used to designate structures that are obviously intended to serve this purpose. The phylactocarp of the genera Aglaophenia and Thecocarpus is the largest and most remarkable of this group of structures, and has received the special name "corbula." The corbula consists of an axial stem or rachis, and of a number of corbula-leaves arising alternately from the rachis, bending upwards and then inwards to meet those of the other side above, the whole forming a pod-shaped receptacle. The gonangia are borne at the base of each of the corbula-leaves. There is some difference of opinion as to the homologies of the parts of the corbula, but the rachis seems to be that of a modified hydrocladium, as it usually bears at its base one or more hydrothecae of the normal type. The corbula-leaves are usually described as modified nematophores (vide infra), but according to Nutting[315] there is no more reason to regard them as modified nematophores than as modified hydrothecae, and he regards them as "simply the modification of a structure originally intended to {277}protect an indefinite person, an individual that may become either a sarcostyle[316] or a hydranth."

The other forms of phylactocarps are modified branches as in Lytocarpus, and those which are morphologically appendages to branches as in Cladocarpus, Aglaophenopsis, and Streptocaulus.

The structures known as "nematophores" in the Calyptoblastea are the thecae of modified zooids, comparable with the dactylozooids of Millepora. They form a well-marked character of the very large family Plumulariidae, but they are also found in species of the genera Ophiodes, Lafoëina, Oplorhiza, Perisiphonia, Diplocyathus, Halecium, and Clathrozoon among the other Calyptoblastea. The dactylozooids are usually capitate or filiform zooids, without tentacles or a mouth, and with a solid or occasionally a perforated core of endoderm. They bear either a battery of nematocysts (Plumularia, etc.), or of peculiar adhesive cells (Aglaophenia and some species of Plumularia). The functions of the dactylozooids are to capture the prey and to serve as a defence to the colony. In the growth of the corbula of Aglaophenia the dactylozooids appear to serve another purpose, and that is, as a temporary attachment to hold the leaves together while the edges themselves are being connected by trabeculae of coenosarc.

In a very large number of Calyptoblastea the gonophore is a reduced Medusa which never escapes from the gonotheca, but in the family Eucopidae the gonophores escape as free-swimming Medusae, exhibiting certain very definite characters. The gonads are situated not on the manubrium, as in the Anthomedusae, but on the sub-umbrellar aspect of the radial canals. The marginal sense-organs may be ocelli or vesiculate statocysts. The bell is usually more flattened, and the velum smaller than it is in the Anthomedusae, and the manubrium short and quadrangular. Such Medusae are called Leptomedusae.

Leptomedusae of many specific forms are found abundantly at the surface of the sea in nearly all parts of the world, but with the exception of some genera of the Eucopidae and a few others, their connexion with a definite Calyptoblastic hydrosome has not been definitely ascertained. It may be an assumption that time will prove to be unwarranted that all the Leptomedusae pass through a Calyptoblastic hydrosome stage.

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Fam. Aequoreidae.—In this family the hydrosome stage is not known except in the genus Polycanna, in which it resembles a Campanulariid. The sense-organs of the Medusae are statocysts. The radial canals are very numerous, and the genital glands are in the form of ropes of cells extending along the whole of their oral surfaces. Aequorea is a fairly common genus, with a flattened umbrella and a very rudimentary manubrium, which may attain a size of 40 mm. in diameter.

Fam. Thaumantiidae.—The Medusae of this family are distinguished from the Aequoreidae by having marginal ocelli in place of statocysts. The hydrosome of Thaumantias alone is known, and this is very similar to an Obelia.

Fam. Cannotidae.—The hydrosome is quite unknown. The Medusae are ocellate, but the radial canals, instead of being undivided, as in the Thaumantiidae, are four in number, and very much ramified before reaching the ring canal. The tentacles are very numerous. In the genus Polyorchis, from the Pacific coast of North America, the four radial canals give rise to numerous lateral short blind branches, and have therefore a remarkable pinnate appearance.

Fam. Sertulariidae.—In this family the hydrothecae are sessile, and arranged bilaterally on the stem and branches. The general form of the colony is pinnate, the branches being usually on opposite sides of the main stem. The gonophores are adelocodonic. Sertularia forms more or less arborescent colonies, springing from a creeping stolon attached to stones and shells. There are many species, several of which are very common upon the British coast. Many specimens are torn from their attachments by storms or by the trawls of fishermen and cast up on the sand or beach with other zoophytes. The popular name for one of the commonest species (S. abietina) is the "sea-fir." The genus has a wide geographical and bathymetrical range. Another common British species frequently thrown up by the tide in great quantities is Hydrallmania falcata. It has slender spirally-twisted stems and branches, and the hydrothecae are arranged unilaterally.

The genus Grammaria, sometimes placed in a separate family, is distinguished from Sertularia by several characters. The stem and branches are composed of a number of tubes which are considerably compressed. The genus is confined to the southern seas.

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Fam. Plumulariidae.—The hydrothecae are sessile, and arranged in a single row on the stem and branches. Nematophores are always present. Gonophores adelocodonic. This family is the largest and most widely distributed of all the families of the Hydrozoa. Nutting calculates that it contains more than one-fourth of all the Hydroids of the world. Over 300 species have been described, and more than half of these are found in the West Indian and Australian regions. Representatives of the family occur in abundance in depths down to 300 fathoms, and not unfrequently to 500 fathoms. Only a few species have occasionally been found in depths of over 1000 fathoms.

The presence of nematophores may be taken as the most characteristic feature of the family, but similar structures are also found in some species belonging to other families (p. 277).

The family is divided into two groups of genera, the Eleutheroplea and the Statoplea. In the former the nematophores are mounted on a slender pedicel, which admits of more or less movement, and in the latter the nematophores are sessile. The genera Plumularia and Antennularia belong to the Eleutheroplea. The former is a very large genus, with several common British species, distinguished by the terminal branches being pinnately disposed, and the latter, represented by A. antennina and A. ramosa on the British coast, is distinguished by the terminal branches being arranged in verticils.

The two most important genera of the Statoplea are Aglaophenia and Cladocarpus. The former is represented by a few species in European waters, the latter is only found in American waters.

Fam. Hydroceratinidae.—The colony consists of a mass of entwined hydrorhiza, with a skeleton in the form of anastomosing chitinous tubes. Hydrothecae scattered, tubular, and sessile. Nematophores present. Gonophores probably adelocodonic.

This family was constituted for a remarkable hydroid, Clathrozoon wilsoni, described by W. B. Spencer from Victoria.[317] The zooids are sessile, and spring from more than one of the numerous anastomosing tubes of the stem and branches. The whole of the surface is studded with an enormous number of small and very simple dactylozooids, protected by tubular nematophores. Only {280}a few specimens have hitherto been obtained, the largest being 10 inches in height by 4 inches in width. In general appearance it has some resemblance to a dark coloured fan-shaped Gorgonia.

Fam. Campanulariidae.—The hydrothecae in this family are pedunculate, and the gonophores adelocodonic.

In the cosmopolitan genus Campanularia the stem is monosiphonic, and the hydrothecae bell-shaped. Several species of this genus are very common in the rock pools of our coast between tide marks. Halecium is characterised by the rudimentary character of its hydrothecae, which are incapable of receiving the zooids even in their maximum condition of retraction. The genus Lafoea is remarkable for the development of a large number of tightly packed gonothecae on the hydrorhiza, each of which contains a blastostyle, bearing a single gonophore and, in the female, a single ovum. This group of gonothecae was regarded as a distinct genus of Hydroids, and was named Coppinia.[318] Lafoea dumosa with gonothecae of the type described as Coppinia arcta occurs on the British coast.

Perisiphonia is an interesting genus from deep water off the Azores, Australia, and New Zealand, with a stem composed of many distinct tubes.

The genus Zygophylax, from 500 fathoms off the Cape Verde, is of considerable interest in having a nematophore on each side of the hydrotheca. According to Quelch it should be placed in a distinct family.

Ophiodes has long and very active defensive zooids, protected by nematophores. It is found in the Laminarian zone on the English coast.

Fam. Eucopidae.—The hydrosome stage of this family is very similar to that of the Campanulariidae, but the gonophores are free-swimming Medusae of the Leptomedusan type.

One of the best-known genera is Obelia, of which several species are among the commonest Hydroids of the British coast.

Clytia johnstoni is also a very common Hydroid, growing on red algae or leaves of the weed Zostera. It consists of a number of upright, simple, or slightly branched stems springing from a creeping hydrorhiza. When liberated the Medusae are globular in form, with four radial canals and four marginal tentacles, but {281}this Medusa, like many others of the order, undergoes considerable changes in form before it reaches the sexually mature stage.

Phialidium temporarium is one of the commonest Medusae of our coast, and sometimes occurs in shoals. It seems probable that it is the Medusa of Clytia johnstoni.[319] By some authors the jelly-fish known as Epenthesis is also believed to be the Medusa of a Clytia.

Fam. Dendrograptidae.—This family includes a number of fossils which have certain distinct affinities with the Calyptoblastea. In Dictyonema, common in the Ordovician rocks of Norway, but also found in the Palaeozoic rocks of North America and elsewhere, the fossil forms fan-shaped colonies of delicate filaments, united by many transverse commissures, and in well-preserved specimens the terminal branches bear well-marked uniserial hydrothecae. In some species thecae of a different character, which have been interpreted to be gonothecae and nematophores respectively, are found.

Other genera are Dendrograptus, Thamnograptus, and several others from Silurian strata.

Order V. Graptolitoidea.

A large number of fossils, usually called Graptolites, occurring in Palaeozoic strata, are generally regarded as the skeletal remains of an ancient group of Hydrozoa.

In the simpler forms the fossil consists of a delicate straight rod bearing on one side a series of small cups. It is suggested that the cups contained hydroid zooids, and should therefore be regarded as the equivalent of the hydrothecae, and that the axis represents the axis of the colony or of a branch of the Calyptoblastea. In some of the forms with two rows of cups on the axis (Diplograptus), however, it has been shown that the cups are absent from a considerable portion of one end of the axis, and that the axes of several radially arranged individuals are fused together and united to a central circular plate. Moreover, there is found in many specimens a series of vesicles, a little larger in size than the cups, attached to the plate and arranged in a circle at the base of the axes. These vesicles are called the gonothecae.

The discovery of the central plate and of the so-called {282}gonothecae suggests that the usual comparison of a Graptolite with a Sertularian Hydroid is erroneous, and that the colony or individual, when alive, was a more or less radially symmetrical floating form, like a Medusa, of which only the distal appendages (possibly tentacles) are commonly preserved as fossils.

The evidence that the Graptolites were Hydrozoa is in reality very slight, but the proof of their relationship to any other phylum of the animal kingdom does not exist.[320] It is therefore convenient to consider them in this place, and to regard them, provisionally, as related to the Calyptoblastea.

The order is divided into three families.

Fam. 1. Monoprionidae.—Cups arranged uniserially on one side of the axis.

The principal genera are Monograptus, with the axis straight, curved, or helicoid, from many horizons in the Silurian strata; Rastrites, with a spirally coiled axis, Silurian; Didymograptus, Ordovician; and Coenograptus, Ordovician.

Fam. 2. Diprionidae.—Cups arranged in two or four vertical rows on the axis.

Diplograptus, Ordovician and Silurian; Climacograptus, Ordovician and Silurian; and Phyllograptus, in which the axis and cups are arranged in such a manner that they resemble an ovate leaf.

Fam. 3. Retiolitidae.—Cups arranged biserially on a reticulate axis.

Retiolites, Ordovician and Silurian; Stomatograptus, Retiograptus, and Glossograptus, Ordovician.

Fossil Corals possibly allied to Hydrozoa.

Among the many fossil corals that are usually classified with the Hydrozoa the genus Porosphaera is of interest as it is often supposed to be related to Millepora. It consists of globular masses about 10-20 mm. in diameter occurring in the Upper Cretaceous strata. In the centre there is usually a foreign body around which the coral was formed by concentric encrusting growth. Running radially from pores on the surface to the centre, there are numerous tubules which have a certain general resemblance to the pore-tubes of Millepora. The monomorphic {283}character of these tubes, their very minute size, the absence of ampullae, and the general texture of the corallum, are characters which separate this fossil very distinctly from any recent Hydroid corals. Porosphaera, therefore, was probably not a Hydrozoon, and certainly not related to the recent Millepora.

Closely related to Porosphaera apparently are other globular, ellipsoidal, or fusiform corals from various strata, such as Loftusia from the Eocene of Persia, Parkeria from the Cambridge Greensand, and Heterastridium from the Alpine Trias. In the last named there is apparently a dimorphism of the radial tubes.

Allied to these genera, again, but occurring in the form of thick, concentric, calcareous lamellae, are the genera Ellipsactinia and Sphaeractinia from the Upper Jurassic.

Another important series of fossil corals is that of the family Stromatoporidae. These fossils are found in great beds of immense extent in many of the Palaeozoic rocks, and must have played an important part in the geological processes of that period. They consist of a series of calcareous lamellae, separated by considerable intervals, encrusting foreign bodies of various kinds. Sometimes they are flat and plate-like, sometimes globular or nodular in form. The lamellae are in some cases perforated by tabulate, vertical, or radial pores, but in many others these pores are absent. The zoological position of the Stromatoporidae is very uncertain, but there is not at present any very conclusive evidence that they are Hydrozoa.

Stromatopora is common in Devonian and also occurs in Silurian strata. Cannopora from the Devonian has well-marked tabulate pores, and is often found associated commensally with another coral (Aulopora or Syringopora).

Order VI. Stylasterina.

The genera included in this order resemble Millepora in producing a massive calcareous skeleton, and in showing a consistent dimorphism of the zooids, but in many respects they exhibit great divergence from the characters of the Milleporina.

The colony is arborescent in growth, the branches arising frequently only in one plane, forming a flabellum. The calcareous skeleton is perforated to a considerable depth by the gastrozooids, dactylozooids, and nutritive canals, and the {284}gastropores and dactylopores are not provided with tabulae except in the genera Pliobothrus and Sporadopora. The character which gives the order its name is a conical, sometimes torch-like projection at the base of the gastropore, called the "style," which carries a fold of the ectoderm and endoderm layers of the body-wall, and may serve to increase the absorptive surface of the digestive cavity. In some genera a style is also present in the dactylopore, in which case it serves as an additional surface for the attachment of the retractor muscles. The pores are scattered on all aspects of the coral in the genera Sporadopora, Errina, and Pliobothrus; in Spinipora and Steganopora the scattered dactylopores are situated at the extremities of tubular spines which project from the general surface of the coral, the gastropores being situated irregularly between the spines. In Phalangopora the pores are arranged in regular longitudinal lines, and in Distichopora they are mainly in rows on the edges of the flattened branches, a single row of gastropores being flanked by a single row of dactylopores on each side. In the remaining genera the pores are arranged in definite cycles, which are frequently separated from one another by considerable intervals, and have, particularly in the dried skeleton, a certain resemblance to the calices of some of the Zoantharian corals.

In Cryptohelia the cycles are covered by a lid-like projection from the neighbouring coenenchym (Fig. 136, l 1, l 2). The gastrozooids are short, and are usually provided with a variable number of small capitate tentacles. The dactylozooids are filiform and devoid of tentacles, the endoderm of their axes being solid and scalariform.

The gonophores of the Stylasterina are situated in large oval or spherical cavities called the ampullae, and their presence can generally be detected by the dome-shaped projections they form on the surface of the coral. The female gonophore consists of a saucer-shaped pad of folded endoderm called the "trophodisc," which serves the purpose of nourishing the single large yolk-laden egg it bears; and a thin enveloping membrane composed of at least two layers of cells. The egg is fertilised while it is still within the ampulla, and does not escape to the exterior until it has reached the stage of a solid ciliated larva. All the Stylasterina are therefore viviparous. The male gonophore has a very much smaller trophodisc, which is sometimes (Allopora) prolonged into a columnar process or spadix, penetrating the {285}greater part of the gonad. The spermatozoa escape through a peculiar spout-like duct which perforates the superficial wall of the ampulla. In some genera (Distichopora) there are several male gonophores in each ampulla.

The gonophores of the Stylasterina have been regarded as much altered medusiform gonophores, and this view may possibly prove to be correct. At present, however, the evidence of their derivation from Medusae is not conclusive, and it is possible that they may have had a totally independent origin.

Distichopora and some species of Stylaster are found in shallow water in the tropics, but most of the genera are confined to deep or very deep water, and have a wide geographical distribution. No species have been found hitherto within the British area.

fig136

Fig. 136.—A portion of a branch of Cryptohelia ramosa, showing the lids l 1 and l 2 covering the cyclosystems, the swellings produced by the ampullae in the lids amp1, amp2, and the dactylozooids, dac. × 22. (After Hickson and England.)

A few specimens of a species of Stylaster have been found in Tertiary deposits and in some raised beaches of more recent origin, but the order is not represented in the older strata.

Fam. Stylasteridae.—All the genera at present known are included in this family.

Sporadopora is the only genus that presents a superficial general resemblance to Millepora. It forms massive, branching white coralla, with the pores scattered irregularly on the surface, and, like many varieties of Millepora, not arranged in cyclosystems. It may, however, be distinguished at once by the presence of a long, brush-like style in each of the gastropores. The ampullae are large, but are usually so deep-seated in the coenenchym that their presence cannot be detected from the surface. It was found off the Rio de la Plata in 600 fathoms of water by the "Challenger."

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In Errina the pores are sometimes irregularly scattered, but in E. glabra they are arranged in rows on the sides of the branches, while in E. ramosa the gastropores occur at the angles of the branches only. The dactylopores are situated on nariform projections of the corallum. The ampullae are prominent. There are several gonophores in each ampulla of the male, but only one in each ampulla of the female. This genus is very widely distributed in water from 100 to 500 fathoms in depth.

Phalangopora differs from Errina in the absence of a style in the gastropore; Mauritius.—Pliobothrus has also no style in the gastropore, and is found in 100-600 fathoms of water off the American Atlantic shores.

Distichopora is an important genus, which is found in nearly all the shallow seas of the tropical and semi-tropical parts of the world, and may even flourish in rock pools between tide marks. It is nearly always brightly coloured—purple, violet, pale brown, or rose red. The colony usually forms a small flabellum, with anastomosing branches, and the pores are arranged in three rows, a middle row of gastropores and two lateral rows of dactylopores on the sides of the branches. There is a long style in each gastropore. The ampullae are numerous and prominent, situated on the anterior and posterior faces of the branches. Each ampulla contains a single gonophore in the female colony and two or three gonophores in the male colony.

Spinipora is a rare genus from off the Rio de la Plata in 600 fathoms. The branches are covered with blunt spines. These spines have a short gutter-like groove at the apex, which leads into a dactylopore. The gastropores are provided with a style and are situated between the spines.

Steganopora[321] from the Djilolo Passage, in about 600 fathoms, is very similar to Spinipora as regards external features, but differs from it in the absence of styles in the gastropores, and in the wide communications between the gastropores and dactylopores.

Stylaster is the largest and most widely distributed genus of the family, and exhibits a considerable range of structure in the many species it contains. It is found in all the warmer seas of the world, living between tide marks at a few fathoms, and extending to depths of 600 fathoms. Many specimens, but especially those from very shallow water, are of a beautiful rose {287}or pink colour. The corallum is arborescent and usually flabelliform. The pores are distributed in regular cyclosystems, sometimes on one face of the corallum only, sometimes on the sides of the branches, and sometimes evenly distributed. There are styles in both gastropores and dactylopores.

Allopora is difficult to separate from Stylaster, but the species are usually more robust in habit, and the ampullae are not so prominent as they are on the more delicate branches of Stylaster. It occurs at depths of 100 fathoms in the Norwegian fjords. A very large red species (A. nobilis) occurs in False Bay, Cape of Good Hope, in 30 fathoms of water. In this locality the coral occurs in great submarine beds or forests, and the trawl that is passed over them is torn to pieces by the hard, thick branches, some of which are an inch or more in diameter.

Astylus is a genus found in the southern Philippine sea in 500 fathoms of water. It is distinguished from Stylaster by the absence of a style in the gastropore.

Cryptohelia is an interesting genus found both in the Atlantic and Pacific Oceans at depths of from 270 to about 600 fathoms. The cyclosystems are covered by a projecting lid or operculum (Fig. 136, l 1, l 2). There are no styles in either the gastropores or the dactylopores. The ampullae are prominent, and are sometimes situated in the lids. There are several gonophores in each ampulla of the female colony, and a great many in the ampulla of the male colony.

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CHAPTER XI

HYDROZOA (CONTINUED): TRACHOMEDUSAE—NARCOMEDUSAE—SIPHONOPHORA

Order VII. Trachomedusae.

The orders Trachomedusae and Narcomedusae are probably closely related to one another and to some of the families of Medusae at present included in the order Calyptoblastea, and it seems probable that when the life-histories of a few more genera are made known the three orders will be united into one. Very little is known of the hydrosome stage of the Trachomedusae, but Brooks[322] has shown that in Liriope, and Murbach[323] that in Gonionema, the fertilised ovum gives rise to a Hydra-like form, and in the latter this exhibits a process of reproduction by gemmation before it gives rise to Medusae. Any general statement, therefore, to the effect that the development of the Trachomedusae is direct would be incorrect. The fact that the hydrosomes already known are epizoic or free-swimming does not afford a character of importance for distinction from the Leptomedusae, for it is quite possible that in this order of Medusae the hydrosomes of many genera may be similar in form and habits to those of Liriope and Gonionema.

The free border of the umbrella of the Trachomedusae is entire; that is to say, it is not lobed or fringed as it is in the Narcomedusae. The sense-organs are statocysts, each consisting of a vesicle formed by a more or less complete fold of the surrounding wall of the margin of the umbrella, containing a reduced clapper-like tentacle loaded at its extremity with a statolith.

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fig137

Fig. 137.—Liriope rosacea, one of the Geryoniidae, from the west side of North and Central America. Size, 15-20 mm. Colour, rose. cp, Centripetal canal; gon, gonad; M, mouth at the end of a long manubrium; ot, statocyst; t, tentacle; to, tongue. (After Maas.)

This statocyst is innervated by the outer nerve ring. There appears to be a very marked difference between these marginal sense-organs in some of the best-known examples of Trachomedusae and the corresponding organs of the Leptomedusae. The absence of a stalk supporting the statolith and the innervation of the otocyst by the inner instead of by the outer nerve ring in the Leptomedusae form characters that may be of supplementary value, but cannot be regarded as absolutely distinguishing the two orders. The statorhab of the Trachomedusae is probably the more primitive of the two types, and represents a marginal tentacle of the umbrella reduced in size, loaded with a statolith and enclosed by the mesogloea. Intermediate stages between this type and an ordinary tentacle have already been discovered and described. In the type that is usually found in the Leptomedusae the modified tentacle is still further reduced, and all that can be recognised of it is the statolith attached to the wall of the statocyst, but intermediate stages between the two types are seen in the family Olindiidae, in which the stalk supporting the statolith passes gradually into the tissue surrounding the statolith on the one hand and the vesicle wall on the other. The radial canals are four or eight in number or more numerous. They communicate at the margin of the umbrella with a ring canal from which a number of short blind tubes run in the umbrella-wall towards the centre of the Medusa (Fig. 137, cp). These "centripetal canals" are subject to {290}considerable variation, but are useful characters in distinguishing the Trachomedusae from the Leptomedusae. The tentacles are situated on the margin of the umbrella, and are four or eight in number or, in some cases, more numerous. The gonads are situated as in Leptomedusae on the sub-umbrella aspect of the radial canals.

In Gonionema murbachii the fertilised eggs give rise to a free-swimming ciliated larva of an oval shape with one pole longer and narrower than the other. The mouth appears subsequently at the narrower pole. The larva settles down upon the broader pole, the mouth appears at the free extremity, and in a few days two, and later two more, tentacles are formed (Fig. 138).

At this stage the larva may be said to be Hydra-like in character, and as shown in Fig. 138 it feeds and lives an independent existence. From its body-wall buds arise which separate from the parent and give rise to similar Hydra-like individuals. An asexual generation thus gives rise to new individuals by gemmation as in the hydrosome of the Calyptoblastea. The origin of the Medusae from this Hydra-like stage has not been satisfactorily determined, but it seems probable that by a process of metamorphosis the hydriform persons are directly changed into the Medusae.[324]

fig138

Fig. 138.—Hydra-like stage in the development of Gonionema murbachii. One of the tentacles is carrying a worm (W) to the mouth. The tentacles are shown very much contracted, but they are capable of extending to a length of 2 mm. Height of zooid about 1 mm. (After Perkins.)

In the development of Liriope the free-swimming larva develops into a hydriform person with four tentacles and an enormously elongated hypostome or manubrium; and, according to Brooks, it undergoes a metamorphosis which directly converts it into a Medusa.

There can be very little doubt that in a large number of Trachomedusae the development is direct, the fertilised ovum giving rise to a medusome without the intervention of a hydrosome stage. In some cases, however (Geryonia, etc.), the tentacles {291}appear in development before there is any trace of a sub-umbrella cavity, and this has been interpreted to be a transitory but definite Hydroid stage. It may be supposed that the elimination of the hydrosome stage in these Coelenterates may be associated with their adaptation to a life in the ocean far from the coast.

During the growth of the Medusa from the younger to the adult stages several changes probably occur of a not unimportant character, and it may prove that several genera now placed in the same or even different families are stages in the development, of the same species. In the development of Liriantha appendiculata,[325] for example, four interradial tentacles appear in the first stage which disappear and are replaced by four radial tentacles in the second stage.

As with many other groups of free-swimming marine animals the Trachomedusae have a very wide geographical distribution, and some genera may prove to be almost cosmopolitan, but the majority of the species appear to be characteristic of the warmer regions of the high seas. Sometimes they are found at the surface, but more usually they swim at a depth of a few fathoms to a hundred or more from the surface. The Pectyllidae appear to be confined to the bottom of the sea at great depths.

The principal families of the Trachomedusae are:—

Fam. Olindiidae.—This family appears to be structurally and in development most closely related to the Leptomedusae, and is indeed regarded by Goto[326] as closely related to the Eucopidae in that order. They have two sets of tentacles, velar and exumbrellar; the statocysts are numerous, two on each side of the exumbrellar tentacles. Radial canals four or six. Manubrium well developed and quadrate, with distinct lips. There is an adhesive disc on each exumbrellar tentacle.

Genera: Olindias, Olindioides, Gonionema (Fig. 139), and Halicalyx.

As in other families of Medusae the distribution of the genera is very wide. Olindias mülleri occurs in the Mediterranean, Olindioides formosa off the coast of Japan, Gonionema murbachii is found in abundance in the eel pond at Wood's Holl, United States of America, and Halicalyx off Florida.

Two genera may be referred to in this place, although their {292}systematic position in relation to each other and to other Medusae has not been satisfactorily determined.

fig139

Fig. 139.—Gonionema murbachii. Adult Medusa, shown inverted, and clinging to the bottom. Nat. size. (After Perkins.)

Limnocodium sowerbyi is a small Medusa that was first discovered in the Victoria regia tanks in the Botanic Gardens, Regent's Park, London, in the year 1880. It has lately made its appearance in the Victoria regia tank in the Parc de la Bête d'Or at Lyons.[327] As it was, at the time of its discovery, the only fresh-water jelly-fish known, it excited considerable interest, and this interest was not diminished when the peculiarities of its structure were described by Lankester and others. It has a rather flattened umbrella, with entire margin and numerous marginal tentacles, the manubrium is long, quadrate, and has four distinct lips. There are four radial canals, and the male gonads (all the specimens discovered were of the male sex) are sac-like bodies on the sub-umbrellar aspect of the middle points of the four radial canals. In these characters the genus shows general affinities with the Olindiidae. The difficult question of the origin of the statoliths from the primary germ layers of the embryo and some other points in the minute anatomy of the Medusa have {293}suggested the view that Limnocodium is not properly placed in any of the other orders. Goto,[328] however, in a recent paper, confirms the view of the affinities of Limnocodium with the Olindiidae.

The life-history of Limnocodium is not known, but a curious Hydroid form attached to Pontederia roots was found in the same tank as the Medusae, and this in all probability represents the hydrosome stage of its development. The Medusae are formed apparently by a process of transverse fission of the Hydroid stock[329] similar in some respects to that observed in the production of certain Acraspedote Medusae. This is quite unlike the asexual mode of formation of Medusae in any other Craspedote form. The structure of this hydrosome is, moreover, very different to that of any other Hydroid, and consequently the relations of the genus with the Trachomedusae cannot be regarded as very close.

Limnocodium has only been found in the somewhat artificial conditions of the tanks in botanical gardens, and its native locality is not known, but its association with the Victoria regia water-lily seems to indicate that its home is in tropical South America.

Limnocnida tanganyicae is another remarkable fresh-water Medusa, about seven-eights of an inch in diameter, found in the lakes Tanganyika and Victoria Nyanza of Central Africa.[330] It differs from Limnocodium in having a short collar-like manubrium with a large round mouth two-thirds the diameter of the umbrella, and in several other not unimportant particulars. It produces in May and June a large number of Medusa-buds by gemmation on the manubrium, and in August and September the sexual organs are formed in the same situation.

fig140

Fig. 140.—Limnocnida tanganyicae. × 2. (After Günther.)

The fixed hydrosome stage, if such a stage occurs in the life-history, has not been discovered; but Mr. Moore[331] believes that {294}the development is direct from ciliated planulae to the Medusae. The occurrence of Limnocnida in Lake Tanganyika is supposed by the same authority to afford a strong support to the view that this lake represents the remnants of a sea which in Jurassic times spread over part of the African continent. This theory has, however, been adversely criticised from several sides.[332]

The character of the manubrium and the position of the sexual cells suggest that Limnocnida has affinities with the Narcomedusae or Anthomedusae, but the marginal sense-organs and the number and position of the tentacles, showing considerable similarity with those of Limnocodium, justify the more convenient plan of placing the two genera in the same family.

Fam. Petasidae.—The genus Petasus is a small Medusa with four radial canals, four gonads, four tentacles, and four free marginal statorhabs. A few other genera associated with Petasus show simple characters as regards the canals and the marginal organs, but as very little is known of any of the genera the family may be regarded as provisional only. Petasus is found in the Mediterranean and off the Canaries.

Fam. Trachynemidae.—In this family there are eight radial canals, and the statorhabs are sunk into a marginal vesicle. Trachynema, characterised by its very long manubrium, is a not uncommon Medusa of the Mediterranean and the eastern Atlantic Ocean. Many of the species are small, but T. funerarium has sometimes a disc two inches in diameter. Homoconema and Pentachogon have numerous very short tentacles.

Fam. Pectyllidae.—This family contains a few deep-sea species with characters similar to those of the preceding family, but the tentacles are provided with terminal suckers. Pectyllis is found in the Atlantic Ocean at depths of over 1000 fathoms.

Fam. Aglauridae.—The radial canals are eight in number and the statorhabs are usually free. In the manubrium there is a rod-like projection of the mesogloea from the aboral wall of the gastric cavity, covered by a thin epithelium of endoderm, which occupies a considerable portion of the lumen of the manubrium. This organ may be called the tongue. Aglaura has an octagonal umbrella, and a manubrium which does not project beyond the velum. It occurs in the Atlantic Ocean and Mediterranean Sea.

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Fam. Geryoniidae.—In this family there are four or six radial canals, the statorhabs are sunk in the mesogloea, and a tongue is present in the manubrium. Liriope (Fig. 137) is sometimes as much as three inches in diameter. It has a very long manubrium, and the tongue sometimes projects beyond the mouth. There are four very long radial tentacles. It is found in the Atlantic Ocean, the Mediterranean Sea, and the Pacific and Indian Oceans. Geryonia has a wider geographical distribution than Liriope, and is sometimes four inches in diameter. It differs from Liriope in having six, or a multiple of six, radial canals. Carmarina of the Mediterranean and other seas becomes larger even than Geryonia, from which it differs in the arrangement of the centripetal canals.

Liriantha appendiculata sometimes occurs on the south coast of England during September, October, or at other times.

Order VIII. Narcomedusae.

The Narcomedusae differ from the Trachomedusae in having the margin of the umbrella divided into a number of lobes, and in bearing the gonads on the sub-umbrellar wall of the gastral cavity instead of upon the radial canals. The tentacles are situated at some little distance from the margin of the umbrella at points on the aboral surface corresponding with the angles between the umbrella lobes. Between the base of the tentacle and the marginal angle there is a tract of modified epithelium called the "peronium." The manubrium is usually short, and the mouth leads into an expanded gastral chamber which is provided with lobular diverticula reaching as far as the bases of the tentacles. The marginal sense-organs are in the form of unprotected statorhabs. Very little is known concerning the life-history of any of the Narcomedusae. In Cunoctantha octonaria the peculiar ciliated larva with two tentacles and a very long proboscis soon develops two more tentacles and creeps into the bell of the Anthomedusan Turritopsis, where, attached by its tentacles, it lives a parasitic life. Before being converted into a Medusa it gives rise by gemmation to a number of similar individuals, all of which become, in time, Medusae. The parasitic stage is often regarded as the representative of the hydrosome stage reduced and adapted to the oceanic habit of the adult.

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In Cunina proboscidea, and in some other species, a very remarkable method of reproduction has been described by Metschnikoff, called by him "sporogony." In these cases young sexual cells (male or female) wander from the gonad of the parent into the mesogloea of the umbrella, where they develop parthenogenetically into ciliated morulae. These escape by the radial canals into the gastric cavity, and there form a stolon from which young Medusae are formed by gemmation. In C. proboscidea these young Medusae are like the genus Solmaris, but in C. rhododactyla they have the form of the parent. In some cases the ciliated larvae leave the parent altogether and become attached to a Geryonia or some other Medusa, where they form the stolon.

This very interesting method of reproduction cannot be regarded as a primitive one, and throws no light on the origin of the order. It might be regarded as a further stage in the degeneration of the hydrosome stage in its adaptation to a parasitic existence.

The Narcomedusae have a wide geographical distribution. Species of Aeginopsis occur in the White Sea and Bering Strait, but the genera are more characteristic of warmer waters. Some species occur in moderately deep water, and Cunarcha was found in 1675 fathoms off the Canaries, but they are more usually found at or near the surface of the sea.

Fam. Cunanthidae.—Narcomedusae with large gastral diverticula corresponding in position with the bases of the tentacles. Cunina and Cunoctantha, occurring in the Mediterranean and in the Atlantic and Pacific Oceans, belong to this family. In Cunina the tentacles may be eight in number, or some multiple of four between eight and twenty-four. In Cunoctantha the number of tentacles appears to be constantly eight.

Fam. Peganthidae.—There appear to be no gastral pouches in this family. The species of Pegantha are found at depths of about 80 fathoms in the Indian and Pacific Oceans.

Fam. Aeginidae.—The large gastral pouches of this family alternate with the bases of the tentacles. Aegina occurs in the Atlantic and Pacific Oceans. Aeginopsis.

Fam. Solmaridae.—In this family the gastral pouches are variable, sometimes corresponding with, sometimes alternating with, the bases of the tentacles. The circular canal is represented {297}in some genera by solid cords of endoderm. Solmaris sometimes appears in the English Channel, but it is probably a wanderer from the warmer regions of the Atlantic Ocean. It is found in abundance during November on the west coast of Ireland.

Order IX. Siphonophora.

In this order the naturalist finds collected together a number of very beautiful, delicate transparent organisms to which the general term "jelly-fish" may be applied, although their organisation is far more complicated and difficult to describe than that of any of the Medusae. In several of the Hydrozoa the phenomenon of dimorphism has already been noticed. In these cases one set of individuals in a colony performs functions of stinging and catching food and another the functions of devouring and digesting it. In many of the Siphonophora there appears to be a colony of individuals in which the division of labour is carried to a much further extent than it is in the dimorphic Hydrozoa referred to above. Not only are there specialised gastrozooids and dactylozooids, but also gonozooids, zooids for propelling the colony through the water ("nectocalyces"), protective zooids ("hydrophyllia"), and in some cases a specialised zooid for hydrostatic functions; the whole forming a swimming or floating polymorphic colony. But this conception of the construction of the Siphonophora is not the only one that has met with support. By some zoologists the Siphonophoran body is regarded not as a colony of individuals, but as a single individual in which the various organs have become multiplied and dislocated.

The multiplication or repetition of organs that are usually single in each individual is not unknown in other Hydrozoa. In the Medusa of the Gymnoblast Syncoryne, usually known as Sarsia, for example, there is sometimes a remarkable proliferation of the manubrium, and specimens have been found with three or four long manubria attached by a tubular stalk to the centre of the umbrella. Moreover, this complex of manubria may become detached from the umbrella and live for a considerable time an independent existence.[333]

If we regard the manubrium of a Medusa as an organ of the {298}animal's body, it might be thought obvious that the phenomenon observed in the Medusae of Syncoryne is a case of a simple repetition of the parts of an individual; but the power that the group of manubria possesses of leading an independent existence renders its interpretation as a group of organs a matter of some inconvenience. If we can conceive the idea that an organ may become detached and lead an independent existence, there is no reason why we should not regard the Medusa itself of Syncoryne as an organ, and we should be driven to the paradoxical conclusion that, as regards several genera and families of Hydrozoa, we know nothing at present of the individuals, but only of their free-swimming organs, and that in others the individual has degenerated, although one of its organs remains.

There is, however, no convincing argument to support either the conception that the Siphonophoran body is a colony of individuals, or that it is an individual with disjointed organs. These two conceptions are sometimes called the "Poly-person" and "Poly-organ" theories respectively. The difficulty is caused by the impossibility of giving any satisfactory definition in the case of the Hydrozoa of the biological terms "organ" and "individual." In the higher animals, where the correlation of parts is far more complex and essential than it is in Coelenterata, a defined limit to the scope of these terms can be laid down, but in the lower animals the conception of what is termed an organ merges into that which is called an individual, and no definite boundary line between the two exists in Nature. The difficulty is therefore a permanent one, and, in using the expression "colony" for the Siphonophoran body, it must be understood that it is used for convenience' sake rather than because it represents the only correct conception of the organisation of these remarkable Coelenterates.

Regarding the Siphonophora as polymorphic colonies, then, the following forms of zooids may be found.

Nectocalyces.—The nectocalyces are in the form of the umbrella of a medusa attached to the stolon of the colony by the aboral pole. They are provided with a velum and, usually, four radial canals and a circular canal. There is no manubrium, and the marginal tentacles and sense-organs are rudimentary or absent. There may be one or more nectocalyces in each colony, {299}and their function is, by rhythmic contractions, to propel the colony through the water (Fig. 142, N).

Gastrozooids.—These are tubular or saccular zooids provided with a mouth and attached by their aboral extremity to the stolon (Fig. 142, G). In some cases the aboral region of the zooid is differentiated as a stomach. It is dilated and bears the digestive cells, the oral extremity or hypostome being narrower and more transparent. In some cases the mouth is a simple round aperture at the extremity of the hypostome, but in others it is dilated to form a trumpet-like lip.

Dactylozooids.—In Velella and Porpita the dactylozooids are similar in general characters to the tentacles of many Medusae. They are arranged as a frill round the margin of the colony, and each consists of a simple tube of ectoderm and endoderm terminating in a knobbed extremity richly provided with nematocysts.

In many other Siphonophora, however, the dactylozooids are very long and elaborate filaments, which extend for a great distance from the colony into the sea. They reach their most elaborate condition in the Calycophorae.

fig141

Fig. 141.—A small Crustacean (Rhinocalanus) caught by a terminal filament (f.t) of a battery of Stephanophyes. b, The proximal end of the battery with the most powerful nematocysts; e, elastic band; S, stalk supporting the battery on the dactylozooid. (After Chun.)

The dactylozooid in these forms has a hollow axis, and the lumen is continuous with the cavity of the neighbouring gastrozooid. Arranged at regular intervals on the axis is a series of tentacles ("tentilla"), and each of these supports {300}a kidney-shaped swelling, the "cnidosac," or battery, which is sometimes protected by a hood. Each battery contains an enormous number of nematocysts. In Stephanophyes, for example, there are about 1700 nematocysts of four different kinds in each battery. At the extremity of the battery there is a delicate terminal filament. The action of the battery in Stephanophyes is, according to Chun,[334] a very complicated one. The terminal filament lassos the prey and discharges its somewhat feeble nematocysts at it (Fig. 141). If this kills it, the dactylozooid contracts and passes the prey to a gastrozooid. If the animal continues its struggles, it is drawn up to the distal end of the battery and receives the discharge of a large number of nematocysts; and if this also fails to put an end to its life, a membrane covering the largest and most powerful nematocysts at the proximal end of the whole battery is ruptured, and a final broadside of stinging threads is shot at it.

The larger nematocysts of these batteries in the Siphonophora are among the largest found in Coelenterata, being from 0.5 to 0.1 mm. in length, and they are frequently capable of inflicting painful stings on the human skin. The species of Physalia, commonly called "Portuguese Men-of-War," have perhaps the worst reputation in this respect, the pain being not only intense but lasting a long time.

Hydrophyllia.—In many Siphonophora a number of short, mouthless, non-sexual zooids occur, which appear to have no other function than that of shielding or protecting other and more vital parts of the colony. They consist of an axis of firm mesogloea, covered by a layer of flattened ectoderm, and they may be finger-shaped or triangular in form. In Agalma and Praya an endoderm canal perforates the mesogloea and terminates in a little mouth at the free extremity. In Athoria and Rhodophysa the hydrophyllium terminates in a little nectocalyx.

Pneumatophore.—In all the Siphonophora, with the exception of the Calycophorae, there is found on one side or at one extremity of the colony a vesicle or bladder containing a gas,[335] which serves as a float to support the colony in the water. {301}This bladder or pneumatophore is probably in all cases a much modified nectocalyx. It shows great variations in size and structure in the group. It is sometimes relatively very large, as in Physalia and Velella, sometimes very small, as in Physophora. It is provided with an apical pore in some genera (Rhizophysa), or a basal pore in others (Auronectidae), but it is generally closed. In the many chambered pneumatophore of the Chondrophoridae there are several pores.

In many forms two distinct parts of the pneumatophore can be recognised—a distal region lined by chitin,[336] probably representing the sub-umbrellar cavity of the nectocalyx, and a small funnel-shaped region lined by an epithelium, the homology of which is a matter of dispute. It is believed that the gas is secreted by this epithelium. In the Auronectidae the region with secretory epithelium is relatively large and of a more complicated histological character. It is remarkable also that in this family the pore communicates, not with the chitin-lined region, but directly with the epithelium-lined region.

There is no pneumatophore in the Calycophorae, but in this sub-order a diverticulum of an endoderm canal secretes a globule of oil which may serve the same hydrostatic function.

The stolon is the common stem which supports the different zooids of the colony. In the Calycophorae the stolon is a long, delicate, and extremely contractile thread attached at one end to a nectocalyx, and bearing the zooids in discontinuous groups. These groups of zooids arranged at intervals on the stolon are called the "cormidia." The stolon is a tube with very thick walls. Its lumen is lined by a ciliated endoderm with circular muscular processes, and the surface is covered with an ectoderm, also provided with circular muscular processes. Between these two layers there is a relatively thick mesogloea showing on the outer side deep and compound folds and grooves supporting an elaborate system of longitudinal muscular fibres. In many Physonectidae the stolon is long and filamentous, but not so contractile as it is in Calycophorae, but in others it is much reduced in length and relatively stouter. The reduction {302}in length of the stolon is accompanied by a complication of structure, the simple tubular condition being replaced by a spongy complex of tubes covered by a common sheath of ectoderm. In the Auronectidae the stolon is represented by a conical or hemispherical spongy mass bearing the zooids, and in the Rhizophysaliidae and Chondrophoridae it becomes a disc or ribbon-shaped pad spreading over the under side of the pneumatophore.

Gonozooids.—The gonozooids are simple tubular processes attached to the stolon which bear the Medusae or the degenerate medusiform gonophores. In the Chondrophoridae the gonozooids possess a mouth, but in most Siphonophora they have neither mouth nor tentacles. In some cases, such as Anthophysa, the colonies are bisexual—the male and female gonophores being borne by separate gonozooids—but in others (e.g. Physalia) the colonies appear to be unisexual.

As a general rule the gonophores of Siphonophora do not escape from the parent colony as free-swimming Medusae, but an exception occurs in Velella, which produces a number of small free-swimming Medusae formerly described by Gegenbaur under the generic name Chrysomitra. This Medusa has a velum, a single tentacle, eight to sixteen radial canals, and it bears the gonads on the short manubrium. The Medusa of Velella has, in fact, the essential characters of the Anthomedusae.

Our knowledge of the life-history of the Siphonophora is very incomplete, but there are indications, from scattered observations, that in some genera, at least, it may be very complicated.

The fertilised ovum of Velella gives rise to a planula which sinks to the bottom of the sea, and changes into a remarkable larva known as the Conaria larva. This larva was discovered by Woltereck[337] at depths of 600-1000 metres in great numbers. It is very delicate and transparent, but the endoderm is red (the colour so characteristic of animals inhabiting deep water), and it may be regarded as essentially a deep-sea larva. The larva rises to the surface and changes into the form known as the Ratarula larva, which has a simple one-chambered pneumatophore containing a gas, and a rudiment of the sail. In contrast to the Conaria, the Ratarula is blue in colour. With the development of the zooids on the under side of this {303}larva (i.e. the side opposite to the pneumatophore), a definite octoradial symmetry is shown, there being for some time eight dactylozooids and eight definite folds in the wall of the pneumatophore. This octoradial symmetry, however, is soon lost as the number of folds in the pneumatophore and the number of tentacles increase.

It is probable that in the Siphonophora, as in many other Coelenterata, the production of sexual cells by an individual is no sign that its life-history is completed. There may possibly be two or more phases of life in which sexual maturity is reached.

An example of a complicated life-history is found in the Calycophoran species Muggiaea kochii. The embryo gives rise to a form with a single nectocalyx which is like a Monophyes, and this by the budding of a second nectocalyx produces a form that has a remarkable resemblance to a Diphyes, but the primary nectocalyx degenerates and is cast off, while the secondary one assumes the characters of the single Muggiaea nectocalyx. The stolon of the Muggiaea produces a series of cormidia, and as the sexual cells of the cormidia develop, a special nectocalyx is formed at the base of each one of them, and the group of zooids is detached as an independent colony, formerly known as Eudoxia eschscholtzii. In a similar manner the cormidia of Doramasia picta give rise to the sexual free-swimming monogastric forms, known by the name Ersaea picta (Fig. 142). In these cases it seems possible that the production of ripe sexual cells is confined to the Eudoxia and Ersaea stages respectively, but it is probable that in other species the cormidia do not break off from the stolon, or may escape only from the older colonies.

fig142

Fig. 142.—Free-swimming Ersaea group of Doramasia picta. B, B, batteries of nematocysts borne by the tentilla; D, dactylozooid; G, gastrozooid; H, hydrophyllium; N, nectocalyx; O, oleocyst; f.t, terminal filament of a battery; t, t, tentilla. The gonozooid is hidden by the gastrozooid. × 10. (After Chun.)

The Siphonophora are essentially free-swimming pelagic {304}organisms. Some of them (Auronectidae) appear to have become adapted to a deep-sea habit, others are usually found in intermediate waters, but the majority occur with the pelagic plankton at or very near the surface of the open sea. Although the order may be said to be cosmopolitan in its distribution, the Siphonophora are only found in great numbers and variety in the sub-tropical and tropical zones. In the temperate and arctic zones they are relatively rare, but Galeolaria biloba and Physophora borealis appear to be true northern forms. The only British species are Muggiaea atlantica and Cupulita sarsii. Velella spirans occasionally drifts from the Atlantic on to our western shores, and sometimes great numbers of the pneumatophores of this species may be found cast up on the beach. Diphyes sp., Physalia sp., and Physophora borealis are also occasionally brought to the British shores by the Gulf Stream.

The Calycophorae are usually perfectly colourless and transparent, with the exception of the oil-globule in the oleocyst, which is yellow or orange in colour. Many of the other Siphonophora, however, are of a transparent, deep indigo blue colour, similar to that of many other components of the plankton.

Most of the Siphonophora, although, strictly speaking, surface animals, are habitually submerged; the large pneumatophores of Velella and Physalia, however, project above the surface, and these animals are therefore frequently drifted by the prevailing wind into large shoals, or blown ashore. At Mentone, on the Mediterranean, Velella is sometimes drifted into the harbour in countless numbers. Agassiz mentions the lines of deep blue Velellas drifted ashore on the coast of Florida; and a small species of blue Physalia may often be seen in long lines on the shore of some of the islands of the Malay Archipelago.

The food of most of the Siphonophora consists of small Crustacea and other minute organisms, but some of the larger forms are capable of catching and devouring fish. It is stated by Bigelow[338] that a big Physalia will capture and devour a full-grown Mackerel. The manner in which it feeds is described as follows:—"It floats on the sea, quietly waiting for some heedless individual to bump its head against one of the tentacles. The fish, on striking, is stung by the nettle-cells, and fastened probably by them to the tentacle. Trying to run away the fish pulls on the {305}tentacle. The tension on its peduncle thus produced acts as a stimulus on apparently some centre there which causes it to contract. The fish in this way is drawn up so that it touches the sticky mouths of the squirming siphons [i.e. gastrozooids]. As soon as the mouths, covered as they are with a gluey substance and provided with nettle-cells, touch the fish they stick fast, a few at first, and gradually more. The mouths open, and their lips are spread out over the fish until they touch, so that by the time he is dead the fish is enclosed in a tight bag composed of the lips of a dozen or more siphon mouths. Here the fish is digested. As it begins to disintegrate partially digested fragments are taken into the stomachs of the attached siphons (gastrozooids). When they have become gorged they detach themselves from the remains of the fish, the process of digestion is completed in the stomachs, and the nutrient fluid is distributed...."

In consequence of the very unsatisfactory state of our knowledge of the life-history of the Siphonophora the classification of the order is a matter of unusual difficulty.

Sub-Order I. Calycophorae.

The character which distinguishes this sub-order is the absence of a pneumatophore.

The colony usually consists of a long, slender, contractile stolon, provided at one end with one, two, or several nectocalyces. Upon the stolon are arranged several groups ("cormidia") of polymorphic zooids.

The nectocalyces have a well-developed velum, four radial canals, and a muscular umbrella-wall. A special peculiarity of the nectocalyx of this sub-order is a diverticulum (oleocyst) from one of the radial canals, containing a coloured globule of oil. The function of this oil-globule is probably similar to that of the pneumatophore, and assists the muscular efforts of the nectocalyces in keeping the colony afloat. One of the nectocalyces of each colony exhibits on one side a deep ectodermic fold, which is frequently converted into a pit. At the bottom of this pit is attached the end of the stolon, the whole of which with its numerous cormidia can be withdrawn into the shelter of the pit when danger threatens. The cormidia consist of at least four {306}kinds of zooids: a gastrozooid with a trumpet-shaped mouth armed with nematocysts, a long dactylozooid provided with a series of tentilla, and a rudimentary gonozooid bearing numbers of male or female medusiform gonophores. These three kinds of zooids are partially covered and protected by a bent shield-shaped phyllozooid or hydrophyllium.

Each of the cormidia is unisexual, but the colony as a whole is usually hermaphrodite, the male and female cormidia regularly alternating, or the male cormidia being arranged on the nectocalycine half and the female cormidia on the opposite half of the stolon.

The families of the Calycophorae are:—

Fam. 1. Monophyidae.—In this family there is a single conical or mitre-shaped nectocalyx. The cormidia become detached as free-swimming Eudoxia or Ersaea forms.

Sub-Fam. 1. Sphaeronectinae.—The primary nectocalyx persists throughout life—Monophyes and Sphaeronectes.

Sub-Fam. 2. Cymbonectinae.—The primary nectocalyx is thrown off, and is replaced by a secondary and permanent nectocalyx—Cymbonectes, Muggiaea, and Doramasia.

Fam. 2. Diphyidae.—The primary mitre-shaped nectocalyx is thrown off and replaced by two secondary rounded, prismatic, or pyramidal, heteromorphic nectocalyces.

This family contains several sub-families, which are arranged in two groups: the Diphyidae Oppositae, in which the two secondary bells are opposite one another, and do not exhibit pronounced ridges; and the Diphyidae Superpositae, in which one of the two secondary nectocalyces is situated in front of the other, and each nectocalyx is provided externally with very definite and often wing-like ridges. In all the Diphyidae Oppositae the cormidia remain attached, whereas in most of the Diphyidae Superpositae they become free-swimming, as in the Monophyidae.

The sub-families of the Diphyidae Oppositae are:—

Sub-Fam. 1. Amphicaryoninae.—One of the two secondary nectocalyces becomes flattened above to form a shield, and at the same time its sub-umbrellar cavity is atrophied, and its radial canals reduced. Mitrophyes, Atlantic Ocean.

Sub-Fam. 2. Prayinae.—The colony exhibits a pair of large, obtuse nectocalyces, with a relatively small sub-umbrellar cavity. Praya, Mediterranean and Atlantic.

{307}

Sub-Fam. 3. Desmophyinae.—The colony bears a large number of reserve or tertiary nectocalyces arranged in two rows. Desmophyes, Indian Ocean.

Sub-Fam. 4. Stephanophyinae.—There are four nectocalyces arranged in a horizontal plane. Each one of the cormidia bears a nectocalyx, which is periodically replaced. This sub-family is constituted for Stephanophyes superba from the Canary Islands. It attains a length of 25 cm., and is probably the largest and most beautiful of all the Calycophoridae.[339]

The group Diphyidae Superpositae contains the following:—

Sub-Fam. 1. Galeolarinae.—Galeolaria.

Sub-Fam. 2. Diphyopsinae.—Diphyes.

Sub-Fam. 3. Abylinae.—Abyla.

These sub-families differ from one another in the character and shape of the nectocalyces and in other characters. They have a world-wide distribution, Diphyes and Galeolaria extending north into the Arctic Seas. Diphyes is British.

Fam. 3. Polyphyidae.—The nectocalyces are numerous, and superposed in two rows. The cormidia remain attached.

The family contains the genera Polyphyes and Hippopodius, both probably cosmopolitan in warm waters.

Sub-Order II. Physophorae.

In this sub-order the primary nectocalyx gives rise to a definite pneumatophore. There are four families.

Fam. 1. Physonectidae.—In this, the largest family of the sub-order, there is a monothalamic pneumatophore supporting a stolon, which in some forms is of great length, but in others is reduced to a stump or pad, on which there are usually found several nectocalyces, hydrophyllia, gastrozooids, gonozooids, and tentilla.

The principal sub-families are:—

Agalminae.—With a long stolon, bearing at the upper end (i.e. the end next to the pneumatophore) two rows of nectocalyces. The other zooids are arranged in cormidia on the stolon, each covered by a hydrophyllium. Dactylozooids with tentilla. Agalma and Cupulita, Mediterranean Sea.

Apoleminae.—Similar to the above, but without tentilla. {308}Apolemia—this genus attains a length of two or three metres. Mediterranean Sea. Dicymba, Indian Ocean.

Physophorinae.—The pneumatophore larger in proportion than it is in the preceding families. The stolon is short, and bears rows of nectocalyces at the upper end. The gastrozooids, dactylozooids, and gonozooids are arranged in verticils on the lower expanded part of the stolon. Hydrophyllia absent. Physophora, cosmopolitan in the areas of warm sea water.

Fam. 2. Auronectidae.—The pneumatophore is large. The stolon is reduced to a spongy mass of tissue on the under side of the pneumatophore, and this bears numerous cormidia arranged in a helicoid spiral. Projecting from the base of the pneumatophore there is a peculiar organ called the "aurophore," provided with an apical pore. This organ has been described as a specially modified nectocalyx, but it is probably a specialised development of the epithelium-lined portion of the pneumatophore of other Physophorae. The Auronectidae are found only at considerable depths, 300 to 1400 fathoms, and are probably specially adapted to that habitat. Rhodalia, Stephalia, Atlantic Ocean.

Fam. 3. Rhizophysaliidae.—The pneumatophore is large, or very large, in this family. The zooids are arranged in horizontal rows on the under side of the pneumatophore (Physalia), or in a helicoid spiral on a short stolon (Epibulia). There are no nectocalyces nor hydrophyllia.

The genus Physalia is the notorious "Portuguese Man-of-War." The pneumatophore is a large bladder-like vesicle, sometimes attaining a length of 12 cm. One species described by Haeckel under the generic name Caravella has a pneumatophore 30 cm. and more in length, and dactylozooids attaining a length of 20 metres. It is a curious fact that only the male colonies of Physalia are known, and it is suggested that the female may have quite a different form.[340] Epibulia has a much smaller bladder than Physalia. Both genera have a cosmopolitan distribution at the surface of the warm seas.

Fam. 4. Chondrophoridae.—This family stands quite by itself in the sub-order Physophorae, and is placed in a separate division of the sub-order by Chun, who gives it the name Tracheophysa. The essential distinguishing characters of the family are {309}the large polythalamic pneumatophore and the single large central gastrozooid.

The colony is disc-shaped, and has a superficial resemblance to a Medusa. On the upper side is the flattened pneumatophore, covered by a fold of tissue continuous with that at the edge of the disc. In Velella a vertical triangular sail or crest rises from the upper side, but this is absent in Porpita.

The mouth of the gastrozooid opens into a large digestive cavity, and between this and the under surface of the pneumatophore there is a glandular spongy tissue called the liver. The liver extends over the whole of the under side of the pneumatophore, and sends processes round the edge of the disc into the tissues of its upper surface. Intimately associated with the liver, and penetrating its interstices, is an organ which appears to be entirely composed of nematocysts, derived from the ectoderm, and called the central organ. At the margin of the disc there is a fringe of simple digitiform dactylozooids, and between the dactylozooids and the centrally placed gastrozooid are numerous gonozooids. Each of the gonozooids is provided with a distinct mouth, and bears the gonophores, which escape before the ripening of the gonads as the free-swimming Medusae called Chrysomitra. The pneumatophore consists of a number of annular chambers arranged in a concentric manner round the central original chamber formed from a modified zooid. These annular chambers are in communication with one another, and have each two pores (pneumatopyles) opening above to the exterior. The most remarkable feature, however, of the system is a series of fine branching tubes ("tracheae"), which pass from the annular chambers of the pneumatophore downwards into the hepatic mass and ramify there.

There are two well-known genera: Velella with a sail, and Porpita without a sail. They are both found at the surface of the warmer regions of the great oceans and in the Mediterranean. Velella sometimes drifts on to British coasts from the Atlantic.

The genus Discalia has a much more simple octoradial structure. It was found at depths of 2600 and 2750 fathoms in the Pacific Ocean.

{310}

CHAPTER XII

COELENTERATA (CONTINUED): SCYPHOZOA = SCYPHOMEDUSAE

CLASS II. SCYPHOZOA = SCYPHOMEDUSAE

The Scyphozoa are jelly-fishes, usually found floating at or near the surface of the sea. A few forms (Stauromedusae) are attached to rocks and weeds by a stalked prolongation of the aboral region of the umbrella. With this exception, however, they are all, in the adult stage, of the Medusa type of structure, having a bell-shaped or discoid umbrella, from the under surface of which depends a manubrium bearing the mouth or (in Rhizostomata) the numerous mouths.

Although many of the species do not exceed an inch or a few inches in diameter, others attain a very great size, and it is among the Scyphozoa that we find the largest individual zooids of the Coelenterata. Some Discophora have a disc three or four feet in diameter, and one specimen obtained by the Antarctic Expedition of 1898-1900 weighed 90 lbs.[341] The common jelly-fish, Aurelia, of our coasts belongs to a species that appears to be very variable in general characters as well as in size. Specimens obtained by the "Siboga" in the Malay Archipelago ranged from 6 to 64 cm. in diameter. The colour is very variable, shades of green, blue, brown, and purple being conspicuous in many species; but a pale milky-blue tint is perhaps the most prevalent, the tissues being generally less transparent than they are in the Medusae of the Hydrozoa. The colour of the Cubomedusae is usually yellow or brown, but Charybdea xaymacana is colourless and transparent. The deep-sea species, particularly the Periphyllidae, have usually an opaque brown or dark red colour. The surface-swimming {311}forms, such as the common Aurelia, Pelagia, Cyanaea, are usually of a uniform pale milky-blue or green colour. Generally the colour is uniformly distributed, but sometimes the surface of the umbrella is freckled with irregular brown or yellow patches, as in Dactylometra and many others. There is frequently a special colour in the statorhabs which renders them conspicuous in the living jelly-fish, and the lips, or parts of the lips, of the manubrium have usually a different colour or tone to that of the umbrella.

There is no reason to believe that the general colour of any of these jelly-fishes has either a protective or a warning significance. Nearly all the larger species, whether blue, green, or brown in colour, can be easily seen from a considerable distance, and the colours are not sufficiently bright or alarming to support the belief that they can serve the purpose of warning either fish or birds of the presence of a dangerous stinging animal. It is possible, however, that the brighter spots of colour that are often noticed on the tips of the tentacles and on the lips may act as a lure or bait in attracting small fish and Crustacea.

Some of the Scyphozoa are phosphorescent, but it is a singular fact that there are very few recorded observations concerning the phosphorescence or the absence of it in most of the species. The pale blue light of Pelagia noctiluca or P. phosphora can be recognised from the deck of a ship in the open ocean, and they are often the most brilliant and conspicuous of the phosphorescent organisms.

The food of the Scyphozoa varies a good deal. Charybdea and Periphylla, and probably many others with large mouths, will capture and ingest relatively large fish and Crustacea; but Chrysaora isosceles[342] apparently makes no attempt to capture either Copepoda or small fish, but preys voraciously upon Anthomedusae, Leptomedusae, Siphonophora, Ctenophora, and pelagic worms. Very little is known about the food of the Rhizostomata, but the small size of the mouths of these forms suggests that their food must also be of minute size. The frequent association of small fish with the larger jelly-fish is a matter of some interest that requires further investigation. In the North Sea young whiting are the constant guests of Cyanaea capillata.[343] Over a {312}hundred young horse-mackerel (Caranx trachurus) may be found sheltering under the umbrella of Rhizostoma pulmo. As the animal floats through the water the little fishes hover round the margin, but on the slightest alarm dart into the sub-umbrella cavity, and ultimately seek shelter in the sub-genital pits.[344]

Two species of fish accompany the American Medusa Dactylometra lactea, one a Clupeoid, the other the young of the Butter-fish (Stromateus triacanthus). According to Agassiz and Mayer[345] this is not an ordinary case of mutualism, as the fish will tear off and devour fragments of the tentacles and fringe of the Medusa, whilst the Medusa will in its turn occasionally capture and devour one of the fish.

A great many of the Scyphozoa, particularly the larger kinds, have the reputation of being able to sting the human skin, and in consequence the name Acalephae[346] was formerly used to designate the order. Of the British species Aurelia aurita is almost harmless, and so is the rarer Rhizostoma pulmo; but the nematocysts on the tentacles of Cyanaea, Chrysaora, and Pelagia can inflict stings on the more delicate parts of the skin which are very painful for several hours, although the pain has been undoubtedly greatly exaggerated in many popular works.

The soft structure of the Medusae does not favour their preservation in the rocks, but the impressions left by several genera, all belonging apparently to the Rhizostomata, have been found in Cambrian, Liassic, and Cretaceous deposits.

There is reason to believe that many Scyphozoa exhibit a considerable range of variation in the symmetry of the most important organs of the body. Very little information is, however, at hand concerning the variation of any species except Aurelia aurita, which has been the subject of several investigations. Browne[347] has found that in a local race of this species about 20 per cent exhibit variations from the normal in the number of the statorhabs, and about 2 per cent in the number of gastric pouches.

The Scyphozoa are not usually regarded as of any commercial or other value, but in China and Japan two species of Rhizostomata (Rhopilema esculenta and R. verrucosa) are used as food. {313}The jelly-fish is preserved with a mixture of alum and salt or between the steamed leaves of a kind of oak. To prepare the preserved food for the table it is soaked in water, cut into small pieces, and flavoured. It is also stated that these Medusae are used by fishermen as bait for file-fish and sea-bream.[348]

In general structure the Scyphozoa occupy an intermediate position between the Hydrozoa and the Anthozoa. The very striking resemblance of the body-form to the Medusa of the Hydrozoa, and the discovery of a fixed hydriform stage in the life-history of some species, led the older zoologists to the conclusion that they should be included in the class Hydrozoa. Recently the finer details of development have been invoked to support the view that they are Anthozoa specially adapted for a free-swimming existence, but the evidence for this does not appear to us to be conclusive.

They differ from the Hydrozoa and resemble the Anthozoa in the character that the sexual cells are matured in the endoderm, and escape to the exterior by way of the coelenteric cavity, and not directly to the exterior by the rupture of the ectoderm as in all Hydrozoa. They differ, on the other hand, from the Anthozoa in the absence of a stomodaeum and of mesenteries.

The view that the Scyphozoa are Anthozoa is based on the belief that the manubrium of the former is lined by ectoderm, and is homologous with the stomodaeum of the latter; and that the folds of mesogloea between the gastric pouches are homologous with the septa.[349]

The Scyphozoa, notwithstanding their general resemblance to the Medusae of Hydrozoa, can be readily distinguished from them by several important characters. The absence of a velum in all of them (except the Cubomedusae) is an important and conspicuous character which gave to the class the name of Acraspeda. The velum of the Cubomedusae can, however, be distinguished from that of the Craspedote Medusae (i.e. the Medusae of the Hydrozoa) by the fact that it contains endodermal canals.

Sense-organs are present in all Scyphozoa except some of the Stauromedusae, and they are in the form of statorhabs (tentaculocysts), bearing statoliths at the extremity, and in many species, {314}at the base or between the base and the extremity, one or more eyes. These organs differ from the statorhabs of the Hydrozoa in having, usually, a cavity in the axial endoderm; but as they are undoubtedly specially modified marginal tentacles, they are strictly homologous in the two classes. In nearly all the Scyphozoa these organs are protected by a hood or fold formed from the free margin of the umbrella, and this character, although not of great morphological importance, serves to distinguish the common species from the Craspedote Medusae. It was owing to this character that Forbes gave the name Steganophthalmata, or "covered-eyed Medusae," to the class.

Another character of some importance is the presence in the coelenteric cavity of all Scyphozoa of clusters or rows of delicate filaments called the "phacellae." These filaments are covered with a glandular epithelium, and are usually provided with numerous nematocysts. They have a considerable resemblance to the acontia of certain Anthozoa, and are probably mainly digestive in function. These three characters, in addition to the very important character of the position and method of discharge of the sexual cells already referred to, justify the separation of the Scyphozoa from the Medusae of the Hydrozoa as a distinct class of Coelenterata.

The umbrella of the Scyphozoa varies a good deal in shape. It is usually flattened and disc-like (Discophora), but it may be almost globular (Atorella), conical (some species of Periphylla), or cubical (Cubomedusae). It is divided into an aboral and a marginal region by a circular groove in the Coronata. The margin may be almost entire, marked only by notches where the statorhabs occur, or deeply lobed as in the Coronata and many Discophora. Marginal tentacles are present in all but the Rhizostomata, and may be few in number, four in Charybdea, eight in Ulmaris (Fig. 143), or very numerous in Aurelia and many others. The tentacles may be short (Aurelia), or very long as in Chrysaora isosceles, in which they extend for a length of twenty yards from the disc.

The manubrium of the Scyphozoa is usually quadrangular in section, and in those forms in which the shape is modified in the adult Medusa the quadrangular shape can be recognised in the earlier stages of development. The four angles of the manubrium are of importance in descriptive anatomy, as the planes drawn {315}through the angles to the centre of the manubrium are called "perradial," while those bisecting the perradial planes and passing therefore through the middle line of the flat sides of the manubrium are called "interradial."

The free extremity of the manubrium in many Scyphozoa is provided with four triangular perradial lips, which may be simple or may become bifurcated or branched, and have frequently very elaborate crenate edges beset with batteries of nematocysts. In Pelagia and Chrysaora and other genera these lips hang down from the manubrium as long, ribbon-like, folded bands, and according to the size of the specimen may be a foot or more in length, or twice the diameter of the disc.

In the Rhizostomata a peculiar modification of structure takes place in the fusion of the free edges of the lips to form a suture perforated by a row of small apertures, so that the lips have the appearance of long cylindrical rods or tubes attached to the manubrium, and then frequently called the "oral arms." The oral arms may be further provided with tentacles of varying size and importance. In many Rhizostomata branched or knobbed processes project from the outer side of the upper part of the oral arms. These are called the "epaulettes."

fig143

Fig. 143.—Ulmaris prototypus. g, Gonad; I, interradial canal; M, the fringed lip of the manubrium; P, perradial canal; S, marginal sense-organ; t, tentacle. × 1. (After Haeckel.)

The lumen of the manubrium leads into a large cavity in the disc, which is usually called the gastric cavity, and this is extended into four or more interradial or perradial gastric pouches. The number of these pouches is usually four, but in this, as in {316}other features of their radial symmetry, the jelly-fish frequently exhibit duplication or irregular variation of the radii.[350]

The gastric pouches may extend to the margin of the disc, where they are united to form a large ring sinus, or they may be in communication at the periphery by only a very narrow passage (Cubomedusae). In the Discophora the gastric pouches, however, do not extend more than half-way to the margin, and they may be connected with the marginal ring-canal by a series of branched interradial canals. Between the gastric pouches in these forms branched perradial canals pass from the gastric cavity to the marginal ring canal, and the system of canals is completed by unbranched "adradial" canals passing between the perradials and interradials from the sides of the gastric pouches to the ring-canal (Fig. 143).

In the Discophora there are four shallow interradial pits or pouches lined by ectoderm on the under side of the umbrella-wall. As these pits correspond with the position of the gonads in the gastric pouches they are frequently called the "sub-genital pits." In the Stauromedusae and Cubomedusae they are continued through the interradial gastric septa to the aboral side of the disc, and they are generally known in these cases by the name "interradial funnels." The functions and homologies of these ectodermic pits and funnels are still uncertain.

The Scyphozoa are usually dioecious, but Chrysaora and Linerges are sometimes hermaphrodite. The female Medusae can usually be distinguished from the male by the darker or brighter colour of the gonads, which are band-shaped, horseshoe-shaped, or circular organs, situated on the endoderm of the interradial gastric pouches. They are, when nearly ripe, conspicuous and brightly coloured organs, and in nearly all species can be clearly seen through the transparent or semi-transparent tissues of the disc. The reproductive cells are discharged into the gastric cavity and escape by the mouth. The eggs are probably fertilised in the water, and may be retained in special pouches on the lips of the manubrium until the segmentation is completed.[351] Asexual reproduction does not occur in the free-swimming or adult stage of any Scyphozoa. In some cases (probably exceptional) the development is direct. In Pelagia, for example, it is known that the fertilised egg gives {317}rise to a free-swimming Medusa similar in all essential features to the parent.

In many species, however, the planula larva sinks to the bottom of the sea, develops tentacles, and becomes attached by its aboral extremity to a rock or weed, forming a sedentary asexual stage of development with a superficial resemblance to a Hydra. This stage is the "Scyphistoma," and notwithstanding its simple external features it is already in all essential anatomical characters a Scyphozoon.

The Scyphistoma may remain as such for some time, during which it reproduces by budding, and in some localities it may be found in great numbers on seaweeds and stones.[352]

In the course of time, however, the Scyphistoma exhibits a ring-like constriction of the body just below the crown of tentacles, and as this deepens the general features of a Scyphomedusa are developed in the free part above the constriction. In time this free part escapes as a small free-swimming jelly-fish, called an "Ephyra," while the attached part remains to repeat the process. In many species the first constriction is followed by a second immediately below it, then a third, a fourth, and so on, until the Scyphistoma is transformed into a long series of narrow discs, each one acquiring, as it grows, the Ephyra characters. Such a stage has been compared in form to a pile of saucers, and is known as the "Strobila."

The Ephyra differs from the adult in many respects. The disc is thin and flat, the manubrium short, the margin of the umbrella deeply grooved, while the statorhabs are mounted on bifid lobes which project outwards from the margin. The stabilisation of the Scyphistoma is a process of reproduction by transverse fission, and in some cases this is supplemented by gemmation, the Scyphistoma giving rise to a number of buds which become detached from the parent and subsequently undergo the process of strobilisation.

fig144

Fig. 144.—The perisarc tubes of a specimen of Spongicola fistularis (N) ramifying in the skeleton of the Sponge Esperella bauriana (Sp.), as seen in a macerated specimen, × 1. (After Schulze.)

The Scyphistoma of Nausithoe presents us with the most {318}remarkable example of this mode of reproduction (Fig. 144), as it forms an elaborate branching colony in the substance of certain species of sponges. The ectoderm secretes a chitinous perisarc, similar to that of the hydrosome stage of many of the Hydrozoa, and consequently Stephanoscyphus (Spongicola), as this Scyphistoma was called, was formerly placed among the Gymnoblastea. It is remarkable that, although the Scyphozoan characters of Spongicola were proved by Schulze[353] in 1877, a similar Scyphistoma stage has not been discovered in any other genus.

Order I. Cubomedusae.

Scyphozoa provided with four perradial statorhabs, each of which bears a statolith and one or several eyes. There are four interradial tentacles or groups of tentacles. The stomach is a large cavity bearing four tufts of phacellae (Fig. 145, Ph), situated interradially. There are four flattened perradial gastric pouches in the wall of the umbrella which communicate with the stomach by the gastric ostia (Go). These pouches are separated from one another by four interradial septa; and the long leaf-like gonads are attached by one edge to each side of the septa. In many respects the Cubomedusae appear to be of simple structure, but the remarkable differentiation of the eyes and the occurrence of a velum (p. 313) suggest that the order is a highly specialised offshoot from a primitive stock.

fig145

Fig. 145.—Vertical section in the interradial plane of Tripedalia cystophora. Go, Gastric ostia; Man, manubrium; Ph, group of phacellae; T, tentacles in four groups of three; tent, perradial sense-organs; V, velum. (After Conant.)

Fam. 1. Charybdeidae.—Cubomedusae with four interradial tentacles.

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Charybdea appears to have a very wide geographical distribution. Some of the species are usually found in deep water and come to the surface only occasionally, but others (C. xaymacana) are only found at the surface of shallow water near the shore. The genus can be easily recognised by the four-sided prismatic shape of the bell and the oral flattened expansion of the base of the tentacles. The bell varies from 2-6 cm. in length (or height) in C. marsupialis, but a giant form, C. grandis,[354] has recently been discovered off Paumotu Island which is as much as 23 cm. in height. The colour is usually yellow or brown, but C. grandis is white and C. xaymacana perfectly transparent.

"Charybdea is a strong and active swimmer, and presents a very beautiful appearance in its movements through the water; the quick, vigorous pulsations contrasting sharply with the sluggish contractions seen in most Scyphomedusae." It appears to be a voracious feeder. "Some of the specimens taken contained in the stomach small fish, so disproportionately large in comparison with the stomach that they lay coiled up, head overlapping tail."[355]

Very little is known of the development, but it is possible that Tamoya punctata, which lacks gonads, phacellae, and canals in the velum, may be a young form of a species of Charybdea.

Fam. 2. Chirodropidae.—Cubomedusae with four interradial groups of tentacles.

This family is represented by the genera Chirodropus from the Atlantic and Chiropsalmus from the Indian Ocean and the coast of North Carolina.

Fam. 3. Tripedaliidae.—Cubomedusae with four interradial groups of three tentacles.

The single genus and species Tripedalia cystophora has only been found in shallow water off the coast of Jamaica. Specimens of this species were kept for some time by Conant in an aquarium, and produced a number of free-swimming planulae which settled on the glass, and quickly developed into small hydras with a mouth and four tentacles. The further development of this sedentary stage is unfortunately not known.

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Order II. Stauromedusae.

This order contains several genera provided with an aboral stalk which usually terminates in a sucker, by means of which the animal is temporarily fixed to some foreign object. There can be little doubt that this sedentary habit is recently acquired, and the wide range of the characteristic features of the order may be accounted for as a series of adaptations to the change from a free-swimming to a sedentary habit.

It is difficult to give in a few words the characters of the order, but the Stauromedusae differ from other Scyphozoa in the absence or profound modification in structure and function of the statorhabs. They are absent in Lucernaria and the Depastridae, and very variable in number in Haliclystus.

The statorhab of Haliclystus terminates in a spherical knob, which is succeeded by a large annular pad or collar bearing a number of glandular cells which secrete a sticky fluid. At the base of the organ there is a rudimentary ocellus. The number is very variable, and sometimes they are abnormal in character, being "crowned with tentacles." There can be little doubt that the principal function of these organs is not sensory but adhesive, and hence they have received the names "colletocystophores" and "marginal anchors," but they are undoubtedly homologous with the statorhabs of other Scyphozoa.

The tentacles are short and numerous, and are frequently mounted in groups on the summit of digitate outgrowths from the margin of the umbrella. They are capitate, except in Tessera, the terminal swelling containing a battery of nematocysts.

Very little is known concerning the life-history and development of the Stauromedusae.

Fam. 1. Lucernariidae.—Marginal lobes digitate, bearing the capitate tentacles in groups. Haliclystus auricula is a common form on the shores of the Channel Islands, at Plymouth, and other localities on the British coast. It may be recognised by the prominent statorhabs situated in the bays between the digitate lobes of the margin of the umbrella. Each of the marginal lobes bears from 15 to 20 capitate tentacles. It is from 2 to 3 cm. in length. The genus occurs in shallow water {321}off the coasts of Europe and North America, extending south into the Antarctic region.

Lucernaria differs from Haliclystus in the absence of statorhabs. It has the same habit as Haliclystus, and is often found associated with it. L. campanulata is British.

Halicyathus is similar in external features to Haliclystus, but differs from it in certain important characters of the coelenteric cavities. It is found off the coasts of Norway, Greenland, and the Atlantic side of North America.

In Capria, from the Mediterranean, the tentacles are replaced by a denticulated membrane bearing nematocysts.

The rare genus Tessera, from the Antarctic Ocean, differs from all the other Stauromedusae in having no stalk and in having only a few relatively long non-capitate tentacles. If Tessera is really an adult form it should be placed in a separate family, but, notwithstanding the presence of gonads, it may prove to be but a free-swimming stage in the history of a normally stalked genus.

Fam. 2. Depastridae.—The margin of the umbrella is provided with eight shallow lobes bearing one or more rows of tentacles. Statorhabs absent.

Depastrum cyathiforme occurs in shallow water at Plymouth, Port Erin, and in other localities on the coasts of Britain and Norway. The tentacles are arranged in several rows on the margin of the umbrella. In Depastrella from the Canaries there is only one row of marginal tentacles.

Fam. 3. Stenoscyphidae.[356]—Stauromedusae with simple undivided umbrella margin. The eight principal tentacles are converted into adhesive anchors. Secondary tentacles arranged in eight adradial groups. Stenoscyphus inabai, 25 cm., Japan.

Order III. Coronata.[357]

The external surface of the umbrella is divided into two regions, an aboral region and a marginal region, by a well-marked circular groove (the coronal groove). The aboral region is usually smooth and undivided, but it is an elongated dome, {322}thimble- or cone-shaped, in marked contrast to the flattened umbrella of the Discophora. The margin is divided into a number of triangular or rounded lobes, and these are continued as far as the coronal groove as distinct areas delimited by shallow grooves on the surface of the umbrella. The tentacles arise from the grooves between the marginal areas, and are provided with expanded bases called the pedalia. The manubrium may be short or moderately long, but it is never provided with long lips.

Fam. 1. Periphyllidae.[358]—Coronata with four or six statorhabs.

In Pericolpa (Kerguelen) there are only four tentacles and four statorhabs. In Periphylla, a remarkable deep-sea genus from 700 to 2000 fathoms in all seas, but occasionally found at the surface, there are twelve tentacles and four statorhabs. The specimens from deep water have a characteristic dark red-brown or violet-brown colour. They are usually small Medusae, but the umbrella of P. regina is over 21 cm. in diameter. Atorella has six tentacles and six statorhabs.

Fam. 2. Ephyropsidae.—Coronata with eight or more than eight statorhabs.

Nausithoe punctata is a small, transparent jelly-fish, not exceeding 10 mm. in diameter, of world-wide distribution. Its Scyphistoma stage is described on p. 317. N. rubra, a species of a reddish colour found at a considerable depth in the South Atlantic and Indian Oceans, is probably an abysmal form. Palephyra differs from Nausithoe in having elongated instead of rounded gonads. Linantha and Linuche differ from the others in having subdivided marginal lobes.

Fam. 3. Atollidae.Atolla is a deep-sea jelly-fish of very wide geographical distribution. It is characterised by the multiplication of the marginal appendages, but the number is very irregular. There may be double or quadruple the usual number of marginal lobes, or an indefinite number. There may be sixteen to thirty-two statorhabs, and the number of tentacles is quite irregular. Some of the species attain a considerable size, the diameter of the umbrella of A. gigantea being 150 mm., of A. valdiviae sometimes 130 mm., and of A. bairdi 110 mm.

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Order IV. Discophora.

This order contains not only by far the greater number of the species of Scyphozoa, but those of the largest size, and all those that are familiar to the seaside visitor and the mariner under the general term jelly-fish.

They may be distinguished from the other Scyphozoa by several well-marked characters. The umbrella is flattened and disc-shaped or slightly domed, but not divided by a coronary groove. The perradial angles of the mouth are prolonged into long lips, which may remain free (Semaeostomata) or fuse to form an elaborate proboscis (Rhizostomata).

Sub-Order I. Semaeostomata.

In this sub-order the mouth is a large aperture leading into the cavity of the manubrium, and is guarded by four long grooved and often tuberculated lips. The margin of the umbrella is provided with long tentacles.

Fam. 1. Pelagiidae.—Semaeostomata with wide gastric pouches, which are not united by a marginal ring sinus. Pelagia, which forms the type of this family, has eight long marginal tentacles. It develops directly from the egg, the fixed Scyphistoma stage being eliminated.[359] It is probably in consequence of this peculiarity of its development and independence of a shore for fixation that Pelagia has become a common and widespread inhabitant of the high seas. In the Atlantic and Indian Oceans P. phosphora occurs in swarms or in long narrow lines many miles in length. It is remarkable for its power of emitting phosphorescent light. In the Atlantic it extends from 50° N. to 40° S., but is rare or absent from the colder regions. P. perla is found occasionally on the west coast of Ireland. Chrysaora differs from Pelagia in the larger number of tentacles. There are, in all, 24 tentacles and 8 statorhabs, separated by 32 lobes of the margin of the umbrella. C. isosceles is occasionally found off the British coast. It passes through a typical Scyphistoma stage in development. Dactylometra, a very {324}common jelly-fish of the American Atlantic shores, differs from Chrysaora in having sixteen additional but small tentacles arranged in pairs at the sides of the statorhabs.

Fam. 2. Cyanaeidae.—Semaeostomata with eight radial and eight adradial pouches, which give off ramifying canals to the margin of the umbrella; but these canals are not united by a ring-canal. The tentacles are arranged in bundles on the margin of the deeply lobed umbrella.

The yellow Cyanaea capillata and the blue C. lamarcki are commonly found on the British coasts.

Fam. 3. Ulmaridae.—The gastric pouches are relatively small, and communicate with a marginal ring-canal by branching perradial and interradial canals and unbranched adradial canals.

In Ulmaris prototypus (Fig. 143, p. 315) there are only eight long adradial tentacles, and the lips of the manubrium are relatively short. It is found in the South Atlantic.

Aurelia is a well-known and cosmopolitan genus, which may be recognised by the eight shallow lobes of the umbrella-margin beset with a fringe of numerous small tentacles.

Sub-Order II. Rhizostomata.

In this sub-order the lips are very much exaggerated in size, and are fused together by their margin in such a manner that the mouth of the animal is reduced to a number of small apertures situated along the lines of suture. Tentacles are absent on the margin of the umbrella. This sub-order contains some of the largest known jelly-fishes, and exhibits a considerable range of structure. The families are arranged by Maas[360] in three groups.

 

Group I. Arcadomyaria.—Musculature of the disc arranged in feather-like arcades. Oral arms pinnate.

Fam. Cassiopeidae.—There are no epaulettes on the arms. Labial tentacles present. Cassiopea is common in the Indo-Pacific seas, and extends into the Red Sea. It includes a great many species varying in size from 4 to about 12 cm. in diameter.

 

Group II. Radiomyaria.—Musculature arranged in radial tracts. Oral arms bifid.

Fam. Cepheidae.—The genera included in this family differ {325}from the Cassiopeidae in the characters of the group. Cephea is found in the Indo-Pacific Oceans and Red Sea. Cotylorhiza is common in the Mediterranean Sea and extends into the Atlantic Ocean.

 

Group III. Cyclomyaria.—The group contains the majority of the Rhizostomata. Musculature arranged in circular bands round the disc. Oral arms primarily trifid, but becoming in some cases very complicated. The principal families are:—

Fam. Rhizostomatidae.—With well-marked epaulettes, and sixteen radial canals passing to the margin of the umbrella.

Rhizostoma pulmo (= Pilema octopus), a widely distributed species, is often found floating at the surface off the western coasts of Scotland and Ireland, and sometimes drifts up the English Channel into the German Ocean in the autumn. The umbrella is about two feet in diameter, and the combined length of the umbrella and arms is four feet. The colour varies considerably, but that of a specimen obtained off Valencia in 1895 was described as follows: "The colour of the umbrella was pale green, with a deep reddish margin. Arms bright blue."[361]

The family includes Stomolophus, of the Pacific and Atlantic coasts of America, in which the oral arms are united at the base, and Rhopilema, the edible Medusa of Japan and China.

Fam. Lychnorhizidae.—Here there are only eight radial canals reaching as far as the margin of the umbrella, and eight terminating in the ring-canal. There are no epaulettes, and the oral tentacles are often very long. The family includes Lychnorhiza from the coast of Brazil, Crambione from the Malay Archipelago, and Crambessa from the Atlantic shores of France and Spain and from Brazil and Australia. The last-named genus has been found in brackish water at the mouth of the Loire.

In the families Leptobrachiidae and Catostylidae there are eight radial canals reaching the margin of the umbrella, and between them a network of canals with many openings into the ring-canal. In a few of the Leptobrachiidae the intermediate canal-network has only eight openings into the ring-canal, as in the Lychnorhizidae.

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CHAPTER XIII

COELENTERATA (CONTINUED): ANTHOZOA = ACTINOZOA—GENERAL CHARACTERS—ALCYONARIA

CLASS III. ANTHOZOA = ACTINOZOA

Among the familiar objects included in this class are the Sea-anemones, the Stony Corals (Madrepores), the Flexible Corals, the Precious Coral, and the Sea-pens. With the exception of a few species of Sea-anemone, Anthozoa are not commonly found on British sea-shores; but in those parts of the tropical world where coral reefs occur, the shore at low tide is carpeted with various forms of this class, and the sands and beaches are almost entirely composed of their broken-down skeletons.

The majority of the Anthozoa are colonial in habit, a large number of individuals, or zooids as they are called, being organically connected together by a network of nutritive canals, and forming a communal gelatinous or stony matrix for their protection and support. Whilst the individuals are usually small or minute, the colonial masses they form are frequently large. Single colonies of the stony corals form blocks of stone which are sometimes five feet in diameter, and reach a height of two or three feet from the ground. From the tree or shrub-like form assumed by many of the colonies they were formerly included in a class Zoophyta or animal-plants.

But whether the individual polyps are large or small, whether they form colonies in the adult condition or remain independent, they exhibit certain characters in common which distinguish them not only from the other Coelenterata, but from all other animals. When an individual zooid is examined in the living and fully expanded condition, it is seen to possess a cylindrical {327}body, attached at one end (the aboral end) to the common colonial matrix or to some foreign object. At the opposite or free extremity it is provided with a mouth surrounded by a crown of tentacles. In these respects, however, they resemble in a general way some of the Hydrozoa. It is only when the internal anatomy is examined that we find the characters which are absolutely diagnostic of the group.

In the Hydrozoa the mouth leads directly into the coelenteric cavity; in the Anthozoa, however, the mouth leads into a short tube or throat, called the "stomodaeum," which opens into the coelenteric cavity. Moreover, this tube is connected with the body-wall, and is supported by a series of fleshy vertical bands called the mesenteries (Fig. 146). The mesenteries not only support the stomodaeum, but extend some distance below it. Where the mesenteries are free from the stomodaeum their edges are thickened to form the important digestive organs known as the mesenteric filaments (mf). It is in the possession of a stomodaeum, mesenteries, and mesenteric filaments that the Anthozoa differ from all the other Coelenterata. There is one character that the Anthozoa share with the Scyphozoa, and that is, that the gonads or sexual cells (G) are derived from the endoderm. They are discharged first into the coelenteric cavity, and then by way of the mouth to the exterior. In the Anthozoa the gonads are situated on the mesenteries.

fig146

Fig. 146.—Diagram of a vertical section through an Anthozoan zooid. B, Body-wall; G, gonads; M, mesentery; mf, mesenteric filament; St, stomodaeum; T, tentacle.

Nearly all the Anthozoa are sedentary in habit. They begin life as ciliated free-swimming larvae, and then, in a few hours or days, they become attached to some rock or shell at the bottom and immediately (if colonial) start the process of budding, which gives rise to the colonies of the adult stage. Many of the Sea-anemones, however, move considerable distances by gliding {328}over the rocks or seaweeds, others habitually burrow in the sand (Edwardsia, Cerianthus), and one family (the Minyadidae) are supported by a gas bladder, and float at the surface of the sea. The Sea-pens, too, although usually partly buried in the sand or mud, are capable of shifting their position by alternate distension and contraction of the stalk.[362] The Anthozoa are exclusively marine. With the exception of a few Sea-anemones that are found in brackish or almost fresh water in river estuaries, they only occur in salt sea water. The presence of a considerable admixture of fresh water, such as we find at the mouths of rivers, seems to interfere very materially with the development and growth of all the reef-forming Corals, as will be noticed again in the chapter on coral reefs. A few genera descend into the greatest depths of the ocean, but the home of the Anthozoa is pre-eminently the shallow seas, and they are usually found in great abundance in depths of 0-40 fathoms from the shores of the Arctic and Antarctic lands to the equatorial belt.

The only Anthozoa of any commercial importance are the Precious Corals belonging to the Alcyonarian family Coralliidae. The hard pink axis of these corals has been used extensively from remote times in the manufacture of jewellery and ornaments. Until quite recently the only considerable and systematic fishery for the Precious Corals was carried on in the Mediterranean Sea, and this practically supplied the markets of the world. In more recent times, however, an important industry in corals has been developed in Japan. In 1901 the value of the coral obtained on the coasts of Japan was over £50,000, the greater part of which was exported to Italy, a smaller part to China, and a fraction only retained for home consumption. The history of the coral fishery in Japan is of considerable interest. Coral was occasionally taken off the coast of Tsukinada in early times. But in the time of the Daimyos the collection and sale of coral was prohibited, for fear, it is said, that the Daimyo of Tosa might be compelled to present such precious treasure to the Shogun. After the Meiji reform, however (1868), the industry revived, new grounds were discovered, improved methods employed, and a large export trade developed.

There is evidence, however, in the art of Japan, of another {329}coral fishery in ancient times, of which the history is lost. Coral was imported into Japan at least two hundred years ago, and used largely in the manufacture of those exquisite pieces of handicraft for which that country is so justly famous. On many of the carved "Netsukes" and other ornaments, however, the coral branches are represented as the booty of dark-skinned, curly-headed fishermen, "kurombo," and never of Japanese fishermen. The coral used in this art-work can hardly be distinguished from Mediterranean coral, and there are some grounds for believing that Japan imported coral from the far West in very early times. But this does not account for the "kurombo." The only coast-dwelling people of the type that is so clearly carved on these ornaments within the area of the Pacific Ocean at the present time are the Melanesians and Papuans, and the suggestion occurs that a coral fishery existed at one time in the Southern Pacific, which has since been lost.[363]

 

The class Anthozoa is divided into two sub-classes:—I. Alcyonaria; II. Zoantharia.

In the Alcyonaria the fully developed zooids have always eight tentacles and eight mesenteries. In the Zoantharia the number of tentacles and the number of mesenteries in the fully developed zooids may be six, twelve, twenty-four, or an indefinite number, but individuals with eight mesenteries and only eight tentacles are not known to occur.

Sub-Class I. Alcyonaria.

This sub-class includes a large number of genera living in shallow sea-water and a few genera that extend down into deep water. With a few doubtful exceptions (Protoalcyonacea) they all form colonies composed of a large number of zooids. These zooids may be connected together by basal plates or a network of basal strands (stolons), or by stolons with additional connecting bars (Clavularia viridis, Syringopora) or by plates (Tubipora). In the majority of the genera the individual zooids are for the greater part of their length, from the base upwards, united together to form a continuous spongy, colonial mass, which determines the shape of the colony as a whole.

In this last-named group of genera there may be {330}distinguished the free distal portions of the zooids bearing the mouths and tentacles (the "anthocodiae") from the common colonial mass perforated by the coelenteric cavities of the individual zooids. The coelenteric cavities are separated by a considerable amount of a substance called the "mesogloea," usually gelatinous in consistency but chemically more closely related to mucin than to gelatin, which is traversed by endodermal canals, rods of endoderm cells and a number of free amoeboid cells. In this substance, moreover, there are found in nearly all cases numerous spicules of carbonate of lime formed by the "scleroblasts" (spicule-forming cells) which have wandered from the superficial ectoderm of the common colonial mass. This common colonial mesogloea with its spicules, endoderm cells, and superficial covering of ectoderm is called the "coenenchym." The form assumed by the colonies is very varied. In some species of Clavularia they form encrusting plates following the irregularity of the rock or stones on which they grow, in Alcyonium they construct lobed masses of irregular form, in Sarcophytum they are usually shaped like a mushroom, in Juncella they are long whip-like rods, in most of the Gorgonacea they are branched in all directions like shrubs or in one plane to form fan-shaped growths, and in many of the Pennatulacea they assume that graceful feather form which gives the order its name.

The consistency and texture of the colonies also varies considerably. In some cases where the spicules are few or very small, the substance of the colony is soft to the touch, and frequently slimy at the surface, in other cases the great number of the spicules makes the colony hard but brittle, whilst in a few genera (Sclerophytum, Heliopora) the colony is so hard that it can only be broken by the hand with difficulty. In some genera (Spongodes and the Muriceidae) projecting spicules cause the surface to be rough or thorny, and in the Primnoidae the zooids and the surface of the general coenenchym are protected by a series of overlapping scales or plates.

In all the Alcyonaria the nematocysts are very minute, and although they can undoubtedly paralyse minute organisms they are unable to penetrate the human skin. None of the Alcyonaria have been described as stinging-corals except the Pennatulid Virgularia rumphii.

Zooids.—The fully formed zooids of the Alcyonaria exhibit {331}a remarkable uniformity of structure. They have eight intermesenteric tentacles containing a cavity continuous with the coelenteron. Each of these tentacles bears at least two rows of simple pinnules, and they are therefore said to be "pinnate" tentacles. In some species of Xenia the tentacles may have three or four rows of pinnules, which give them a much more feathery appearance than is usually the case. In the great majority of species a single row of from eight to fourteen pinnules is found disposed laterally on each side of the tentacle. The mouth is usually small and slit-like with a slight rounded gape at the ventral extremity. The stomodaeum is usually very short, but in Xenia and in the autozooids of some Pennatulids it is relatively much longer. It is not known how far the stomodaeum is of importance in the digestion of the food. In Xenia[364] it has probably some importance, as shown by its unusual length and the numerous large goblet cells (mucus cells) which it exhibits, associated with the fact that the mesenteric filaments are relatively very small. In Alcyonium and other Alcyonaria gland cells also occur in the stomodaeum, and it is probable that they secrete a fluid capable of digesting to some extent the food as it passes through. The most important part of the digestion, however, is performed by the six "ventral" mesenteric filaments.

Attention has already been drawn to the fact (p. 330) that two regions of the zooids of the colonial Alcyonaria can be recognised. At the oral end there is a region, which in the fully expanded condition consists of a crown of eight tentacles surrounding the mouth, and a body-wall free from its immediate neighbours. This region is called the "anthocodia." The anthocodia is continuous with a region which forms a part of the common colonial mass. Some genera seem to have very little power of contracting the tentacles or of withdrawing the anthocodiae. The zooids of Stereosoma, of Xenia, of Umbellula, and of a few other genera may be described as non-retractile. In many cases, however, the tentacles can be considerably contracted, bent over the mouth, and withdrawn into the shelter of the subjacent body-wall. In such a condition the surface of the colony exhibits a number of tubular, conical, or convex protuberances, called "verrucae," and the colony is said to be partially retractile. In many genera, however, the whole of the {332}anthocodiae can be withdrawn below the general surface of the coenenchym, so that the position of the zooids in the colony is indicated only by star-like holes, or simple key-hole slits in the superficial coenenchym. Such colonies are said to be completely retractile (Fig. 147).

It is often very difficult to determine whether a particular species is or is not completely retractile, unless observations can be made upon the living colony; and there are many instances of confusion in the work of systematists due to a species being described as partially retractile in one instance, and completely retractile in another. The complete retraction of the anthocodiae may be effected very slowly, and after continuous irritation only. If the colony is killed too quickly, the anthocodiae remain in a state of partial retraction. An example of this may be found in the common British Alcyonium digitatum. Specimens of this species which are put into a bucket of sea water and allowed to roll about with the movements of a small boat in a rough sea, undergo complete retraction; but if the same specimens be allowed to expand in the aquarium, and then plunged into spirit, or allowed to dry in the sun, they will die in a condition of partial retraction.

fig147

Fig. 147.—Diagram of a vertical section of a portion of a lobe of Alcyonium to show the mode of retraction of the anthocodiae. 1, Anthocodia of a zooid fully expanded; 2, in the first stage of retraction; 3, in the second stage; 4, in the third stage, leaving a shallow prominence or "verruca" on the surface; 5, final stage, the verruca flattened down and the coenenchym closed. can, Canal system; d.m.f, dorsal mesenteric filament of a zooid; si, siphonoglyph.

The phenomenon of dimorphism occurs in some Alcyonaria. A certain number of the zooids of a colony are arrested in their development, and are known as the "siphonozooids." They may be distinguished from the fully formed zooids, which, in these {333}cases, are called the "autozooids," by the absence of tentacles, by the absence of the six ventral and lateral mesenteric filaments, and by the incomplete development of the muscles on the mesenteries, and of the mesenteries themselves. They are, moreover, frequently distinguished by the greater development and extent of the ciliated groove or siphonoglyph on the ventral side of the stomodaeum.

It is often difficult to distinguish between true siphonozooids and young autozooids, and consequently dimorphism has been attributed to some genera in which it almost certainly does not occur. Simple dimorphism undoubtedly occurs in the genera Heteroxenia, Sarcophytum, Anthomastus, Lobophytum, Acrophytum, and Paragorgia. It has also been said to occur in Corallium (Moseley and Kishinouye), Melitodes (Ridley), and some species of Dasygorgiidae.

The Pennatulacea are trimorphic. The main shaft of these colonies is the much modified first formed or axial zooid, adapted for the support of all the other zooids. It usually exhibits no mouth, no tentacles, and only four of the original eight mesenteries. It has no mesenteric filaments and no stomodaeum, and bears no sexual cells. The other zooids of the colony are similar in structure to the autozooids and siphonozooids of the dimorphic Alcyonaria.

There are eight mesenteric filaments in all Alcyonarian zooids. They have the appearance of thickenings of the free edges of the mesenteries. Two of them, called the "dorsal" mesenteric filaments, are straight when the anthocodia is expanded, and extend from the edge of the stomodaeum for a long distance down into the coelenteron of the zooid; the other six, called the "ventral" mesenteric filaments (i.e. the ventral and ventro-lateral and dorso-lateral), are usually short and are almost invariably slightly convoluted. The dorsal filaments are built up of columnar cells provided with long cilia, and have usually no gland cells, the others may show a few cilia but are principally composed of non-ciliated gland cells. When the bolus of food has passed through the stomodaeum it is seized by these ventral filaments and rapidly disintegrated by the secretion of its cells. The function of the dorsal mesenteric filaments is mainly respiratory. During life their cilia produce a current which flows towards the stomodaeum. On the ventral side of the {334}stomodaeum itself there is a groove called the "siphonoglyph" composed of a specialised epithelium bearing long powerful cilia. But the current produced by the siphonoglyph flows from the mouth downwards into the coelenteric cavity and is thus in the opposite direction to that produced by the dorsal mesenteric filaments. It is very probable that these two currents on the opposite sides of the zooids maintain the circulation of water in the deep-seated parts of the colony which is necessary for the respiration of the tissues.

On each of the eight mesenteries there is a longitudinal ridge due to the presence of a band of retractor muscles. The position of these muscles on the ventral surfaces of the mesenteries only is one of the characteristic features of the sub-class (Fig. 148, and p. 329). They vary considerably in thickness and extent according to the power of retractility possessed by the zooids, but they never vary in their position on the mesenteries.

fig148

Fig. 148.—Diagrammatic transverse sections of an Alcyonarian. A, through the stomodaeum; B, below the level of the stomodaeum. DD, Dorsal directive; dlmf, dorso-lateral mesenteric filament; dmf, dorsal mesenteric filament; gon, gonad; Si, siphonoglyph; V.D, ventral mesentery; V.L, ventro-lateral mesentery. The upper half of the section in B is taken at a higher level than the lower half.

The skeleton of Alcyonaria may consist of spicules of calcium carbonate, of a horny substance frequently impregnated with calcium carbonate and associated with spicules of the same substance, or in Heliopora alone, among recent forms, of a continuous crystalline corallum of calcium carbonate.

The spicules constitute one of the most characteristic features of the Alcyonaria. They are not found in Cornularia, Stereosoma, in a recently discovered genus of Gorgoniidae (Malacogorgia), in certain Pennatulacea and in Heliopora; and it is probable that they may be absent in some local varieties of certain species of Clavularia.

The spicules of Alcyonaria consist of an organic matrix {335}supporting a quantity of crystalline calcium carbonate. In some cases (Xenia) the amount of inorganic salt is so small that the spicule retains its shape after prolonged immersion in an acid; but generally speaking the relative amount of calcium carbonate is so great that it is only by the careful decalcification of the spicules in weak acetic acid that the delicate fibrous organic matrix can be demonstrated.

The spicules vary in size from minute granules to long spindles 9 mm. in length (Spongodes, sp.). They exhibit so many varieties of shape that an attempt must be made to place them in groups. The most prevalent type perhaps is that called the spindle. This is a rod-shaped spicule with more or less pointed extremities. They are usually ornamented with short simple or compound wart-like tubercles (Fig. 149, 5). Spicules belonging to this type are found in all the principal subdivisions of the group except the Pennatulacea.

In the Pennatulacea a very characteristic form of spicule is a long rod or needle marked with two or three slightly twisted ridges, frequently a little knobbed or swollen at the extremities. In the same group, in Xenia and Heteroxenia among the Alcyonacea, and in the family Chrysogorgiidae the spicules are in the form of minute discs or spheres, and in some genera the discs may be united in couples (twins) or in threes (triplets) by short connecting bars (Fig. 149, 10). More irregular calcareous corpuscles of minute size are found in some genera of Pennatulacea.

Other characteristic spicules are the warted clubs of Juncella, the torch-like spicules of Eunicella (Fig. 149, 3), the clubs with irregular leaf-like expansions at one extremity ("Blattkeulen") of Eunicea, and the flat but very irregular scales of the Primnoidae. There are also many genera exhibiting spicules of quite irregular form (Fig. 149, 8).

In the greater number of cases the spicules lie loosely in the mesogloea and readily separate when the soft tissues of the colony decay or are dissolved in a solution of potash. In a few noteworthy examples the spicules become in their growth tightly wedged together to form a compact skeleton, which cannot subsequently be disintegrated into its constituent elements. In the Precious corals (Coralliidae) the spicules of the axial region fuse together to form a solid mass of lime almost as hard and compact as the substance of a pearl.

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fig149

Fig. 149.—Spicules of Alcyonaria. 1, Club of Juncella; 2, warted cross of Plexaurella; 3, torch of Eunicella; 4, needle of Renilla; 5, warted spindle of Gorgonella; 6, spicule of Pennatula; 7, foliate club of Eunicea; 8, irregular spicule of Paramuricea; 9, scale of Primnoa; 10, spicules of Trichogorgia. (5 and 10 original, the remainder after Kölliker.)

In Paragorgia and some other closely related genera the spicules of the axis of the colony also become tightly wedged together, but the core thus formed is far more porous and brittle than it is in the Coralliidae. In Tubipora (the organ-pipe coral) and in Telesto rubra the spicules of the body-walls of the zooids fuse to form perforated calcareous tubes. In some species of Sclerophytum the large spicules of the coenenchym become so closely packed that they form dense stony masses, almost as hard as a Perforate Madreporarian coral. The horny substance, allied chemically to keratin, plays an {337}important part in the building up of skeletal structures in many Alcyonaria. In Clavularia viridis and in Stereosoma a change in the chemical character of the mesogloea of the body-walls of the polyps leads to the formation of a horny tube, which in the former case is built up of interlacing fibres, and in the latter is formed as a homogeneous sheath. In many of the Alcyonacea which have a compact axial skeleton the spicules are cemented together by a horny matrix.

In the Gorgonellidae and some others the hard axis is formed of a horny substance impregnated with a crystalline form of calcium carbonate; but in the Gorgoniidae, many of the Pennatulacea and some other genera very little or no carbonate of lime is found in the horny axis.

The skeleton of the genus Heliopora differs from that of all the other Alcyonaria in its development, structure, and form. In the words of Dr. G. C. Bourne,[365] "the calcareous skeleton of Heliopora is not formed from spicules developed within cells but is a crystalline structure formed by crystallisation of carbonate of lime, probably in the form of aragonite, in an organic matrix produced by the disintegration of cells which I have described as calicoblasts." It is further characterised by its blue colour. A peculiar form of the axial skeleton (Fig. 155), consisting of alternate nodes mainly composed of keratin, and internodes mainly composed of calcium carbonate, is seen in the families Isidae and Melitodidae. In the Melitodidae the nodes contain a considerable number of loose spicules, and the internodes are mainly composed of spicules in close contact but firmly cemented together by a sparse horny matrix. In the Isidae the scanty calcareous substance of the nodes, and the bulk of the substance of the internodes, is formed of amorphous crystalline limestone.

The Alcyonaria exhibit a great variety of colour. Very little is known at present of the chemistry of the various pigments found in the group, but they may conveniently be arranged in two sections, the soluble pigments and the insoluble pigments. To the former section belong various green and brown pigments found in the anthocodiae and superficial coenenchym of many genera. These are related to chlorophyll, and may be very largely the product, not of the Alcyonarians themselves, but of the {338}symbiotic "Algae" (cf. p. 261) they carry. A diffuse salmon-pink colour soluble in spirit occurs in the living Primnoa lepadifera of the Norwegian fjords, and a similar but paler pink colour occurs in some varieties of the common Alcyonium digitatum. Gilchrist[366] states that when he was preserving specimens of Alcyonium purpureum from Cape waters a considerable quantity of a soluble purple pigment escaped.

But the predominant colour of Alcyonarians is usually due to the insoluble pigments of the calcareous spicules. These may be of varying shades of purple, red, orange, and yellow. The colours may be constant for a species or genus, or they may vary in different specimens of one species, or even in different parts of a single colony. Thus the skeletons of Tubipora musica from all parts of the world have a red colour, the species of the genus Anthomastus have always red spicules. On the other hand, we find in Melitodes dichotoma red and yellow varieties in the same locality, and in M. chamaeleon some of the branches of a colony are red and others yellow. In Chironephthya variabilis the colour of the spicules in any one specimen varies considerably, but in a collection of several specimens from a single locality a kaleidoscopic play of colours may be seen, no two specimens being exactly the same in the arrangement of their colour pattern. The influences that determine the colour of the spicules is at present quite unknown, and in view of the great variability that occurs in this respect, colour must be regarded as a most uncertain guide for the determination of species. The blue colour of the genus Heliopora is due to a peculiar pigment which shows characteristic bands in the spectrum.[367]

Phosphorescence.—A great many Alcyonaria are known to be phosphorescent. Moseley says that "All the Alcyonarians dredged by the 'Challenger' in deep water were found to be brilliantly phosphorescent when brought to the surface." The phosphorescence of the common British Pennatula phosphorea has attracted more attention than that of any other species, and has been well described by Panceri, Forbes, and others. Forbes[368] says, "The pen is phosphorescent only when irritated by touch; the phosphorescence appears at the place touched, and {339}proceeds thence in an undulating wave to the extremity of the rachis, but never in the opposite direction; it is only the parts at and above the point of stimulation that show phosphorescence, the light is emitted for a longer time from the point of stimulation than from the other luminous parts; detached portions may show phosphorescence. When plunged in fresh water, the Pennatula scatters sparks about in all directions—a most beautiful sight."

Panceri was of opinion that the mesenteric filaments were the organs of phosphorescence, but the whole question of the cause and localisation of the light in these colonies requires further investigation.

Food.—Very little is known about the food of Alcyonaria, but it is very probable that it consists entirely of minute larvae and other living organisms. When the coelenteric cavities of preserved Alcyonaria are examined, food is very rarely found in them, although fragments of Crustacean appendages have occasionally been seen in the neighbourhood of the mesenteric filaments. Experimenting upon Alcyonium digitatum, Miss Pratt[369] has found that the zooids seize and swallow various small organisms of a surface-net gathering, and that they will also swallow finely minced fragments of the muscle of fish, but that they reject many kinds of fish ova. In many tropical and some extra-tropical species the superficial canal systems and the inter-mesenterial spaces of the zooids contain a large number of Zooxanthellae, and their presence seems to be associated in some cases with a decided degeneration of the digestive organs. It has been suggested that these symbiotic "Algae" prepare food materials after the manner of plants, and that these are absorbed by the hosts, but it appears improbable that in any case this source of food supply is sufficient. It must probably be supplemented in some degree by food obtained by the mouth, and digested in the coelenteric cavity.

The question whether the Alcyonaria can form an important part of the dietary of fish or other carnivorous animals may be economically important. Fragments of the Pennatulid Virgularia have been found in the stomachs of cod and other fish, but with this exception there is no evidence that any genus is systematically or even occasionally preyed upon by any animal. With a very {340}few exceptions Alcyonaria show no signs of having been torn, bitten, or wounded by carnivorous animals. It is improbable that the presence of nematocysts in the tentacles can account for this immunity, as it is known that some predaceous animals do feed upon Coelenterates provided with much larger nematocysts than any Alcyonarian possesses. All Alcyonaria, however, have a characteristic disagreeable odour, and it is possible, as in many other cases, that this is accompanied by an unpleasant taste. But if the Alcyonaria themselves are immune, it is possible that their large yolk-laden eggs may form a not unimportant source of food supply. In places where large colonies flourish, an immense number of eggs or embryos must be discharged into the water during the spawning season, and of these only a minute fraction can survive long enough to found a new colony.

Reproduction.—The formation of colonies by gemmation has frequently been mentioned above. The young buds of a colony arise from the endoderm canals in the body-wall of the zooids, in the general coenenchym, or in the stolon. They never arise from evagination of the coelenteric cavities of the zooids. There is no evidence that fission of a colony to form secondary colonies ever occurs. Gemmation leads to the increase in the number of zooids forming a colony, but not to an increase in the number of colonies.

Fission of the zooids is of extremely rare occurrence; a single case, however, has been recorded by Studer in the genus Gersemia. Sexual reproduction usually occurs once in a year; it is doubtful whether it ever occurs continuously. The colonies appear to be nearly always dioecious, only one case of hermaphroditism having yet been recorded.[370] The ova and sperm sacs are usually formed and matured on the six ventral mesenteries, rarely on the dorsal pair of mesenteries (Fig. 148, B) as well. The spawning season varies with the locality. Alcyonium digitatum spawns at Plymouth at the end of December, and somewhat later at Port Erin. The Pennatulid Renilla and the Gorgonid Leptogorgia spawn in the summer months on the coast of North America. In the Mediterranean Alcyonium palmatum spawns in September and October (Lo Bianco), Gorgonia cavolinii in May and June.

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It is not known for certain when the fertilisation of the ova is effected, but in Alcyonium digitatum, and in the majority of the Alcyonarians, it probably takes place after the discharge of the ova from the zooids. A few forms are, however, certainly viviparous, the larvae of Gorgonia capensis being retained within the coelenteric cavity of the parent zooid until they have grown to a considerable size. The other viviparous Alcyonarians are Corallium nobile (de Lacaze Duthiers), the "Clavulaires petricoles," and Sympodium coralloides (Marion and Kowalevsky), and three species of Nephthya found at depths of 269 to 761 fathoms (Koren and Danielssen). The general features of the development are very similar in all Alcyonarians that have been investigated. The egg contains a considerable amount of yolk, and undergoes a modified form of segmentation. The free-swimming larva is called a "sterrula." It consists of an outer layer of clear ciliated ectoderm cells, surrounding a solid endodermic plasmodium containing the yolk. As the yolk is consumed a cavity appears in the endoderm, and the larva is then called a "planula" (Fig. 150). The mouth is subsequently formed by an invagination of the ectoderm at the anterior pole. The development of the mesenteries has not yet been fully described.

fig150

Fig. 150.—Ciliated "planula" larva of Alcyonium digitatum. Ec, Ectoderm; End, endoderm.

Classification.—The sub-class Alcyonaria may conveniently be classified as follows:—

Grade A. Protalcyonacea.

Grade B. Synalcyonacea.

Order 1. Stolonifera.

Order 2. Coenothecalia.

Order 3. Alcyonacea.

Order 4. Gorgonacea.

Order 5. Pennatulacea.

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Grade A. Protoalcyonacea.

This Grade includes those genera which, like many sea-anemones, do not reproduce by continuous gemmation to form colonies.

Several genera have been described, and they have been placed together in one family called the Haimeidae.

Haimea funebris, M. Edwards, was found off the coast of Algeria; H. hyalina, Koren and Danielssen, in Norway; Hartea elegans, Wright, from the Irish coast; Monoxenia darwinii, Haeckel, from the Red Sea, and a large new species found by the "Siboga" Expedition in deep water off Ceram. All these species, however, are very rare, and there is no satisfactory evidence at present that they remain solitary throughout life.

Grade B. Synalcyonacea.

The sub-division of the Synalcyonacea into orders presents many difficulties, and several different classifications have been proposed. Only two orders of the five that are here recognised are clearly defined, namely, the Coenothecalia, containing the single living genus Heliopora, and the Pennatulacea or Sea-pens; the others are connected by so many genera of intermediate characters that the determination of their limits is a matter of no little difficulty.

Order I. Stolonifera.

These are colonial Alcyonaria springing from a membranous or ribbon-like stolon fixed to a stone or some other foreign object. The body-walls of the individual zooids may be free or connected by a series of horizontal bars or platforms (autothecalous); never continuously fused as they are in other orders (coenothecalous).

In the simplest form of this order, Sarcodictyon catenatum Forbes, the ribbon-like strands of the stolon meander over the surface of stones, forming a red or yellow network, from the upper surface of which the clear transparent anthocodiae of the zooids protrude. When retracted the anthocodiae are drawn down below the surface of the general coenenchym, and their position is indicated by small cushion-like pads on the stolon. {343}Sarcodictyon is found in depths of 10 to 22 fathoms in the Irish Sea, off the west coast of Scotland, the Shetlands, and off the Eddystone Lighthouse, South Devon.

Another very important genus is Tubipora, in which the tubular body-wall of each zooid is very much longer in proportion to its diameter than it is in Sarcodictyon, and the anthocodia is retracted not into the stolon, but into the basal part of the body-wall. The zooids are connected together by horizontal platforms on which new zooids are formed by gemmation. Both horizontal platforms and the body-walls of the zooids are provided with a skeleton of fused spicules of a red colour.

This genus is the well-known Organ-pipe coral, and is found sometimes in immense quantities on the coral reefs of both the old and new world.

It may be seen in pools on the edge of the reefs at low tides in colonies frequently a foot or more in diameter. The tentacles are often of a bright emerald green colour, and as the anthocodiae stand expanded in the clear water they contribute a brilliant patch of colour to the many beauties of their surroundings. When the coral is disturbed, or the water shallows and the anthocodiae are retracted, the dull red colour of the skeleton gradually takes the place of the bright green of the tentacles.

fig151

Fig. 151.—Tubipora musica, a young colony growing on a dead Madrepore branch (M). Hp, The connecting horizontal platforms; p, p, the skeletal tubes of the zooids; St, the basal stolon.

It is probable that this order of Alcyonaria was better represented on the reefs of some of the earlier periods of the world's history than it is at present. The fossil Syringopora, which is found abundantly in the carboniferous limestone and other strata, was probably an Alcyonarian belonging to this order. It resembles Tubipora in its mode of growth, but in place of the horizontal platforms connecting the zooids there are rods or bars from which new zooids spring (Fig. 152). Similar connecting bars are found in the recent Clavularia (Hicksonia, Delage) {344}viridis of the East Indian reefs (Fig. 153). Other fossil forms belonging to the order are Favosites, a very abundant coral of the Upper Silurian rocks, and possibly Columnaria.

fig152

Fig. 152.—Syringopora, a fossil, showing autothecalous tubes (th), funnel-shaped tabulae (tab), and tubular cross-bars (t).

fig153

Fig. 153.—Clavularia (Hicksonia) viridis, with creeping stolon and transverse connecting tubes.

The principal families of the Stolonifera are:—

Fam. 1. Cornulariidae.—Without spicules; Cornularia, Lamarck, Mediterranean; Stereosoma, Hickson, Celebes.

Fam. 2. Clavulariidae.—Clavularia, Quoy and Gaimard; Sarcodictyon, Forbes, British; Sympodium, Ehrb.; Syringopora, Goldfuss, fossil.

Fam. 3. Tubiporidae.—Tubipora, Linnaeus, tropical shallow water.

Fam. 4. Favositidae.—Favosites, Lamarck; Syringolites, Hinde; Stenopora, King.

Order II. Coenothecalia.

This order contains the single genus and species Heliopora coerulea among recent corals, but was probably represented by a large number of genera and species in earlier periods.

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It is found at the present day in many localities in the warm shallow waters of the tropical Pacific and Indian Oceans. It usually flourishes on the inside of the reef, and may form masses of stone five or six feet in diameter. The coral may easily be recognised, as it is the only one that exhibits a blue colour. This colour usually penetrates the whole skeleton, but in some forms is absent from the superficial layers.

The skeleton consists of a number of parallel tubes with imperforate walls, which are fused together in honey-comb fashion. On making a vertical section through a branch of the coral it is found that the tubes are divided into a series of chambers by transverse partitions or "tabulae." The soft living tissues of the coral, the zooids and coenosarc, are confined to the terminal chambers, all the lower parts being simply dead calcareous skeleton supporting the living superficial layer. Among the parallel tubes there may be found a number of larger chambers that seem to have been formed by the destruction of the adjacent walls of groups of about nineteen tubes. These chambers are provided with a variable number of pseudo-septa, and have a remarkable resemblance to the thecae of some Zoantharian corals. That Heliopora is not a Zoantharian coral was first definitely proved by Moseley, who showed that each of these larger chambers contains an Alcyonarian zooid with eight pinnate tentacles and eight mesenteries. The zooids arise from a sheet of coenosarc that covers the whole of the living branches of the coral mass, and this sheet of coenosarc bears a plexus of canals communicating on the one hand with the zooids, and on the other with a series of blind sacs, each of which occupies the cavity of one of the skeletal tubes as far down as the first tabula. The zooids of Heliopora are very rarely expanded during the day-time, and it has been found very difficult to get them to expand in an aquarium. The coral, however, is frequently infested with a tubicolous worm allied to the genus Leucodora, which freely expands and projects from the surface. So constant and so numerous are these worms in some localities that it has actually been suggested that Heliopora should be regarded as a Polychaete worm and not as an Alcyonarian. According to Mr. Stanley Gardiner, however, these worms do not occur in association with the Heliopora found on the reefs of the Maldive Archipelago.

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There is very strong reason to believe that certain fossil corals were closely related to Heliopora; that Heliopora is in fact the solitary survivor of a group of Alcyonarian corals that in past times was well represented on the reefs, both in numbers and in species. The evidence is not so convincing that other fossil corals are closely related to Heliopora, and their true zoological position may remain a matter for surmise. The order may be classified as follows:—

Fam. 1. Heliolitidae.[371]—Coenothecalia with regular, well-developed septa, generally twelve in number, in each calicle.

Heliolites, Dana, Silurian and Devonian. Cosmiolithus, Lindström, Upper Silurian. Proheliolites, Klaer, Lower Silurian. Plasmopora, Edwards and Haime, Upper Silurian. Propora, E. and H., Upper Silurian. Camptolithus, Lindström, Upper Silurian. Diploëpora, Quenst, Upper Silurian. Pycnolithus, Lindström, Upper Silurian.

Fam. 2. Helioporidae.[372]—Coenothecalia with small irregularly arranged coenosarcal caeca, and a variable number of septa or septal ridges. Heliopora, de Blainville, recent, Eocene and Upper Cretaceous. Polytremacis, d'Orbigny, Eocene and Upper Cretaceous. Octotremacis, Gregory, Miocene.

The family Coccoseridae is regarded by Lindström as a sub-family of the Heliolitidae, and the families Thecidae and Chaetetidae are probably closely related to the Helioporidae.

Order III. Alcyonacea.

This order contains a large number of genera of great variety of form. The only characters which unite the different genera are that the body-walls of some groups of zooids, or of all the zooids, are fused together to form a common coenenchym penetrated by the coenosarcal canals, and that the spicules do not fuse to form a solid calcareous, or horny and calcareous, axial skeletal support.

The affinities with the order Stolonifera are clearly seen in the genera Xenia and Telesto. Some species of Xenia form flattened or domed colonies attached to stones or corals, with non-retractile anthocodiae and body-walls united for only a {347}short distance at the base. Young Xenia colonies are in fact Stolonifera in all essential characters. In Telesto prolifera we find a network of stolons encrusting coral branches and other objects after the manner of the stolons of many species of Clavularia, although the zooids do not arise from these stolons singly, but in groups, with their body-walls fused together for a certain distance. In Telesto rubra the spicules of the body-walls are fused together to form a series of perforated tubes very similar in some respects to the tubes of Tubipora.

A remarkable genus is Coelogorgia. Here we find a branching colony arising from a basal stolon, and the axis of the main stem and of each branch consists of a single very much elongated zooid bearing on its thickened walls the branches of the next series and other zooids. It is true that in this genus there is very little fusion of neighbouring zooids, and the amount of true coenenchym is so small that it can hardly be said to exist at all. Bourne[373] has united this genus with Telesto into a family Asiphonacea, which he joins with the Pennatulida in the order Stelechotokea; but their affinities seem to be closer with the Alcyonacea than with the Pennatulacea, from which they differ in many important characters.

fig154

Fig. 154.—Alcyonium digitatum, a single-lobed specimen, with some of the zooids expanded.

The genus Alcyonium not only contains the commonest British Alcyonarian (A. digitatum), but it is one of the most widely distributed genera of all Alcyonaria that occur in shallow water.

The genera Sarcophytum and Lobophytum occur in shallow water in the tropics of the old world. The former frequently consists of huge toad-stool shaped masses, soft and spongy in {348}consistency, of a green, brown, or yellow colour. On some reefs the colonies of Sarcophytum form a very conspicuous feature, and from their very slimy, slippery surface, add to the minor dangers of wading in these regions. Both genera are dimorphic. Some species of the genus Sclerophytum,[374] which occur in the Indian Ocean, are so hard and brittle that they might readily be mistaken for a Zoantharian coral. This character is due to the enormous number of tightly packed spicules borne by the coenenchym. Some of these spicules in S. querciforme are 7 mm. × 1.7 mm.; the largest, though not the longest (vide p. 335) of any spicules occurring in the order.

Another very important genus occurring on coral reefs, and of very wide distribution, is Spongodes. This genus forms bushy and rather brittle colonies of an endless variety of beautiful shapes and colours. Arising from the neck of each anthocodia there are one or two long, sharp, projecting spicules, which give the surface a very spiny or prickly character.

The genera Siphonogorgia and Chironephthya form large brittle, branching colonies which might readily be mistaken for Gorgonians. The strength of the branches, however, is mainly due to the large, densely packed, spindle-shaped spicules at the surface of the coenenchym, the long coelenteric cavities of the zooids penetrating the axis of both stem and branches. Siphonogorgia is usually uniformly red or yellow in colour. Chironephthya, on the other hand, exhibits a great variety of colour in specimens from the same reef, and indeed in different branches of the same colony.

Fam. 1. Xeniidae.—Alcyonacea with non-retractile zooids. Spicules very small discs, usually containing a relatively small proportion of lime.

Xenia, Savigny; Indian Ocean and Torres Straits. Heteroxenia, Kölliker; Red Sea, Cape of Good Hope, and Torres Straits.

Fam. 2. Telestidae.—Colonies arising from an encrusting membranous or branching stolon. The erect stem and branches are formed by the body-walls of two or three zooids only, from which secondary zooids and branches of the next order arise.

Telesto, Lamouroux, widely distributed in warm waters of the Atlantic, Pacific, and Indian Oceans. The genus Fascicularia, Viguier, from the coast of Algiers, seems to be related to Telesto, {349}but the groups of zooids are short, and do not give rise to branches.

Fam. 3. Coelogorgiidae.—The colony arborescent, attached by stolon-like processes. The stem formed by an axial zooid with thickened body-walls. Branches formed by axial zooids of the second order, and branchlets by axial zooids of the third order, borne either on two sides or in spirals by the main stem. Genus Coelogorgia, Zanzibar.

Fam. 4. Alcyoniidae.—The colonies of this family are usually soft and fleshy, and the spicules, evenly distributed throughout the coenenchym, do not usually fuse or interlock to form a continuous solid skeleton. They may be unbranched or lobed, never dendritic in form. The principal genera are:—Alcyonium, Linnaeus, cosmopolitan, but principally distributed in temperate and cold waters. Alcyonium digitatum is the commonest British Alcyonarian. It is found in shallow water, from the pools left at low spring tides to depths of 40 or 50 fathoms, at most places on the British shores. It is stated by Koehler to descend into depths of over 300 fathoms in the Bay of Biscay. There are two principal varieties; one is white or pale pink in the living condition, and the other yellow. In some localities the two varieties may be found in the same pools. Another species, Alcyonium glomeratum, placed in a distinct genus (Rhodophyton) by Gray, and distinguished from the common species by its red colour and long digitate lobes, is found only off the coast of Cornwall. Paralcyonium, Milne Edwards; Mediterranean. Sclerophytum, Pratt; sometimes dimorphic, Indian Ocean. Sarcophytum, Lesson; dimorphic, principally tropical. Lolophytum, Marenzeller; dimorphic, tropical. Anthomastus, Verrill; dimorphic, Atlantic Ocean, deep water. Acrophytum, Hickson; dimorphic, Cape of Good Hope.

Fam. 5. Nephthyidae.—Colonies dendritic. Usually soft and flexible in consistency. Nephthya, Savigny; Indian and Pacific Oceans. Spongodes, Lesson; widely distributed in the Indian and Pacific Oceans.

Fam. 6. Siphonogorgiidae.—Colonies often of considerable size. Dendritic. Spicules usually large and abundant, giving a stiff, brittle consistency to the stem and branches. Siphonogorgia, Kölliker; Red Sea, Indian, and Pacific tropics. Chironephthya, Wright and Studer; Indian and Pacific Oceans. Lemnalia, {350}Gray; Zanzibar. Agaricoides, Simpson;[375] Indian Ocean, 400 fathoms.

Order IV. Gorgonacea.

This order contains a very large number of dendritic and usually flexible corals occurring in nearly all seas and extending from shallow waters to the very great depths of the ocean. A large proportion of them are brightly coloured, and as the principal pigments are fixed in the spicules, and are therefore preserved when the corals are dead and dried, they afford some of the most attractive and graceful objects of a natural history museum.

The only character that separates them from the Alcyonacea is that they possess a skeletal axis that is not perforated by the coelenteric cavities of the zooids. The coelenteric cavities are usually short. The order may conveniently be divided into two sub-orders.

Sub-Order 1. Pseudaxonia.

The axis in this sub-order consists of numerous spicules tightly packed together, or cemented together by a substance which is probably allied to horn in its chemical composition. This substance may be considerable in amount, in which case it remains after decalcification as a spongy, porous residue; or it may be so small in amount, as in Corallium, that the axis appears to be composed of solid carbonate of lime. The statement is usually made that the axis is penetrated by nutritive canals in certain genera, but the evidence upon which this is based is unsatisfactory and in some cases unfounded. There can be no doubt, however, that in some genera the axis is porous and in others it is not, and this forms a useful character for the separation of genera.

Fam. 1. Briareidae.—The medullary substance consists of closely packed but separate spicules embedded in a soft horny matrix, which is uniform in character throughout its course. Nearly all the genera form dendritic colonies of considerable size.

The principal genera are:—Solenocaulon, Gray; Indian Ocean and North Australia. Many of the specimens of this genus have fistulose stems and branches. The tubular character of the stem and branches is probably caused by the activity of a Crustacean, {351}Alpheus, and may be regarded as of the nature of a gall-formation.[376] Paragorgia, M. Edwards; Norwegian fjords, in deep water. This genus forms very large tree-like colonies of a ruby-red or white colour. It is perhaps the largest of the dendritic Alcyonarians. It is dimorphic. Spongioderma, Kölliker; Cape of Good Hope. The surface of this form is always covered by an encrusting sponge. Iciligorgia, Ridley; Torres Straits. The stem and branches are compressed and irregular in section.

Fam. 2. Sclerogorgiidae.—The medullary mass forms a distinct axis consisting of closely packed elongate spicules with dense horny sheaths.

Suberogorgia, Gray, has a wide distribution in the Pacific Ocean, Indian Ocean, and the West Indies. Keroeides, W. and S., comes from Japan.

Fam. 3. Melitodidae.—The axis in this family exhibits a series of nodes and internodes (Fig. 155), the former consisting of pads formed of a horny substance with embedded spicules, the latter of a calcareous substance with only traces of a horny matrix. The internodes are quite rigid, the nodes however give a certain degree of flexibility to the colony as a whole. Neither the nodes nor the internodes are penetrated by nutritive canals, but when dried the nodes are porous.

fig155

Fig. 155.—Melitodes dichotoma, showing the swollen nodes and the internodes.

The principal genera are:—Melitodes, Verrill; widely distributed in the Indian and Pacific Oceans, Cape of Good Hope, etc. This genus is in some localities extremely abundant and exhibits great brilliancy and variety of colour. The branching is usually dichotomous at the nodes. Wrightella, Gray. This is a delicate dwarf form from Mauritius and the coast of South Africa. Parisis, Verrill; Pacific Ocean from Formosa to Australia but not very common. One species from Mauritius. The branches arise from the internodes.

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Fam. 4. Coralliidae.—The axis is formed by the fusion of spicules into a dense, solid, inflexible, calcareous core.

Corallium, Lamarck. Corallium nobile, Pallas, the "precious coral," occurs in the Mediterranean, chiefly off the coast of North Africa, but also on the coasts of Italy, Corsica, Sardinia, and it extends to the Cape Verde Islands in the Atlantic Ocean. C. japonicum, Kishinouye, called Akasango by the fishermen, occurs off the coast of Japan, and C. reginae, Hickson, has recently been described from deep water off the coast of Timor.[377] The genus Pleurocorallium, Gray, is regarded by some authors as distinct, but the characters that are supposed to distinguish it, namely, the presence of peculiar "opera-glass-shaped spicules," and the occurrence of the verrucae on one side of the branches only, are not very satisfactory. The following species are therefore placed by Kishinouye[378] in the genus Corallium:—C. elatius, Ridley (Momoirosango); C. konojoi, Kishinouye (Shirosango); C. boshuensis, K.; C. sulcatum, K.; C. inutile, K.; and C. pusillum, K.,—all from the coast of Japan. Of the coral obtained from these species, the best kinds of Momoirosango vary in price from £30 per pound downwards according to the quality. The Shirosango is the least valuable of the kinds that are brought into the market, and is rarely exported.[379] Three species of Corallium (Pleurocorallium) have been described from Madeira,[380] and one of these, C. johnsoni, has recently been found in 388 fathoms off the coast of Ireland.[381] Other species are C. stylasteroides, from Mauritius; C. confusum, Moroff,[382] from Sagami Bay in Japan; and an undescribed species obtained by the "Siboga," off Djilolo. These corals range from shallow water to depths of 300-500 fathoms. Pleurocoralloides, Moroff, differs from the others in having very prominent verrucae and in the character of the large spindle-shaped and scale-like spicules. It was found in Sagami Bay, Japan. Specimens attributed to the genus Pleurocorallium have been found fossil in the white chalk of France, but Corallium has been found only in the tertiaries.[383]

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Sub-Order 2. Axifera.

The axis in this sub-order may be horny, or horny with a core of calcium carbonate, or composed of horn impregnated with calcium carbonate, or of nodes of horn alternating with internodes of calcium carbonate. It may be distinguished from the axis of the Pseudaxonia by the fact that in no case have definite spicules been observed to take part in its formation. It has been suggested that as the Axifera represent a line of descent distinct from that of the Pseudaxonia they should be placed in a separate order. Apart from the character of the axis, however, the two sub-orders show so many affinities in their general anatomy that it is better to regard the two lines of descent as united within the Gorgonacean limit. It is very improbable that the two groups sprang independently from a stoloniferous ancestor.

Fam. 1. Isidae.—This family includes all those Axifera in which the axis is composed of alternate nodes of horn and internodes of calcareous substance.

There can be little doubt of the close affinities of many of the genera of this family with the Melitodidae among the Pseudaxonia. In both the coenenchym is thin and the coelenteric cavities short. No important differences have been observed between the structure of the zooids of the two families, and now that we know that the "nutritive canals" of Melitodes do not perforate the nodes there is no important difference left between the coenosarcal canal systems. The structure and method of calcification of the internodes of the two families are very similar. The main difference between them is that the nodes of the Isidae are purely horny, whereas in the Melitodidae the horny substance of the nodes contains calcareous spicules.

The principal genera are:—Isis, Linnaeus; Pacific Ocean. This genus forms substantial fan-shaped colonies with, relatively, a thick coenenchym, short stout internodes and black horny nodes. Mopsea, Lamouroux; Coast of Australia. The verrucae are club-shaped and are arranged in spiral rows round the stem. Acanella, Gray; principally found in deep water in the Atlantic Ocean but also in the Pacific. The internodes are long and the branches arise from the nodes. Most of the species occur in deep water, some in very deep water (A. simplex, 1600 to 1700 fathoms). In this and the following genera the coenenchym is {354}thin and the zooids imperfectly or not retractile. Ceratoisis, Wright; Atlantic Ocean, extending from shallow to deep water. The branches arise from the nodes. Chelidonisis, Studer; deep water off the Azores. Isidella, Gray; Mediterranean Sea. Bathygorgia, Wright; off Yokohama, 2300 fathoms. This genus is unbranched, with very long internodes and short nodes. The zooids are arranged on one side only of the stem.

Fam. 2. Primnoidae.—This is a well-marked family. The axis of the colonies is horny and calcareous. The coenenchym and the non-retractile zooids are protected by scale-like spicules, which usually overlap and form a complete armour for the protection of the soft parts. On the aboral side of the base of each tentacle there is a specialised scale, and these fit together, when the tentacles are folded over the peristome, to form an operculum.

The principal genera are:—Primnoa, Lamouroux; Atlantic Ocean, occurring also in the Norwegian fjords. This genus is usually found in moderately deep water, 100 to 500 fathoms. Primnoella, Gray. This genus seems to be confined to the temperate seas of the southern hemisphere. It is unbranched. The zooids are arranged in whorls round the long whip-like stem. Plumarella, Gray; southern hemisphere, in moderately deep water. This is branched pinnately in one plane. The zooids are small and arise at considerable intervals alternately on the sides of the branches. Stenella, Gray; widely distributed in deep water. The zooids are large and are arranged in whorls of three situated at considerable distances apart. Stachyodes, W. and S.; Fiji, Kermadecs, Azores, in deep water. Colony feebly branched. Zooids in regular whorls of five. Other genera belonging to this group of Primnoidae are Thouarella, Gray, and Amphilaphis, Antarctic seas.

The following genera are placed in separate sub-families:—Callozostron, Wright; Antarctic Sea, 1670 fathoms. The axis is procumbent and the zooids are thickly set in rows on its upper surface. The zooids are protected by large imbricate scales, of which those of the last row are continued into long spine-like processes. Calyptrophora, Gray; Pacific Ocean, in deep water. The base of the zooids is protected by two remarkably large scales. Primnoides, W. and S.; Southern Ocean. The opercular scales are not distinctly differentiated and the calyx is therefore imperfectly protected.

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Fam. 3. Chrysogorgiidae.[384]—The axis in this family is composed of a horny fibrous substance with interstratified calcareous particles, and it springs from a calcareous plate, which sometimes gives off root-like processes. It may be unbranched or branched in such a way that the branches of the second, third, and subsequent orders assume in turn the direction of the base of the main axis. The axis is frequently of a metallic iridescent appearance. The zooids usually arise in a single straight or spiral row on the branches, and are not retractile. The coenenchym is thin. The spicules vary considerably, but in a very large proportion of the species they are thin, oval, or hour-glass plates (Fig. 149, 10, p. 336).

By some authors this family is considered to be the simplest and most primitive of the Axifera; but the delicate character of the axis of the main stem and branches, the thinness of the coenenchym, the position of the zooids on one side of the branches only, and the tenuity of the calcareous spicules may be all accounted not as primitive characters, but as special adaptations to the life in the slow uniform currents of deep water.

The principal genera are:—Lepidogorgia, Verrill; Atlantic and Pacific Oceans, 300 to 1600 fathoms. Axis unbranched. Zooids large and arranged in a single row. Trichogorgia, Hickson; Cape of Good Hope, 56 fathoms. Colony branching in one plane. Zooids numerous and on all sides of the branches. Chrysogorgia, D. and M.; deep water. Axis branched. Spicules on the zooids always large. Metallogorgia, Versluys; Atlantic Ocean, 400 to 900 fathoms. Basal part of the stem unbranched (monopodial). Iridogorgia, Verrill. Spiral stem and branches. Pleurogorgia, Versluys. Axis branched in one plane. Coenenchym thick. Riisea, D. and M. Monopodial stem and thick coenenchym.

Fam. 4. Muriceidae.—This is a large family, exhibiting very great variety of habit. The spicules are often very spiny, and project beyond the surface of the ectoderm, giving the colony a rough appearance. A great number of genera have been described, but none of them are very well known. The family requires careful revision.

The more important genera are:—Acanthogorgia, Gray; principally in deep water in the Atlantic Ocean. The calices are {356}large, cylindrical, and spiny. Villogorgia, D. and M.; widely distributed. Delicate, graceful forms, with thin coenenchym. Echinomuricea, Verrill; Muricea, Lamouroux; Paramuricea, Köll; Acamptogorgia, W. and S.; Bebryce, Philippi.

Fam. 5. Plexauridae.—In this family we find some of the largest and most substantial Gorgonids. The axis is usually black, but its horny substance may be impregnated with lime, particularly at the base. The coenenchym is thick, and the zooids are usually completely retractile, and the surface smooth. The species of the family are principally found in shallow water in warm or tropical regions.

The principal genera are:—Eunicea, Lamouroux. The calices are prominent, and not retractile. Plexaura, Lamouroux; Euplexaura, Verrill. Eunicella, Verrill. With an outer layer of peculiar torch-shaped spicules. The only British species of this order is Eunicella cavolini (formerly called Gorgonia verrucosa). It is found in depths of 10 to 20 fathoms off the coast of the English Channel and west of Scotland. Occasionally specimens are found in which a gall-like malformation with a circular aperture is seen, containing a Barnacle. Such gall formations, common enough in some species of Madreporaria, are rarely found in Alcyonaria.

fig156

Fig. 156.—Eunicella cavolini. Some branches of a large dried specimen, showing a gall formed by a Cirripede.

Fam. 6. Gorgoniidae.—This family contains some of the commonest and best-known genera of the order. They usually form large flexible branched colonies with delicate horny axes and thin coenenchym. The zooids are usually completely retractile.

The principal genera are:—Gorgonia, Linn. This genus {357}includes Gorgonia (Rhipidogorgia) flabellum, the well-known fan Gorgonia with intimately anastomosing branches, from the warm waters of the Atlantic Ocean. The genera Eugorgia, Verrill, and Leptogorgia, Milne Edwards, differ from Gorgonia in the character of the spicules. In Xiphigorgia, Milne Edwards, from the West Indies, the branches are much compressed, forming at the edges wing-like ridges, which bear the zoopores in rows. Malacogorgia, Hickson, has no spicules. Cape of Good Hope.

Fam. 7. Gorgonellidae.—In this family the horny axis is impregnated with lime. The surface of the coenenchym is usually smooth, and the spicules small. The colonies are sometimes unbranched (Juncella). In the branching forms the axis of the terminal branches is often very fine and thread-like in dimensions.

fig157

Fig. 157.—Verrucella guadaloupensis, with an epizoic Brittle star (Oph.) of similar colour.

The principal genera are:—Gorgonella, with a ramified flabelliform axis; Ctenocella, with a peculiar double-comb manner of branching; and Juncella, which forms very long unbranched or slightly branched colonies, with club-shaped spicules. All these genera are found in shallow water in the tropical or semi-tropical regions of the world. Verrucella is a genus with delicate anastomosing branches found principally in the shallow tropical waters of the Atlantic shores. Like many of the Gorgonacea, with branches disposed in one plane (flabelliform) Verrucella frequently carries a considerable number of epizoic Brittle stars, which wind their flexible arms round the branches, and thus obtain a firm attachment to their host. There is no reason to suppose that these Brittle stars are in any sense parasitic, as a specimen that bears many such forms shows no sign of injury or degeneration, and it is possible they may even be of service to {358}the Verrucella by preying upon other organisms that might be injurious. An interesting feature of the association is that the Brittle stars are of the same colour as the host, and the knob-like plates on their aboral surface have a close resemblance to the verrucae (Fig. 157).

Order V. Pennatulacea.

The Sea-pens form a very distinct order of the Alcyonaria. They are the only Alcyonarians that are not strictly sedentary in habit, that are capable of independent movement as a whole, and exhibit a bilateral symmetry of the colony. No genera have yet been discovered that can be regarded as connecting links between the Pennatulacea and the other orders of the Alcyonaria. Their position, therefore, is an isolated one, and their relationships obscure.

The peculiarities of the order are due to the great growth and modification in structure of the first formed zooid of the colony. This zooid (Oozooid, Hauptpolyp, or Axial zooid) increases greatly in length, develops very thick fleshy walls, usually loses its tentacles, digestive organs, and frequently its mouth, exhibits profound modification of its system of mesenteries, and in other ways becomes adapted to its function of supporting the whole colony.

fig158

Fig. 158.—Diagram of a Sea-pen. L, leaves composed of a row of autozooids; R, rachis; St, stalk; T, anthocodia of the axial zooid, usually suppressed. (After Jungersen.)

The axial zooid shows from an early stage of development a division into two regions: a distal region which produces by gemmation on the body-wall numerous secondary zooids, and becomes the rachis of the colony; and a proximal region which becomes the stalk or peduncle, and does not produce buds (Fig. 158). The secondary zooids are of two kinds: {359}the autozooids and the siphonozooids. The former have the ordinary characters of an Alcyonarian zooid, and produce sexual cells; the latter have no tentacles, a reduced mesenteric system, and a stomodaeum provided with a very wide siphonoglyph.

The arrangement of the autozooids and siphonozooids upon the axial zooid is subject to great modifications, and affords the principal character for the classification of the order. In the Pennatuleae the autozooids are arranged in two bilaterally disposed rows on the rachis, forming the leaves or pinnae of the colony (Fig. 158). The number in each leaf increases during the growth of the colony by the addition of new zooids in regular succession from the dorsal to the ventral side of the rachis[385] (Fig. 159). In other Pennatulacea the autozooids are arranged in rows which do not unite to form leaves (Funiculina), in a tuft at the extremity of a long peduncle (Umbellula), scattered on the dorsal side of the rachis (Renilla, Fig. 160), or scattered on all sides of the rachis (Cavernularia, Fig. 161). In those forms in which the autozooids are scattered the bilateral symmetry of the colony as a whole becomes obscured. The siphonozooids may be found on the leaves (Pteroeides), but more frequently between the leaves or rows of autozooids, or scattered irregularly among the autozooids. Usually the siphonozooids are of one kind only, but in Pennatula murrayi there is one specially modified siphonozooid at the base of each leaf,[386] which appears to have some special but unknown function.

fig159

Fig. 159.—Diagram of a portion of a rachis of a Sea-pen, aut, The rows of autozooids; 1-6, the order of age of the autozooids composing a leaf; D, the dorsal side of the rachis; Si, the siphonozooids; V, the ventral side of the rachis. (After Jungersen.)

In Umbellula gracilis each siphonozooid bears a single pinnate tentacle, and in some other species of the same genus there is a tentacle which is not pinnate.[387]

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The zooids and coenenchym are usually protected by a crust of coloured or colourless, long, smooth, needle-like, calcareous spicules, situated principally in the superficial layer, so as to leave the subjacent tissues soft and spongy in texture. In some cases the spicules are smooth double clubs, rods, discs, or irregular granules, and in Sarcophyllum, Chunella, some species of Umbellula and others, there is no calcareous skeleton. The tuberculated spindles, so common in other Alcyonaria, are not found in any species. In most genera a horny, or calcified horny rod is embedded in the central part of the axial polyp, serving as a backbone or support for its muscles. It is absent, however, in Renilla, and reduced or absent in Cavernularia.

The sexual organs are borne by the mesenteries of the autozooids only, and each colony is either male or female. There is no record of hermaphroditism in the order. The eggs contain a considerable amount of yolk, and fertilisation is effected in the sea-water after their discharge. The segmentation is irregular, and the free-swimming ciliated larva (of Renilla) shows the rudiments of the first buds from the axial polyp before it settles down in the mud.

The Sea-pens are usually found on muddy or sandy sea-bottoms, from a depth of a few fathoms to the greatest depths of the ocean. It is generally assumed that their normal position is one with the peduncle embedded in the mud and the rachis erect. Positive evidence of this was given by Rumphius, writing in 1741, in the case of Virgularia rumphii and V. juncea at Amboina,[388] and by Darwin in the case of Stylatula darwinii at Bahia Blanca.[389]

"At low water," writes Darwin, "hundreds of these zoophytes might be seen projecting like stubble, with the truncate end upwards, a few inches above the surface of the muddy sand. When touched or pulled they suddenly drew themselves in with force so as nearly or quite to disappear."

It is not known whether the Pennatulids have the power of moving from place to place when the local conditions become unfavourable. It is quite probable that they have this power, but the accounts given of the Sea-pens lying flat on the sand do not appear to be founded on direct observation. The fable of {361}Pennatula swimming freely "with all its delicate transparent polypi expanded, and emitting their usual brilliant phosphorescent light, sailing through the still and dark abyss by the regular and synchronous pulsations of the minute fringed arms of the whole polypi," appears to be based on a statement made by Bohadsch in 1761, and picturesque though it be, is undoubtedly erroneous.

The brilliant phosphorescence of many species of Pennatulacea has been observed by many naturalists, and it is very probable that they all exhibit this property to some degree. The phosphorescence appears to be emitted by the mesenteric filaments of the autozooids, but it is not yet determined whether the phenomenon is confined to these organs or is more generally distributed.

The Pennatulacea are usually devoid of epizoites, but occasionally the parasitic or semi-parasitic Entomostracan Lamippe is found in the zooids. A small crab is also frequently found between the large leaves of species of Pteroeides. The most remarkable case of symbiosis, however, has recently been observed in the form of an encrusting Gymnoblastic Hydroid[390] living on the free edge of the leaves of a species of Ptilosarcus.

 

The order Pennatulacea is divided into four sections.

Sect. 1. Pennatuleae.—In this section the colony is distinctly bilaterally symmetrical, and the autozooids are arranged in rows with their body-walls fused to form leaves.

The genus Pteroeides, the representative genus of the family Pteroeididae, is a fleshy Sea-pen found in shallow sea water in the warm waters of the Pacific Ocean and in the Mediterranean. It has large leaves with long spiny, projecting spicules, and the siphonozooids are borne by the leaves. Pennatula, the representative genus of the family Pennatulidae, has a wider distribution in area and in depth. Pennatula phosphorea is a common British species, found in depths of 10 to 20 fathoms in many localities off our coasts. It is about 5 inches in length. There are several varieties of this species distributed in Atlantic waters. Pennatula grandis is a magnificent species found in Norwegian fjords, in the Faeroe Channel, and off the northern coasts of N. America, in depths of from 50 to 1255 fathoms. Specimens have been {362}obtained no less than 2½ feet in length. P. murrayi and P. naresi are species of the genus found at depths of a few hundred fathoms in tropical seas.

The genus Virgularia, belonging to the family Virgulariidae, is represented in the British seas by V. mirabilis, a long slender Sea-pen found in many localities off the Scottish coasts.

Sect. 2. Spicatae.—This section includes those Sea-pens in which the autozooids are arranged bilaterally on the axial zooid in rows or more irregularly, but do not unite to form leaves. It is a large section and contains many widely divergent genera.

The family Funiculinidae is represented on our coasts by Funiculina quadrangularis, a long and slender Sea-pen 2 to 3 feet in length. The autozooids are arranged in oblique rows, and the siphonozooids are on the ventral side of the rachis. There is one point of special interest in this genus. The siphonozooids appear to change as the colony grows and to become autozooids. If this is the case it may be more correct to describe the genus as devoid of true siphonozooids.

The family Anthoptilidae contains the species Anthoptilum grandiflorum, which has a wide distribution in depths of 130 to 500 fathoms in the N. and S. Atlantic Ocean. It is perhaps the largest of all the Pennatulacea, specimens having been obtained from the Cape of Good Hope over 4 feet long with expanded autozooids, each more than half an inch in length.

The family Kophobelemnonidae contains a number of forms with remarkably large autozooids arranged in irregular rows on the two sides of the rachis. The siphonozooids are numerous and scattered, and their position is indicated by small papilliform calices on the coenenchym. The surface of these pens is usually rough, owing to the presence of numerous coarse projecting spicules. Kophobelemnon occurs in the Mediterranean in deep water, off the coasts of Ireland and Scotland, and in other regions.

The family Umbellulidae contains some of the most remarkable and interesting examples of the deep-sea fauna. The peduncle is very long and the rachis stunted and expanded. The autozooids are of great size, non-retractile, and arranged in a cluster or rosette on the terminal rachis. There is a wide structural range between the species. Some species have numerous large spicules, others have none. In some species the siphonozooids have a single pinnate or digitate tentacle, in others the siphonozooids {363}are of the usual type. Umbellula appears to be a somewhat rare but cosmopolitan genus in deep water, extending from the Arctic to the Antarctic region in water ranging from 200 to 2500 fathoms.

The interesting genus Chunella was discovered by the German "Valdivia" Expedition at a depth of about 420 fathoms off the coast of E. Africa, and subsequently by the Dutch "Siboga" Expedition at a depth of about 500 fathoms in the Malay Archipelago. According to Kükenthal,[391] this genus with another closely allied genus Amphianthus should form a new section of Pennatulacea, the Verticilladeae. Chunella has a long and very delicate rachis and peduncle, and the former terminates in a single autozooid and has five or six whorls of three autozooids, situated at considerable distances from one another. Spicules are absent. The full description of this genus has not yet been published, but it is clear that it occupies a very isolated position in the order.

fig160

Fig. 160.—Renilla reniformis, a small specimen (34 mm.), showing the dorsal side of the expanded rachis. A, autozooid; H, the mouth of the axial zooid; s, siphonozooid; St, the short stalk. (After Kölliker.)

Sect. 3. Renilleae.—This section contains a single family Renillidae and a single genus Renilla (Fig. 160). The rachis is expanded into a flattened cordate form set at an angle to the peduncle, and the zooids are confined to the dorsal surface, which is uppermost in the natural position of the colony. The peduncle is short and does not contain an axial skeleton. The colour of {364}this Sea-pen is usually violet when dried or preserved. Specimens of Renilla are very abundant in shallow water in some localities on the Atlantic and Pacific coasts of N. America, but the genus has also been obtained from the Red Sea and the coast of Australia. A popular name for this genus is "Sea pansy."

Sect. 4. Veretilleae.—This section contains a number of genera in which the bilateral arrangement of the zooids is obscured by their gradual encroachment on the dorsal side of the axial polyp. The rachis and peduncle are thick and fleshy, and the autozooids and siphonozooids are irregularly distributed all round the rachis. The genus Cavernularia is not uncommonly found in moderate depths of water in the Indian and Pacific Ocean, and is distinguished from the other genera by the reduction of the skeletal axis. Other genera are Veretillum, Mediterranean and Atlantic Ocean, and Lituaria, Indian Ocean.

fig161

Fig. 161.—Cavernularia obesa. Au, autozooid; Si, siphonozooid; St, stalk. (After Kölliker.)

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CHAPTER XIV

ANTHOZOA (CONTINUED): ZOANTHARIA

Sub-Class II. Zoantharia.

The Zoantharia exhibit a great deal more diversity of form and structure than the Alcyonaria. The sub-class is consequently difficult to define in a few words, and it may be taken to include all the Anthozoa which do not possess the typical Alcyonarian characters.

All the orders, with the exception of the Antipathidea and Zoanthidea, contain genera of solitary zooids, and the orders Edwardsiidea and Cerianthidea contain no genera that form colonies. In the Madreporaria, Zoanthidea, and Antipathidea, on the other hand, colonies are formed composed of a very large number of individuals which frequently attain to a very great size. The term "Sea-anemone" is commonly used in writing about the solitary Zoantharia which do not form any skeletal structures, and the term "Coral" is applied to all those Zoantharia which do form a skeleton.

In a scientific treatise, however, these popular terms can no longer be satisfactorily employed. The "Sea-anemones" exhibit so many important differences in anatomical structure that they must be placed in at least three distinct orders that are not closely related, and the organisms to which the term Coral has been applied belong to so many organisms—such as Alcyonaria, Hydrozoa, Polyzoa, and even Algae—that its use has become indeterminate.

Whilst these terms must disappear from the systematic part of Zoology, they may still be employed, however, in the description of a local fauna or coral reef to signify the soft solitary zooids on {366}the one hand, and the organisms, animals or plants, which form large, massive skeletons of carbonate of lime, on the other.

The form of the solitary zooids and of the colony of zooids in the Zoantharia, then, may be very divergent. In the Actiniaria we find single soft gelatinous zooids of considerable size adherent to rocks or half-buried in the sand. Among the Madreporaria we find great branching colonies of thousands of zooids supported by the copious skeleton of carbonate of lime that they have secreted. Among the Antipathidea, again, we find a dendritic skeleton of a dark horny substance, formed by a colony of small zooids that cover it like a thin bark. The majority of the Zoantharia are, like other zoophytes, permanently fixed to the floor of the ocean. Where the embryo settles, there must the adult or colony of adults remain until death. Some of the common Sea-anemones can, however, glide slowly over the surface on which they rest, and thus change their position according to the conditions of their surroundings. Others (the Minyadidae) float upside down in the sea, and are carried hither and thither by the currents. Others, again (Cerianthus, Edwardsia, Peachia), burrow in the sand or mud at the sea-bottom.

The structure of the zooid varies considerably, but in the following characters differs from the zooid of the Alcyonaria. The tentacles are usually simple finger-like processes, and when they bear secondary pinnae these can readily be distinguished from the rows of secondary pinnules of the Alcyonarian tentacle. The number of tentacles is very rarely eight (young Halcampa), and in these cases they are not pinnate. The number of tentacles may be six (many Antipathidea and some zooids of Madrepora), twelve (Madrepora), some multiple of six, or an indefinite number. In the Thalassianthidae and some other families of Actiniaria the tentacles are plumose, but do not exhibit the regular pinnate form of the tentacles of Alcyonaria.

fig162

Fig. 162.—Large (A) and small (B) plumose tentacles of Actinodendron plumosum. Large (C) and small (D) plumose tentacles of A. glomeratum. (After Haddon.)

As regards the number of mesenteries, the Zoantharia exhibit {367}very great variety. It has been shown that there is frequently a stage in their development during which there are only eight mesenteries. This stage is usually called the Edwardsia stage. These eight mesenteries are arranged in bilateral pairs as follows:—One pair is attached to the body-wall and reaches to the dorsal side of the stomodaeum, and is called the pair of dorsal directives; a corresponding pair attached to the ventral side of the stomodaeum is called the pair of ventral directives. The other two pairs are the lateral mesenteries. To these four pairs are added, at the close of the Edwardsia stage, two additional pairs, making in all twelve mesenteries (cf. Fig. 163).

These six primary pairs of mesenteries, conveniently called the "protocnemes" by Duerden, may be traced in the development and recognised in the adult of the majority of Zoantharia. But the number of the mesenteries is usually increased in the later stages by the addition of other mesenteries called the "metacnemes." The metacnemes differ from the protocnemes in that they usually appear in unilateral pairs, that is to say, in pairs of which both members arise on the same side of the stomodaeum, and the number is very variable throughout the group. The space enclosed by a pair of mesenteries is called an "entocoele," and the space between two pairs of mesenteries is called an "ectocoele."

The twelve protocnemes are usually complete mesenteries, that is to say, they extend the whole distance from the body-wall to the stomodaeum, while the metacnemes may be complete or incomplete; in the latter case extending only a part of the distance from the body-wall towards the stomodaeum.

We find, therefore, in making a general survey of the anatomy of the Zoantharia that there is no general statement to be made, concerning the number or arrangement of the mesenteries, which holds good for the whole or even for a considerable portion of the genera.

The bands of retractor muscles are, as in the Alcyonaria, situated on one face only of the mesenteries (except in the Antipathidea and Cerianthidea), but an important character of the Zoantharia is that the muscle bands on the ventral pair of directives are situated on the dorsal faces of these mesenteries, and not on the ventral faces as they are in Alcyonaria.

In the Edwardsiidea there are only eight complete mesenteries, {368}but a variable number of other rudimentary and incomplete mesenteries have recently been discovered by Faurot.[392] In the Zoanthidea the mesenteries are numerous, but the order is remarkable for the fact that the dorsal directives are incomplete, and that, of the pairs of metacnemes that are added, one mesentery becomes complete and the other remains incomplete. In most of the genera of the Antipathidea there are only ten mesenteries, but in Leiopathes there are twelve, and as they bear no bands of retractor muscles it is difficult to determine accurately their true relation to the mesenteries of other Zoantharia.

fig163

Fig. 163.—Diagrams of transverse sections of 1, Alcyonarian; 2, Edwardsia; 3, Cerianthus; 4, Zoanthus; 5, Favia; 6, Madrepora. DD, the dorsal directive mesenteries; VD, the ventral directives; I-VI, the protocnemes in order of sequence.

In the Cerianthidea the mesenteries are very numerous, and increase in numbers by the addition of single mesenteries alternately right and left in the ventral inter-mesenteric chamber throughout the life of the individual. These mesenteries do not bear retractor muscles.

In the Actiniaria and Madreporaria, with the exception of the genera Madrepora, Porites, and a few others, there are also very many mesenteries. The two pairs of directives are usually present, but they may not occur in those zooids that are produced {369}asexually by fission (see p. 388). The metacnemes are frequently formed in regular cycles, and in many genera appear to be constantly some multiple of six (Fig. 163, 5).

In Madrepora and Porites[393] the two pairs of directives and two pairs of lateral protocnemes are complete; the other two pairs of protocnemes are, however, incomplete; and metacnemes are not developed (Fig. 163, 6).

The stomodaeum is usually a flattened tube extending some distance into the coelenteric cavity and giving support to the inner edges of the complete mesenteries; in many of the Madreporaria, however, it is oval or circular in outline. In most of the Actiniaria there are deep grooves on the dorsal and ventral sides of the stomodaeum, but in Zoanthidea the groove occurs on the ventral side only and in the Cerianthidea on the dorsal side only. In the Madreporaria these grooves do not occur or are relatively inconspicuous.[394] In the Alcyonaria the siphonoglyph exhibits a very marked differentiation of the epithelium (see Fig. 148, p. 334), and the cilia it bears are very long and powerful. It has not been shown that the grooves in the Zoantharia show similar modifications of structure, and they are called by the writers on Zoantharia the sulci. There is no difference in structure, and rarely any difference in size, between the dorsal sulcus and the ventral sulcus in the Actiniaria, and the use of the word—sulculus—for the former is not to be commended.

The mesenteries bear upon their free edges the mesenteric filaments. These organs are usually more complicated in structure than the corresponding organs of the Alcyonaria, and the dorsal pair of filaments is not specialised for respiratory purposes as it is in that group.

In many genera the mesenteric filaments bear long, thread-like processes—the "acontia"—armed with gland cells and nematocysts which can be protruded from the mouth or pushed through special holes (the "cinclides") in the body-wall.

The gonads in the Zoantharia are borne upon the sides of the mesenteries and are usually in the form of long lobed ridges instead of being spherical in form, and situated at the edges of the mesenteries as they are in the Alcyonaria.

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Nearly all the zooids and even the colonies of the Zoantharia are unisexual, but some species, such as Manicina areolata (Wilson), Meandrina labyrinthica (Duerden), Cerianthus membranaceus, and others, are hermaphrodite. Mr. J. S. Gardiner has recently given reasons for believing that the genus Flabellum is protandrous.

Skeleton.—The soft tissues of the Zoantharian zooids may be supported or protected by hard skeletal structures of various kinds. In the Zoanthidea and the Actiniaria there are many species that have no skeletal support at all, and are quite naked. These seem to be sufficiently well protected from the attacks of carnivorous animals by the numerous nematocysts of the ectoderm, and perhaps in addition by a disagreeable flavour in their tissues. Anemones do not seem to be eaten habitually by any fish, but cases have been described of Peachia hastata being found in the stomach of the Cod, and of Edwardsia in the stomach of the Flounder.[395] On the Scottish coasts Anemones are occasionally used with success as a bait for cod.[396] The body-wall of Edwardsia, however, is protected to a certain extent by the secretion of a mucous coat in which grains of sand and mud are embedded. Some Anemones, such as Urticina, Peachia, and others, lie half-buried in the sand, and others form a cuticle, like that of Edwardsia, to which foreign bodies are attached.

Cerianthus is remarkable for constructing a long tube composed of a felt-work of discharged nematocysts mixed with mud and mucus, into which it retires for protection. In the Zoanthidea the body-wall is frequently strengthened by numerous and relatively large grains of sand, which are passed through the ectoderm to lie in the thick mesogloea.

In the Madreporaria a very elaborate skeleton of carbonate of lime is formed. In the solitary forms it consists of a cup-shaped outer covering for the base and column of the zooid called the "theca," of a series of radial vertical walls or "septa" projecting into the intermesenteric chambers carrying the endodermal lining of the coelenteric cavity with them, and in some cases a pillar, the "columella," or a series of smaller pillars, the "pali" projecting upwards from the centre of the base of the {371}theca towards the stomodaeum. In the colonial forms the theca of the individual zooids is continuous with a common colonial skeleton called the "coenosteum." This is solid in the Imperforate corals, and it supports at the surface only a thin lamina of canals and superficial ectoderm. In the Perforate corals, however, the coenosteum envelopes and surrounds the canals during its formation, and thereby remains perforated by a network of fine channels. In the colonial Madreporaria the skeletal cups which support and protect the zooids are called the "calices."

The skeleton of the Antipathidea is of a different nature. It is composed of a horny substance allied to keratin. When it is old and thick, it usually has a polished black appearance, and is commonly termed "black coral." The surface of this kind of coral is ornamented with thorny or spiny projections, but it is never perforated by calices or canal systems. It forms a solid axis for the branches of the corals, and all the soft parts of the zooids and coenosarc are superficial to it.

It was formerly considered that this type of coral, which shows no trace of the shape and form of the living organisms that produce it, is of a different character to the calcareous skeleton which exhibits calices, septa, pores, and other evidence of the living organism, and it was called a "sclerobase" to distinguish it from the "scleroderm" of the Madreporaria.

It is now known that both the sclerobasic skeleton and the sclerodermic skeleton are products of the ectoderm, and consequently these expressions are no longer in general use.

Asexual reproduction in the Zoantharia may be effected by continuous or discontinuous fission or gemmation.

In the Edwardsiidea, Actiniaria, and Cerianthidea, that is to say in the animals popularly known as Sea-anemones, asexual reproduction does not commonly occur, but nevertheless a good many instances of it are now known in individual genera. In Actinoloba (Metridium), for example, Parker has described a case of complete longitudinal fission, and Duerden states that it occurs in the West Indian Anemones Actinotryx and Ricordea. A still more remarkable form of asexual reproduction known as transverse fission has been described in the genus Gonactinia.[397] In this case, the body of the Anemone becomes constricted in {372}the middle, a circlet of tentacles is formed below the constriction, and division takes place. The upper half floats away with the original tentacles and stomodaeum and becomes attached by the base in another place; the lower half remains behind and develops a new stomodaeum, mesenteric filaments, and sexual organs. In some of the Actiniaria another form of asexual reproduction occurs, known as "Pedal laceration." In the common British Actinoloba, for example, so often kept in aquaria, the pedal disc sometimes spreads on the glass or rock upon which the animal rests, in the form of a thin membrane or film of an irregular circular shape, nearly twice the diameter of the column. As the Anemone glides along, the film remains behind and breaks up into a number of hemispherical droplets, which in a few days develop tentacles, a mouth, mesenteries, and the other organs of a complete and independent Anemone. A similar method of reproduction has been observed in several species of Sagartia. A true process of discontinuous gemmation has also been observed in Gonactinia, in Corynactis, and in Actinoloba.

fig164

Fig. 164.—Longitudinal fission of Actinoloba. (After Agassiz and Parker.)

In the Madreporaria, Zoanthidea and Antipathidea, the usual method of reproduction to form the colonies is continuous gemmation. The new zooids that are added to the colony as it grows arise as buds, either from the superficial canals of the coenenchym, or from the base or body-wall of the older zooids. In these cases the young zooids acquire the same number of mesenteries, and the same characters of the stomodaeum as the original parent. Some further particulars of asexual reproduction in the Madreporaria are given on p. 387.

The sexual reproduction of a great many species of Zoantharia has now been observed. The eggs are, as a general rule, ripened in batches, and fertilisation is effected before their discharge from the body. In some cases the sexual condition is seasonal. In temperate climates the generative organs ripen in the spring and {373}summer months, and remain small and relatively inconspicuous in the colder weather; but British Sea-anemones, when kept in an aquarium and regularly fed, will breed nearly all the year round. The corals of the tropics living in warmer water of a more regular temperature show considerable variety in their breeding habits. Thus Duerden found that colonies of Favia, Manicina, Siderastraea and Porites are fertile at nearly all times, whereas colonies of Madrepora, Orbicella and Cladocora were rarely so. In nearly all cases the fertilisation is effected, and segmentation of the ovum occurs within the body of the parent, the young Zoantharian beginning its independent life as an oval or pear-shaped ciliated larva.

There are a great many cases among the Actiniaria in which the embryos are retained within the coelenteron, or in special brood pouches of the parent (p. 379), until a stage is reached with twelve or more tentacles.

The oval or pear-shaped larva swims about for a few days or hours, and then settles down on its aboral end. In swimming, the aboral end is always turned forwards. In the larva of Lebrunia coralligens and Rhodactis sancti-thomae, a distinct sense organ has been observed upon the aboral extremity, and a similar but less distinct organ on the larva of Actinia equina. These organs are of considerable interest, as they are probably the only specialised sense organs known to occur in the Zoantharia.

The larvae of Zoantharia present, as a rule, very little variation from the type described, and live but a short time if they fail to find a suitable place for fixation. The colour is usually white and opaque, but in some species the endoderm may be coloured yellow by Zooxanthellae (cf. pp. 86, 125).

The larvae of the Cerianthidea, however, are remarkable and exceptional. After the larva of these animals has passed through the gastrula stage, a certain number of mesenteries and tentacles are formed, and it rises in the water to live a pelagic life of some duration. This larva is known as Arachnactis, and is not unfrequently found in the plankton.

The character of the food of the Zoantharia varies with the size of the zooids, the occurrence of Zooxanthellae in the endoderm, and local circumstances; but in general it may be said to consist mainly of small living animals.

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Sea-anemones kept in an aquarium will readily seize and devour pieces of raw beef or fragments of mussel that are offered to them; but they may also be observed to kill and swallow the small Crustacea that occur in the water. When a living animal of a relatively small size comes within range of the tentacles, it appears to be suddenly paralysed by the action of the nematocysts and held fast. The tentacles in contact with it, and others in the neighbourhood but to a lesser extent, then bend inwards, carrying the prey to the mouth. The passage of the food through the stomodaeum is effected partly by ciliary, and partly by muscular action, and the food is then brought to the region of the mesenteric filaments where it is rapidly disintegrated by the digestive fluids they secrete. Any unsavoury or undigested portions of the food are ejected by the mouth.

Very little is known concerning the food of the Madreporarian Corals. Many investigators have noticed that the zooids of preserved specimens very rarely contain any fragments of animal or plant bodies that could possibly be regarded as evidence of food. It is possible that many Corals derive a part, perhaps in some cases a considerable part, of their nourishment from the symbiotic Zooxanthellae (pp. 86, 125) which flourish in the endoderm; but it is improbable that in any case this forms the only source of food supply. The absence of food material in the cavities of the zooids may perhaps be accounted for by the fact that nearly all the Corals are fully expanded, and therefore capable of catching their food only at night. Corals are usually collected during the daytime, and therefore during the period of rest of the digestive organs.

It is true that nearly all Corals do exhibit Zooxanthellae in their endoderm, but there are some species from which they are nearly or wholly absent, such as Astrangia solitaria and Phyllangia americana on the West Indian reefs,[398] and the Pocilloporidae. The absence of any signs of degeneration in the tentacles or digestive organs of those corals with Zooxanthellae as compared with those without them suggests, at any rate, that the Zooxanthellae do not supply such a large proportion of the food necessary for the support of the colonies as to warrant any relaxation of the efforts to obtain food by other means. Mr. Duerden found that when living Annelids are placed upon the {375}tentacles of a living Siderastraea—a genus with Zooxanthellae, the tentacles at once close upon them and prevent their escape. The general conclusion seems to be, therefore, that the Madreporarian Corals feed upon small animals in much the same way as the Sea-anemones, whether they have Zooxanthellae or not, but that in general they feed only at night.

Age.—It is known that Sea-anemones kept in an aquarium and regularly fed will live for a considerable number of years without showing signs of weakness or failing health. Dalyell kept in an aquarium a specimen of Actinia mesembryanthemum, which lived for sixty-six years and then died a natural death; and specimens of Sagartia, still living, are known to be about fifty years old.[399] The unnatural conditions of life in an aquarium may have favoured the longevity of these specimens, and it would not be reasonable to conclude from these records that the average life of a full-grown Anemone on the rocks is more than thirty or thirty-five years, and perhaps it is a good deal less.

As regards the Madreporarian Corals, we know but little concerning their duration of life. An examination of any living coral reef is sufficient to convince an observer that the power of asexual reproduction of the colonial forms is not unlimited; that colonies, like individuals, have a definite span of life, and that they grow old, senile, and then die a natural death if spared in their youth from accident and disease. Mr. Gardiner has calculated that the duration of life in solitary Corals like Flabellum is about twenty-four years, in colonial forms such as Goniastraea, Prionastraea, Orbicella, and Pocillopora, from twenty-two to twenty-eight years.

Order I. Edwardsiidea.

This order contains only a few genera and species of small size living in shallow water in various parts of the world. In external features they closely resemble several genera of the Actiniaria, particularly those belonging to the family Halcampidae. The distinguishing character of the order is to be found in the system of mesenteries. In all the species only eight mesenteries are complete, namely, the first two pairs of protocnemes, and the two pairs of directives (Fig. 163, 2), {376}and these usually support such large and powerful muscle-bands that they appear to be the only mesenteries present. A careful examination of transverse sections, however, reveals the fact that other mesenteries are present. The fifth and sixth pairs of protocnemes seem to be invariably represented, and two or three pairs of metacnemes can also be traced in some species.

The tentacles are variable in number. In Edwardsia beautempsii, for example, they may be 14-16 in number, arranged in a single row round the oral disc. In E. timida they vary from 20 to 24. The normal number appears to be eight tentacles of the first cycle, corresponding to the eight primary inter-mesenteric chambers, plus 6 or 12 tentacles, corresponding with the chambers limited by the more rudimentary mesenteries,—making a total of 14 or 20 tentacles; but by the suppression of the two primary dorso-lateral tentacles, or by the addition of tentacles of another cycle, the actual number is found to vary considerably. The Edwardsiidea are not fixed to the bottom, but are usually found deeply embedded in sand, the aboral extremity being pointed and used for burrowing purposes. The general colour of the body is yellow or yellowish brown, but it is partly hidden by a short jacket of mud or sand and mucous secretion. The oral crown frequently shows beautiful colours. De Quatrefages relates that in Edwardsia beautempsii the oral cone is golden yellow, and the tentacles, transparent for the greater part of their extent, terminate in opaque points of a beautiful yellowish red colour.

fig165

Fig. 165.—Edwardsia beautempsii. Nat. size. (After de Quatrefages.)

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Fam. 1. Edwardsiidae.—Several species of this family have been found in the British area. They are very local in their distribution, but sometimes occur in great numbers.

Edwardsia beautempsii occurs in shallow water near the shores of the English Channel and has been found in Bantry Bay; and E. carnea and E. timida have also been found in the Channel. E. tecta is a recently described species from the S. Irish coast, and E. allmani and E. goodsiri are found in Scottish waters.

Fam. 2. Protantheidae.—This family, constituted for the reception of three remarkable genera, is now usually included in the order Edwardsiidea on the ground that not more than eight mesenteries are complete.

The genus Gonactinia exhibits the very exceptional character of having a thick layer of muscles in the body-wall (cf. Cerianthidea, p. 409), and it is also remarkable for the frequency with which it reproduces itself asexually by longitudinal and, more rarely, by transverse fission. It has been found in Norway, the Mediterranean, and on the reefs of New Caledonia. The other genera of the family are Oractis from California, and Protanthea from the coast of Sweden.

Order II. Actiniaria.

This order contains nearly all the animals popularly known as Sea-anemones. They are usually found in shallow water, attached by a broad basal disc to shells, stones, or sea-weeds. In the Halcampidae, however, the aboral extremity ends in a blunt point as in the Cerianthidea and Edwardsiidea, and the animals live half-buried in sand or mud. The Minyadidae of the southern oceans are pelagic in habit, floating near the surface of the sea with the mouth turned downwards. They are supported in the water by a bladder, formed by an involution of the pedal disc, and filled with gas.

Many of the Sea-anemones are found in symbiotic association with other animals. The common Adamsia of the British coasts is found on whelk shells containing hermit crabs. The crab is probably protected from the attacks of some of its enemies by the presence of the Anemone, which in its turn has the advantage of securing some fragments of the food captured and torn to {378}pieces by the crab. The association, therefore, seems to be one of mutual advantage to the messmates. It is a noteworthy fact that in these associations the species of Sea-anemone associated with a particular hermit crab is nearly always constant. Thus in the English Channel, Adamsia palliata is almost invariably found associated with Eupagurus prideauxii, and Adamsia rondeletii with Eupagurus bernhardus. But, perhaps, the most remarkable association of this kind is to be seen in the case of the little shore crab of the Indian Ocean, Melia tesselata, which invariably holds in each of its large claws a small Sea-anemone. Möbius, who originally described this case, relates that when the crab is robbed of its Anemone it appears to be greatly agitated, and hunts about on the sand in the endeavour to find it again, and will even collect the pieces, if the Anemone is cut up, and arrange them in its claw.[400]

Another very interesting association is that of certain fish and Crustacea with the large Sea-anemones of the tropical Australian coast.[401] Thus Stoichactis kenti almost invariably contains two or more specimens of the Percoid fish Amphiprion percula. This fish is remarkable for its brilliant colour, three pearly white cross-bands interrupt a ground plan of bright orange-vermilion, and the ends of the cross-bands as well as the fins are bordered with black. In another species a prawn of similar striking colours is found. These companions of the giant Anemones swim about among the tentacles unharmed, and when disturbed seek refuge in the mouth. It has been suggested that these bright and attractive animals serve as a lure or bait for other animals, which are enticed into striking distance of the stinging threads of the Anemone, but how the commensals escape the fate of the animals they attract has yet to be explained.

In a considerable number of Sea-anemones, such as Actinoloba marginata and A. dianthus, some species of Sagartia, Actinia cari, Anemonia sulcata, and Calliactis parasitica, the fertilisation of the eggs and their subsequent development take place in the sea water.[402] In a great many others, such as Bunodes (several species), Cereactis aurantiaca, Sagartia troglodytes, Bunodactis {379}gemmacea, etc., the embryos are discharged into the water from the body-cavity of the parent, at a stage with six or twelve tentacles. In the Arctic species of the genera Urticina and Actinostola, however, the embryos are retained within the body of the parent until several cycles of tentacles are developed, and in Urticina crassicornis the young have been found with the full number of tentacles already formed. In Epiactis prolifera from Puget Sound, the young Anemones attach themselves to the body-wall of the parent after their discharge, and in Epiactis marsupialis, Pseudophellia arctica, Epigonactis fecunda, and other species from cold waters, the young are found in numerous brood sacs opening in rows on the body-wall. It is not known for certain how these embryos enter the brood sacs, but it is possible that each sac is formed independently for a young embryo that has settled down from the outside upon the body-wall of the parent. The most specialised example of this kind of parental care in the Sea-anemones is seen in Marsupifer valdiviae from Kerguelen, in which there are only six brood sacs, but each one contains a great many (50-100) embryos.

The wonderful colours of our British Sea-anemones are familiar to most persons who have visited the sea-side. The common Actinia mesembryanthemum of rock pools, for example, is of a purple red colour. The base is usually green with an azure line. Around the margin of the disc there are some twenty-five turquoise blue tubercles. On each side of the mouth there is a small purple spot, and the numerous tentacles forming a circlet round the mouth are of a pale roseate colour. Nothing could be more beautiful than the snowy-white Actinoloba dianthus or the variegated Urticina crassicornis.

Similar wonderful variety and beauty of colour are seen in the Sea-anemones of other parts of the world. Thus Saville Kent[403] in describing a species of the gigantic Stoichactis of the Australian Barrier Reef says, "the spheroidal bead-like tentacles occur in irregularly mixed patches of grey, white, lilac, and emerald green, the disc being shaded with tints of grey, while the oral orifice is bordered with bright yellow."

The order Actiniaria contains a large number of families, presenting a great variety of external form and of detail in general anatomy. The definitions of the families and their {380}arrangement in larger groups have presented many difficulties, and have led to considerable differences of opinion; and even now, although our anatomical knowledge has been greatly extended, the classification cannot be regarded as resting on a very firm basis. The families may be grouped into two sub-orders:—

Sub-Order 1. Actiniina.—The tentacles are simple and similar, and there is one tentacle corresponding to each intermesenteric chamber (endocoel).

Sub-Order 2. Stichodactylina.—The tentacles are simple and similar, or provided with teat-like or ramified pinnules. One or more tentacles may correspond with an endocoel, and there may be two kinds of tentacles (marginal and accessory) in the same genus.

Sub-Order 1. Actiniina.

Fam. 1. Halcampidae.—This family is clearly most closely related to the Edwardsiidea. There are, however, twelve complete mesenteries of the first cycle, and a second cycle of more or less incomplete mesenteries. The tentacles are usually twelve in number, but may be twenty or twenty-four. There is no pedal disc, but the base is swollen and rounded or pointed at the end.

The genus Halcampa includes a considerable number of small species occurring in the shallow waters of the temperate northern hemisphere, and of the Kerguelen Islands in the south. Three British species have been described, of which Halcampa chrysanthellum alone is common. The larva with eight tentacles and eight mesenteries has been found living on the Medusa Thaumantias.

Peachia is a genus containing Anemones of much larger size (10-25 cm.). It is remarkable for the very large siphonoglyph on the ventral side of the stomodaeum, prolonged into a papillate lip projecting from the mouth called the "conchula." The genera Scytophorus from 150 fathoms off Kerguelen and Gyractis from Ceylon, although showing some remarkable peculiarities of their mesenteric system, appear to be closely related to this family.

Ilyanthus mitchellii is a large Anemone with a vesicular base, forty-eight tentacles and mesenteries, occurring in the English Channel, but it is not very common. It is usually {381}placed in a separate family, but is in many respects intermediate in character between the Halcampidae and the Actiniidae.

Fam. 2. Actiniidae.—This family contains some of the commonest British Sea-anemones. There is a large flat pedal disc by which the body is attached to stones and rocks. The body-wall is usually smooth, and not perforated by cinclides. The edge of the disc is usually provided with coloured marginal tubercles. There are no acontia.

Actinia.—This genus contains the widely distributed and very variable species Actinia mesembryanthemum, one of the commonest of the Sea-anemones found in rock pools on the British coast. The colours of this species are often very beautiful (see p. 379) but variable.

Anemonia is a genus with remarkably long tentacles which are not completely retractile. A. sulcata (sometimes called Anthea cereus) is very common in the rock pools of our southern coasts.

Bolocera tuediae is, next to Actinoloba dianthus, the largest of the British Anemones. It has very much the same colour as the common varieties of Actinia mesembryanthemum, but the body-wall is studded with minute, rounded warts. It is found between tide marks in the Clyde sea-area, but usually occurs in deeper water.

Fam. 3. Sagartiidae.—This family includes several genera with a contractile pedal disc, with the body-wall usually perforated by cinclides, and provided with acontia.

The genera may be arranged in several sub-families distinguished by well-marked characters. Among the well-known Sea-anemones included in the family may be mentioned:—

Sagartia troglodytes, a very common British species found in hollows in rocks. It is usually of an olive green or olive brown colour, and the upper third or two-thirds of the body-wall is beset with numerous pale suckers. Adamsia palliata has a white body-wall spotted with bright red patches, and is associated with the hermit crab Eupagurus prideauxii.

Actinoloba (frequently called Metridium) dianthus is considered the handsomest of all the British Sea-anemones. It has a lobed disc frilled with numerous small tentacles, and is uniformly coloured, creamy-white, yellow, pale pink, or olive brown. It lives well in captivity, and sometimes reaches a length of 6 inches with a diameter of 3 inches (Fig. 164).

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Aiptasia couchii is a trumpet-shaped Anemone, found under stones at low-water mark in Cornwall and the Channel Islands, with relatively slight power of retraction.

Gephyra dohrnii is an interesting species with twelve tentacles, which was supposed at one time to form a connecting link between the Actiniaria and the Antipathidea. It is found attached to the stems and branches of various Hydrozoa and Alcyonaria, sometimes in such numbers and so closely set that it gives the impression of having formed the substance of its support. Haddon[404] has described specimens found on the stems of Tubularia from deep water off the south and south-west coasts of Ireland. It also occurs in the Mediterranean and the Bay of Biscay.

Fam. 4. Aliciidae.—The members of this family have a large flat contractile base and simple tentacles. The body-wall is provided with numerous simple or compound outgrowths or vesicles, usually arranged in vertical rows. Alicia mirabilis is a rare Anemone from Madeira with a very broad base, capable of changing its position with considerable activity, and of becoming free and floating upside down at the surface of the sea. Other genera of the family are Bunodeopsis and Cystiactis. The genus Thaumactis, described by Fowler,[405] from the Papeete reefs, has many peculiarities, but is probably capable of crawling rapidly and of floating at the surface like other members of the family. The remarkable Anemone Lebrunia from the West Indies may be included in this family.

Fam. 5. Phyllactidae.—These are distinguished by the presence of a broad collar of foliaceous or digitate processes outside the circle of tentacles. The processes have some resemblance to the foliaceous tentacles of the Stichodactylinae. They are found in the Mediterranean, Red Sea, and on the shores of the Atlantic Ocean, but have not yet been found in the British area.

Fam. 6. Bunodidae.—This family is characterised by prominent verrucae and tubercles of the body-wall. It contains several British species, of which Bunodes gemmacea found between tide marks on our southern shores is fairly common. The very common British species Urticina (Tealia) crassicornis is usually placed in this family, but exhibits some peculiarities which seem {383}to warrant its removal to another division of the Actiniaria. It is found in tide pools attached to rocks, but is usually partially hidden by adherent sand or small stones.

Fam. 7. Minyadidae.—This family contains a number of floating Anemones. The basal disc is folded over to form a gas bladder lined by a cuticular secretion. The species are principally found in the seas of the southern hemisphere.

Sub-Order 2. Stichodactylina.

Fam. 1. Corallimorphidae.—In this family the marginal cycle of tentacles and accessory tentacles are all of the same kind. The accessory tentacles are arranged in radial rows. All the tentacles are knobbed at the extremity. The musculature is weak. Capnea sanguinea, Corynactis viridis, and Aureliania heterocera belong to the British fauna. They are all small Anemones of exquisite colours, but are not very common. The genus Corallimorphus is principally found in the southern hemisphere.

Fam. 2. Discosomatidae.—The tentacles are all of one kind and are very numerous. The mesenteries are also very numerous. The sphincter muscle is strong.

This family includes a rather heterogeneous assembly of forms, and will probably require some rearrangement as our knowledge increases. Nearly all the species are found in the shallow waters of the tropics, and among them are to be found some of the largest Anemones of the world. Stoichactis kenti, from the Barrier Reef, is from one to four feet in diameter across the disc. In the West Indies these Anemones do not attain to such a great size, but Homostichanthus anemone from Jamaica is sometimes 8 inches in diameter.

Fam. 3. Rhodactidae.—In this family the body-wall is smooth and the oral disc greatly expanded. The tentacles are of two kinds. On the margin there is a single cycle of minute tentacles, while on the disc there are numerous tuberculate or lobed tentacles. Many of the species of this family are quite small, but Actinotryx mussoides from Thursday Island has an oral disc 8 inches in diameter. The genera and species are widely distributed in the warm, shallow waters of the world.

Fam. 4. Thalassianthidae.—The tentacles are simple or {384}ramified (Fig. 166), and in some cases very long (Actinodendron arboreum). Many of the specimens of A. plumosum and Megalactis griffithsi are of very large size, 8 to 12 inches in diameter. Of the former of these two species Saville Kent remarks: "The colours are lacking in brilliancy, being chiefly represented by varying shades of light brown and white, which are probably conducive to its advantage by assimilating it to the tint of its sandy bed. When fully extended the compound tentacles are elevated to a height of 8 or 10 inches, and bear a remarkable resemblance to certain of the delicately branching, light brown sea-weeds that abound in its vicinity." The same author calls attention to their stinging, which is "nearly as powerful as the ordinary stinging nettle."

fig166

Fig. 166.—Actinodendron plumosum. D, disc of attachment; Si, siphonoglyph; t, t, lobes of the marginal disc bearing the tentacles; W, body-wall. Height of the column 200 mm. (After Haddon.)

Order III. Madreporaria.

The Madreporaria form a heterogeneous group of Zoantharia characterised by a single common feature, the formation of an extensive skeletal support of carbonate of lime. In a great many cases the skeleton exhibits cups or "calices" into which the zooids may be completely or partially retracted, and these calices usually exhibit a series of radially disposed vertical laminae, the "septa," corresponding with the inter-mesenteric spaces of the zooids. Calices and structures simulating septa also occur in Heliopora, which is an Alcyonarian, and in certain fossil corals which are probably not Zoantharians. The anatomy of the zooids of a great many Madreporaria is now known, and, {385}although a great deal of work yet remains to be done, it may be said that the Madreporaria exhibit close affinities in structure with the Actiniaria. The chief points in the anatomy of the zooids are described under the different sub-divisions, but a few words are necessary in this section to explain the principal features exhibited by the skeleton.

There is no more difficult task than the attempt to explain upon any one simple plan the various peculiarities of the Madreporarian skeleton.[406] The authorities upon the group are not agreed upon the use of the terms employed, nor are the current theories of the evolution of the skeleton consistent. It is necessary, however, to explain the sense in which certain terms are employed in the systematic part that follows, and in doing so to indicate a possible line of evolution of the more complicated compound skeletons from the simple ones.

fig167

Fig. 167.—Series of diagrams to illustrate the structure of the Madreporarian skeleton. A, young stage of a solitary coral with simple protheca (p.t). B, solitary coral, with theca (th), epitheca (e.t), and prototheca (p.t). C, young stage of colonial coral, showing coenosteum (coe) and theca (th), and the formation of the theca of a bud (b). D, two zooids of a more advanced stage of a colonial coral. coe, Coenosteum; th, theca. The black horizontal partitions are the tabulae. E, transverse section of a calyx. c, Costa; col, columella; d, dissepiment; g, septum; p, pali.

There can be no doubt whatever that the whole of the skeleton of these animals is formed by the ectoderm, and is external to their bodies. If we could get rid of the influence of tradition upon our use of popular expressions we should call this skeleton a shell. There can be little doubt, moreover, that this skeleton is formed by a single layer of specialised ectoderm cells called the "calicoblasts."

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The calicoblasts form, in the first instance, a skeletal plate at the aboral end of the coral embryo, which becomes turned up at the edges to form a shallow saucer or cup. This cup is called the "prototheca."[407] At this stage the body-wall of the living zooid may or may not overflow the edge of the prototheca. In the former case the growth of the rim of the prototheca is brought about by the calicoblasts of an inner and outer layer of epiblast, and the cup is then called the "theca." In the latter case, the growth of the rim of the prototheca is continued by the calicoblasts of one layer of epiblast only, and it is called the "epitheca" (Flabellum). With the continued growth of the theca the tissues that have overflowed—the "episarc"—retreat from the base, and in doing so the ectoderm of the edge and, to some extent, the outer side of the episarc secrete a layer of epitheca which becomes more or less adherent to the theca. Thus the cup may have a double wall, the theca and the epitheca (Caryophyllia).

fig168

Fig. 168.—Diagram of a vertical section of a young Caryophyllia, showing the septa (S) covered with endoderm projecting into the coelenteric cavity. M, mouth; St, stomodaeum. (After G. von Koch.)

fig169

Fig. 169.—A young Caryophyllia, viewed from above, showing the tentacles (t) and the stomodaeum (St). The letter m points to a space between a pair of mesenteries, and the darker shading in this place shows a septum projecting radially from the wall of the theca. (After G. von Koch.)

With the growth of the theca and epitheca a certain number of radially disposed laminae of lime rise from the walls and grow centripetally. These are the "septa." Additional ridges on {387}the inner wall of the cup between the septa are called the "dissepiments." Corresponding with the septa there may be a circle of columns or bands rising from the basal parts of the prototheca—the "pali"; and from the actual centre a single column called the "columella." The longitudinal ridges on the outside of the theca, corresponding in position with the septa inside, are called the "costae" (Fig. 167, E, c).

We may imagine that in the primitive forms that gave rise to colonies, the episarc of the primary zooid overflowed on to the substance to which it was attached, and gave rise to successive layers of epithecal skeleton, which may be called the "coenosteum." The ectoderm at the base of the original prototheca is in some corals periodically dragged away from the skeleton, and forms another cup or platform of lime at a little distance from it—the "tabula." New zooids are developed at some distance from the primary one by a process of gemmation in the episarc, and independent thecae, septa, etc., are formed in it; the skeleton of the new zooid thus originated being connected with that of the primary zooid by the coenosteum.

There are many modifications of this simple description of skeleton formation to be considered before a thorough knowledge of coral structure can be understood, but sufficient has been said to explain the use of the terms that it is necessary to employ in the description of the families. When it is necessary to speak of the cup in which the zooid is situated without expressing an opinion as to the homology of its wall, it is called the calyx.

There are many forms of asexual reproduction observed in the Madreporaria. Of these the most frequent is gemmation. The buds are formed either on the episarc or on the canals running between zooids at the surface of the coenenchym. When the young zooids that have been formed by gemmation reach maturity they have the same characters as their parents. Fission occurs in the production of a great many colonies of Madreporaria. It occurs occasionally in such genera as Madrepora and Porites, where reproduction by gemmation prevails, but it is said that gemmation never occurs in those forms such as the Astraeidae Fissiparantes where fission is the rule. In fission a division of the zooid takes place in a vertical plane passing through the stomodaeum and dividing the zooid into two equal parts. In some cases these two parts become separated during the further {388}growth of the coral. In other cases, however, further divisions of the stomodaeum occur before the separation of the zooids, and then elongated, serpentine polyps are produced (as in Meandrina, etc.), which consist of a number of imperfectly separated zooids, each with a distinct mouth and stomodaeum but with continuous coelenteric cavities. Two kinds of fission must be distinguished from each other. In Madrepora and Porites the plane of fission passes dorso-ventrally through the zooids, that is, between the dorsal and ventral pairs of directive mesenteries. In these cases the zooids produced by fission are similar to the parent form. In most Madreporaria, however, the plane of fission appears to be more or less at right angles to this, and the resulting zooids are unlike the original parent form in having either no directive mesenteries at all or only one pair of them.

fig170

Fig. 170.—Diagrammatic transverse sections of Porites to illustrate the process of fission. A, before division; B, fission nearly completed. In A four bilateral pairs (a, b, c, d) of mesenteries have appeared in the entocoele of the ventral directives (VD). These are increased to six pairs and then fission commences as seen in B, the plane of fission passing through the entocoeles of the last pair of secondary mesenteries (f) and of the dorsal directives (DD). I, II, V, VI, the protocnemes in the order of their development. (After Duerden.)

The section Fungacea presents us with some exceptional and remarkable forms of asexual reproduction. The embryo Fungia gives rise to a conical fixed coral called a "trophozooid." The upper part of the calyx of this trophozooid expands and becomes disc-shaped. This is called the "anthocyathus," and after it has reached a certain size it breaks away from the rest of the trophozooid as an adult Fungia. Several anthocyathi may be formed in succession from one trophozooid. This may be described as a process of successive transverse fission. In Diaseris the disc divides into four quadrants, and each quadrant appears to be capable of acquiring the shape and size of the undivided parent.

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Without doubt a process of sexual reproduction occurs in all Madreporaria. In some genera sexual reproduction appears to be almost continuous throughout the year; in others the sexual organs are formed only at periods separated by considerable intervals of sterility. According to the researches of Duerden the Madreporaria appear to be usually viviparous, the early stages of development are passed through within the body of the parent, and the young coral is discharged into the water as a free-swimming ciliated larva. The larvae are spheroidal, oval, or pear-shaped, but change their shape a good deal, and sometimes become elongated, straight, or spirally twisted rods. The larvae are at first dense and opaque, but subsequently they become distended by the absorption of water, and more nearly transparent. They swim about for one or two days, and then settle down by the aboral pole and become fixed. The tentacles are not formed, in any species that has yet been observed, during the free-swimming stage of existence.

fig171

Fig. 171.—A fixed stage in the development of Fungia. The trophozooid has become differentiated into a discoid crown, the anthocyathus (Cy) and a pedicle, the anthocaulus (Ca). (After G. C. Bourne.)

Distribution of Reef Corals.—The principal reef-forming corals reach their greatest size and grow with greatest rapidity in the warm, shallow waters of the world, but they are not confined to this habitat. A species of Madrepora has been found in the very cold waters of Archangel, and Manicina areolata occurs in Simon's Bay, Cape of Good Hope, many degrees south of the region of the East African coral reefs. As regards the distribution of these corals in depth, very little is known at present. The face of the growing coral reef that is turned towards the open sea is so steep that it has been found impossible to determine to what depth the living reef corals actually extend.

The survey of the Macclesfield bank proved that a considerable number of reef corals are to be found alive at depths {390}ranging from 30 to 50 fathoms.[408] To give one example:—In the dredging No. 50, depth 32 to 35 fathoms, living examples of the following genera of corals were obtained: Madrepora, Montipora, Psammocora, Pavonia, and Astraeopora.

Coral Reefs and Atolls.—In many regions of the tropical seas, banks and islands are found which are built up of blocks of coral, coral detritus, and altered or modified limestone. These are the famous coral reefs of which so much has been said and written during the last half-century. There can be little doubt that the superficial strata of these formations are entirely due to the action of coral-forming animals and plants living in warm, shallow sea-water.

fig172

Fig. 172.—Plan of Minikoi Atoll in Laccadive Archipelago. A, the land elevated above the level of high-water mark; Ch, the boat channel; 5 fm, the five fathom line; 2 fm, the two fathom line; L, the lagoon with a maximum depth of 7 fathoms; R, the reef continuing the circle on the east side of the atoll, awash at high tides. (After Stanley Gardiner.)

Three classes of coral reefs are usually recognised: the "fringing reefs" which follow the contour of the coast at a distance of a few hundred yards, and are separated from the beach at low tide by sand flats or a shallow lagoon; the "barrier reefs," following the contour of the coast less regularly than the fringing reefs, but at a much greater distance, and separated from the beach by a lagoon of sufficient depth to serve as a harbour for ships of great size; and, finally, the "atolls," which are ring-shaped, or broken circlets of low islands enclosing a lagoon which is, in some cases, of considerable depth.

It was observed by the early surveyors that in many cases the sea-bottom slopes downwards steeply or almost precipitously from the outer edge of the barrier reefs and atolls to very great {391}depths—to depths, in fact, at which reef-forming corals do not live.

It seems obvious, therefore, that the atolls and barrier-reefs are resting upon some stratum which could not possibly have been formed by reef-building organisms at the same relative position it has now, and the questions arose, What is the substratum and how was it formed?

If this stratum is a coral rock, it is clear that it must have been formed at a time when it was nearer to the surface of the sea than it is now, and that it must have subsided subsequently to greater depths. If, on the other hand, it is a primitive rock, we must assume that in such regions as the Indian Ocean and the South Pacific, where the archipelagoes of atolls extend for hundreds of miles, there are chains of mountain ranges with peaks reaching to a uniform level beneath the surface of the sea. "But we cannot believe that a broad mountain summit lies buried at the depth of a few fathoms beneath every atoll, and nevertheless that throughout the immense areas above named not one point of rock projects above the level of the sea. For we may judge of mountains beneath the sea by those on land, and where can we find a single chain, much less several such chains many hundred miles in length, and of considerable breadth, with broad summits attaining the same height from within 120 to 180 feet?"[409]

To account for the observed facts of the atolls and barrier-reefs, Darwin conceived and expounded the subsidence theory. According to this theory, the regions where atolls now occur were at one time dry land, or an archipelago of volcanic islands surrounded by fringing reefs of the ordinary type. A gradual subsidence of the land took place, and the area of the land diminished; but the area enclosed by the coral reefs did not diminish in a corresponding degree, and the young corals growing on the débris of the older ones as they sank continued the growth of the reef in a direction nearly vertical to the sea-bottom. The fringing reefs thus became barrier reefs, and they were separated from the land by a lagoon of considerable depth. Finally, when the mountain peaks disappeared beneath the waves, a ring-shaped reef or atoll was all that was left to mark the position of the former land.

The fundamental assumption in the subsidence-theory is that {392}the substratum of the coral reefs and islands is coral-formed limestone. To test the truth of this assumption an expedition was sent out to obtain, by boring, evidence of the character of the substratum of a typical atoll. The island of Funafuti in the Ellice group of the Pacific Ocean was selected, and after several attempts a successful boring was made to a depth of 1114 feet. The material from the boring was found to consist of rocks or sands entirely derived from the calcareous skeletons of marine Invertebrate animals and calcareous Algae.[410] Moreover, in the cores from various depths down to the lowermost the fossilised skeletons of the common genera of recent corals, and very few or no representatives of genera of corals now extinct were discovered.

fig173

Fig. 173.—Section of the outer edge of one of the Maldive Atolls. A, foundation of primitive rock cut down by the currents; B, upgrowth of the rim by the deep-sea-, intermediate depth- and (B') reef-organisms; C, extension outwards by means of the talus slope; D, lagoon. Scale in fathoms. (After Stanley Gardiner.)

These facts, therefore, prove the justice of Darwin's assumption as to the nature of the substratum—and give support to the subsidence-theory as applied to this particular island. A strong opinion has, however, been expressed by several authors of recent years that the subsidence-theory cannot account for the formation of all the atolls and barrier reefs that have now been investigated, and alternate hypotheses have been put forward to account for particular cases. The main chain of the Maldive Archipelago in the Indian Ocean, for example, presents special difficulties to the acceptance of the subsidence-theory as one of general application.[411] The main chain of these islands is more than 300 miles long, and lies at right angles to the monsoon currents of the {393}Indian Ocean. Here the action of the currents appears to have cut down a great tract of land to form a plateau more than 100 fathoms in depth. The outer rim of this plateau may have grown in height by the deposit of the skeletons of surface-swimming animals, and the skeletons of deep-sea corals, until it reached a level where reef-forming corals can thrive. A certain number of channels would be retained and even deepened as the rim grew up, and thus the coral would eventually reach the surface not as a single large atoll, but as a series of coral islands. When the coral reef has thus reached the surface and cannot grow farther in height, it spreads radially like a fairy ring on the talus formed by broken corals that have fallen down the slopes. The central parts, no longer protected by living organisms, are continually subject to the solvent action of the sea water penetrating the porous substratum, and sink to form the lagoon.

It is not only in the reefs of the Indian Ocean, however, but in many of the archipelagoes of the Pacific Ocean, where there is evidence of very extensive elevation of the land areas in the neighbourhood of atolls and barrier reefs, that the subsidence-theory does not satisfactorily account for all the observed facts. It appears probable, therefore, that although a gradual subsidence of the land may have been the primary cause of coral reef formation in some areas, similar reefs may have been formed in other areas by other natural methods.

Fossil Corals.—A great number of the genera of corals found in the newer Tertiary deposits, and a smaller number of those occurring in the older Tertiary and Cretaceous strata clearly belong to families now represented by recent corals. In the earlier strata, however, fossils are found which cannot be placed in our system with any degree of certainty. Attempts have been made from time to time to arrange these corals in their proper positions by the careful study and comparison of their skeletal features, but the reasons given are not convincing. The genus Syringopora, and the families Favositidae, Heliolitidae, and Coccoseridae have been noticed in the chapter on Alcyonaria (pp. 343-346). The family Zaphrentidae will be noticed when dealing with the order Zoanthidea.

Among the families of fossil corals of uncertain position which may still be included in the order Madreporaria, the more important are:—

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Cyathophyllidae, a family of solitary and colonial corals with numerous radially arranged septa, extending from the Silurian to the Carboniferous limestone. It includes the genera Cyathophyllum, which was very abundant in Devonian times, and Lithostrotion, which, in the times of the formation of the Carboniferous limestone, occurred in continuous masses extending over great areas of the sea-bottom. The Cyathophyllidae may possibly be ancestral to the representatives of both Astraeidae and Fungiidae, which appeared in the Triassic strata.

The Cyathaxoniidae form a family of solitary turbinate or horn-shaped corals, with septa showing a regular, radial arrangement, and may have been the ancestors of the modern family Turbinoliidae. They have the same geological range as the Cyathophyllidae.

The Cystiphyllidae.—This family consists of solitary corals with very thin septa; the interseptal spaces are filled with an abundant vesicular substance called the "stereoplasm." The systematic position of this family is very doubtful, as the structure is evidently much destroyed, but by some authors it is supposed to be ancestral to the family Eupsammiidae.

These three families, together with the Zaphrentidae (p. 406), were formerly grouped together as the Tetracoralla or Rugosa.

Sub-Order 1. Entocnemaria.

Madreporaria forming perforate coralla, with calices that do not project above, or project only slightly above the surface of the coenosarc. The zooids of each colony are usually small and crowded. The mesenteries arise in bilateral pairs, and the increase in their number takes place in the chamber between the ventral or the dorsal pairs of directives. The corals included in this order are among the most important of the reef-builders. On many of the recent coral reefs they occur in enormous numbers and of great individual size. But although so prevalent upon recent reefs, they appear to have played a far less important part in the formation of the reefs of the early Tertiary times, and in the reefs of times antecedent to the Tertiary they were rare or absent.

Judging from the structure of the skeleton and the palaeontological history alone it might be thought that the Entocnemaria {395}represent the most recent types of Madreporarian structure, but the anatomy of the zooids points to a contrary conclusion. The zooids are of very simple structure; the mesenteries are found only in bilateral pairs, and all the new mesenteries formed after the protocnemes originate in one of the directive chambers. These are characters indicating a very ancient history, suggesting affinities with the Edwardsiidea on the one hand, and some ancient type of Cerianthidea on the other. There can be little doubt that it was owing to the evolution of a porous skeleton of rapid growth that these corals have caught up and passed the Astraeidae and other more specialised forms in the struggle for predominance on the coral reefs.

Fam. 1. Madreporidae.—The calices of the corallum are small and contain a few perfectly distinct septa. The coenosteum is porous and contains a plexus of the coenosarcal canals, which connects the cavities of neighbouring zooids. This family is divided into a number of sub-families, but it is only necessary here to mention the peculiarities of a few of the well-known genera.

Madrepora.—This genus is represented by an immense number of forms on the coral reefs of both the old and new world. Attempts have been made at various times to divide these forms into specific groups, and a large number of species have been defined and named. The differences between these species, however, are such as may be due to varying conditions of life upon the reefs and not to characters transmitted from generation to generation by heredity. There can be no doubt that when our knowledge of the soft tissues of these corals is extended the number of species will be greatly reduced. There are, however, three principal forms of growth or facies in the genus.

1. The flabellate or palmate colonies with large flat or concave fronds, radiating from an encrusting base: Forma palmata.

2. Much branched colonies, several branches radiating obliquely from a common centre: Forma prolifera.

3. Large and more erect colonies, less branched except towards the periphery: Forma cervicornis.

On some reefs one of these forms of growth predominates, and for miles the reef seems to be built up mainly of corals of this shape. On other reefs two or sometimes all three of these forms may be found within a stone's throw of one another. {396}Notwithstanding the difficulty of distinguishing the species, the genus itself is quite well defined. The calices project slightly from the surface of the branches and contain six septa, of which the pair that is parallel with the axis of the branch is the strongest. This strong pair of septa can usually be well seen when a slender branch of a Madrepore is examined by a lens by transmitted light. At the apex of each branch there is a terminal zooid and in the skeleton an apical calyx. The terminal zooid is (in some species at least) different from the lateral or radial zooids. The former is radially symmetrical and has six long equal digitiform tentacles, the latter have usually twelve tentacles, of which six are larger than the others. These tentacles alternate, but they are so arranged on the disc as to give a distinctly bilateral appearance to the zooids.

The colour of the West Indian Madrepores appears to be entirely due to Zooxanthellae (pp. 86, 125). They are lighter or darker shades of brown, sometimes becoming green, yellow, or orange. On the Australian barrier reef and other reefs of the eastern seas the growing points of the branches are variable and often brilliantly coloured, emerald green, violet, or red; giving some of the most wonderful colour effects for which the reef pools are famous. The cause of these brilliant apical colours has not yet been ascertained.

The genus is found in shallow water of all seas of the tropical belt except on the western side of the continent of America.

Montipora.—In this genus the calices are small and situated in depressions in the coenosteum, and there are six, sometimes twelve, septa of approximately equal size. There is no terminal calyx at the apex of the branches. This is a genus of very variable form and wide distribution in all tropical seas except on the shores of the Atlantic Ocean.

Turbinaria.—This genus is usually cup-shaped or foliaceous and twisted in form. The septa may be six to thirty in number. Some of the species of this genus attain to a very great size in favourable localities. There is a specimen in the British Museum that is 16 feet in circumference and weighed, when dried, 1500 lbs.

Fam. 2. Poritidae.—The corallum is usually encrusting, foliaceous, lobed or tufted, rarely dendritic. The whole skeleton is built up of a system of trabeculae and stout cross bars, and in {397}section the limits of the calices are not well defined. The septa are represented by twelve trabeculae. The zooids are small and are usually provided with twelve tentacles. The most important genus is Porites, which is so abundant on many reefs that it may be said to rival Madrepora itself in the luxuriance of its growth. On the Australian barrier reef a species of Porites builds up coralla over twenty feet in length and as many in height. According to Saville Kent they are usually found on the outer side of the reef and form a basis of support for the high-level Madreporas and other corals.[412]

The colours of Porites are very variable and often beautiful. In Jamaica[413] the prevailing colours are bright blue, pale yellow, and yellowish green. In Australia the colours are less brilliant perhaps, but among the prevailing tints are light or bright lilac, a delicate pink, dark yellow, and brown. The genus Porites occurs in Eocene and Miocene deposits, and is now found on all the more important coral reefs of the world.

The genus Alveopora is usually placed with the Poritidae. According to Bernard,[414] however, its affinities with this family are remote, and it is more closely related to the Favositidae (see p. 344). The walls of the calices are contiguous and the septa are reduced to rows of spines, as in the Favositidae. It is found in shallow water in the Pacific, the Indian Ocean, and the Red Sea.

Sub-Order 2. Cyclocnemaria.

Madreporaria forming perforate or imperforate coralla. Solitary or colonial. The zooids have usually a large number of mesenteries arranged in two or more cycles. The mesenteries beyond the protocnemic pairs arise in unilateral pairs in chambers other than those between the directives.

Sect. 1. Aporosa.—Cyclocnemaria in which the theca and septa are not perforated. The zooids of the colonial forms may communicate by means of superficial canals of the coenosarc, or they may be in contact with one another only at their edges.

Several families are included in this section, of which the more important are:—

{398}

Fam. 1. Turbinoliidae.—The corals included in this family are mostly solitary forms attached to foreign objects, or living partly embedded in sand. In some cases a small colony is formed by gemmation.

The genus Flabellum is a solitary coral of a compressed top shape. It has a large number of septa arranged radially on the cup-wall. This cup-wall is not a true theca but an epitheca. In some forms root-like tubes grow out from the sides of the cup near its base and may serve to support the coral on solid objects. In some remarkably fine specimens recently obtained from the Persian Gulf these tubes served to attach the coral to a telegraph cable. Flabellum seems to be cosmopolitan in its distribution. It is usually found in deep or moderately deep water, but some specimens have been dredged in water of 2 to 9 fathoms.

fig174

Fig. 174.—Side view of Trochocyathus hastatus, with exsert septa, well-marked costae (c), and with three spinous projections (Sp) at the base formed by outgrowths from primary costae. (After G. C. Bourne.)

Caryophyllia is a conical coral fixed by a slightly expanded base. The cup-wall is a true theca covered below by an epitheca. There is a spongy columella surrounded by a single circle of pali. There is one British species, C. smithii. It is found attached to shells at a depth of about thirty fathoms near the Eddystone Lighthouse and in other localities in the English Channel. It also occurs between tide marks in the Scilly Islands, and is found off the Shetlands, on the west coast of Scotland, and the south-west of Ireland. The genus is widely distributed and extends from shallow water to depths of 1500 fathoms. Caryophyllia sometimes occurs in clusters which have the appearance of an incipient colony. This may be due to the embryos fixing themselves upon the epitheca of existing individuals and developing there. It is doubtful whether the species ever reproduce asexually either by gemmation or by fission. When the zooid is fully expanded it projects some distance above the corallum and shows a very transparent body-wall with a crown of some fifty tentacles. Each tentacle terminates in a globose head (Fig. 169) charged with nematocysts. The general colour is pale pink, and there is a broad brown circle {399}round the mouth. Large specimens may be three-quarters of an inch in diameter.

Turbinolia is a common Eocene fossil genus found in England and France, and is stated to occur in the Caribbean Sea. The columella stands up like a stylet and the septa are "exsert," i.e. project above the rim of the theca.

Trochocyathus is a genus with well-marked "costae" occurring in tropical shallow water (Fig. 174).

Fam. 2. Oculinidae.—Colonial forms, dendritic or encrusting, with relatively large and rather prominent calices separated by considerable stretches of compact coenosteum. The zooids bear a crown of ten to forty-eight or more capitate tentacles.

Neohelia has a fistulose stem lined internally by a horny membrane. There seems to be some reason for supposing that this membrane is formed by the zooids themselves. A similar membrane is found in the fistulose stems of Amphihelia and perhaps other Oculinidae. If this membrane is really formed by the activity of the corals it forms an exception to the general rule that the skeleton of the Madreporaria is entirely calcareous. Others maintain, however, that this membrane is formed by the Chaetopod worms which are found in the tubes, and that the fistulose stem of the coral is formed by folding round and encrusting the horny tubes of the worm. Neohelia is found in the Pacific Ocean.[415]

Lophohelia is a genus forming dendritic colonies of considerable size. The calices have thick walls and are very deep. Lophohelia prolifera has been found in deep water off the island of Skye and in other localities off the west coast of Scotland. It is also not uncommon in some of the Norwegian fjords and in other parts of the world.

Oculina is another widely distributed genus found in the shallow tropical waters of the West Indies, the Indian and Pacific Oceans. It forms dendritic colonies of considerable size. The calices are usually arranged in a spiral manner on the branches. The colour of the West Indian species is stated to be light or dark brown when alive. The tentacles are arranged in three cycles, and are usually twenty-four in number. Asexual reproduction takes place by budding at the apex of the branches.

Fam. 3. Astraeidae.—This is a very large family, and {400}authorities are not agreed as to its limits or classification. Excluding the simple forms for the present, the family may be said to be distinguished by having the calices so closely crowded that there is little or no coenosteum between them. The corallum is compact and massive, unless bored and perforated by algae, worms, and other coral-destroying organisms.

The genera of Astraeidae that form colonies may be divided into two groups: the Gemmantes and the Fissiparantes. In the group Gemmantes asexual reproduction is effected by gemmation, and each zooid of a colony is a distinct individual with two pairs of directive mesenteries. Among the best known of recent corals included in this group may be mentioned Galaxea. In this genus there is a good deal more coenosteum between the calices than there is in most of the Astraeidae. The calices are long and project some distance above the coenosteum. The septa are exsert. In Galaxea esperi examined by Fowler[416] there are twelve septa, twelve pairs of mesenteries, and twenty-four tentacles, of which twelve are very small and twelve rather larger. The colour is green or brown. The genus is found in shallow water in the tropics of the old world.

In Astrangia solitaria the zooids are either isolated or more generally united by thin strands of perithecal tissue to form encrusting colonies. The septa are not exsert as in Galaxea. Six are prominent and belong to the first cycle, six smaller ones form a second cycle, and an incomplete third and fourth cycle may be seen. Corresponding with each septum there is a tentacle. The tentacles of the innermost cycle are the longest (3 mm. in length). All the tentacles terminate in a knobbed apex. The living zooids are colourless throughout, or display only very delicate tints within restricted areas.[417] This genus occurs principally on the coasts of the American continent, extending as far south as the Straits of Magellan. Other well-known genera of Astraeidae Gemmantes are Orbicella, Cladocora, Phyllangia.

In the group Fissiparantes asexual production takes place by fission without the production of morphologically complete zooids. The tentacles, mesenteries, and septa, when fission is established, are not arranged in regular hexameral cycles, and no {401}new directive mesenteries arise. In some cases very large corals are formed, and, if our conception is correct, these must be regarded, not as a colony of zooids, but as a single individual zooid divided into a considerable number of incompletely separated parts. Among the well-known genera belonging to this group are Euphyllia, Mussa, Meandrina, Coeloria, Favia, and Goniastraea.

In such genera as Euphyllia the parts of the colony become separated by deep grooves, and have the superficial appearance of being distinct individuals; but in the Brain-coral Coeloria and others the surface of the coral presents a series of more or less bent or curved grooves, each with a row of slit-shaped mouths and bordered by rows of tentacles.

A number of genera of solitary corals united in the subfamily Trochosmiliacea are generally included in the family Astraeidae. The study of their skeletal characters has suggested[418] that they are more closely allied to the Turbinoliidae. The principal genera thus transferred would be Trochosmilia, Placosmilia, Parasmilia, and Asterosmilia. As these genera and their allies are nearly all extinct, and nothing is known of the structure of the living zooids, their removal from the Astraeidae may be regarded as not fully justified.

Fam. 4. Pocilloporidae.—The general anatomy of the zooids of this family of corals has some resemblance to that of the Entocnemaria, and it is possible that they will eventually find a place in our classification near to, if not actually within that group. The fact, however, that the skeleton is imperforate is sufficient for the present to justify the inclusion of the family in the section Aporosa. There are but two genera at present known, and in both of them the zooids have twelve tentacles, twelve mesenteries, and only two mesenterial filaments. The zooids are connected together by an elaborate system of canals running in the superficial coenosarc. The calices are bilaterally symmetrical, and in Seriatopora the septa which are parallel with the axis of the branch are united in the centre of the calyx, and are very much larger than the others, as in Madrepora. In all these characters the family shows affinities with the Entocnemaria. In the characters of the skeleton, which is imperforate and tabulate, the affinities are rather with the {402}Cyclocnemaria. The two genera are widely distributed on the coral reefs of the old world, and in some localities are very abundant. Neither genus is found in the West Indies. They are both of recent origin, but Pocillopora occurs in the Miocene. It is a remarkable feature of the family that both genera may be attacked by the gall-forming crab Hapalocarcinus. From some reefs nearly all the Pocilloporidae show crab-galls on a large number of their branches, whereas other Madreporaria are free from them.

fig175

Fig. 175.—A portion of a colony of Pocillopora from the Maldive Archipelago.

fig176

Fig. 176.—A single calyx of Pocillopora septata, showing Co, the columella; S, S, the septa; Th, the theca wall. (After Gardiner.)

Pocillopora is a coral that forms encrusting masses, rising into lobes or branches of considerable size, terminating in blunt apices. Seriatopora is much more slender and ramified, the branches terminating in sharp points.

Sect. 2. Fungacea.—This section of Cyclocnemaria contains a number of solitary and colonial corals of very varied form united in the possession of a number of cross-bars called "synapticula" connecting the septa, and thereby giving strength to the calyx apart from any increase in the thickness of the calyx-wall. The family Fungiidae shows many peculiarities which separate it very distinctly from both the Cyclocnemaria and the Aporosa. The Eupsammiidae, however, approach the {403}Cyclocnemaria in many respects, and the Plesiofungiidae form a connecting link with the Astraeidae. It is very probable that this section had a dual origin, and therefore does not represent a single line of descent.

Fam. 5. Plesiofungiidae.—This family is related to the Aporosa in the possession of septa that are generally solid and imperforate, and to the Astraeidae in particular in the possession of dissepiments. They differ from them, however, in the presence of synapticula and in certain peculiarities of the tentacles.

The genus Siderastraea has recently been studied by Duerden.[419] The colony is usually massive and encrusting in habit. The zooids when expanded do not rise much above the level of the corallum. The tentacles are short and are arranged in irregular cycles on the disc. They terminate in knobbed extremities, and those of the inner cycles are bifurcated. The colour of S. sideraea is reddish-brown when alive. Siderastraea is found in shallow water on the coral reefs, and is widely distributed.

In Agaricia the colony is more foliaceous. The tentacles are rudimentary or small. The colour of the living zooids is very similar to that of Siderastraea. Epistrelophyllum is a solitary coral, from the Jurassic series, belonging to the family.

Fam. 6. Fungiidae.Fungia is an unattached solitary coral of a flat disc-like shape with very numerous exsert imperforate septa. It is frequently of considerable size (six to twelve inches in diameter). On many of the coral reefs of the old world it is extremely abundant, and consequently it is one of the commonest corals of our collections. When alive the corallum is almost hidden by the disc, which is studded all over with very numerous long tentacles.[420] The colour varies in different species, but is usually brown. One species on the Australian barrier reef, F. crassitentaculata, is of a dark olive green colour, the tentacles terminating in white knobs.

The free adult Fungias are derived from a fixed stock called the trophozooid, from which the young Fungias are detached by transverse fission (see p. 388). The thecal wall of the young Fungia when detached from the trophozooid is perforated, but {404}the pores become largely filled up during the later growth of the coral.

There are several genera of colonial Fungiidae of less frequent occurrence, such as Halomitra, Herpetolitha, and Cryptabacia.

Fam. 7. Cycloseridae.—These are solitary or colonial Fungacea with an imperforate theca. Bathyactis occurs at great depths. Diaseris, shallow water on coral reefs.

Fam. 8. Plesioporitidae.—The septa in this family are trabeculate and perforate, resembling in this respect the septa of Poritidae. Leptophyllia, Microsolena, extinct.

Fam. 9. Eupsammiidae.—This family of perforate corals is usually placed with the Madreporidae and Poritidae in the old group Perforata. The researches of Fowler and Gardiner have shown that the arrangement of the mesenteries is that of the Cyclocnemaria, and the presence of synapticula connecting the septa suggests affinities with the Fungacea. The synapticula of the Eupsammiidae, however, are peculiar in being arranged, not in a vertical series, but alternately with one another or quite irregularly in position. The members of this family are solitary or colonial in habit.

Stephanophyllia is a flattened disc-shaped coral, with perforate and dentate septa, found in the Pacific Ocean and as a fossil in various strata since Cretaceous times.

In Leptopenus, from depths of about 1500 fathoms, the perforations are much larger than in the last-named genus, and the skeleton is reduced to a system of slender trabeculae.

Rhodopsammia has a conical shape, and gives rise by gemmation to a number of young zooids, which remain attached for some time to the parent form before becoming free.

Among the colonial genera are Dendrophyllia, Coenopsammia, and the well-known Mediterranean genus Astroides.

Order IV. Zoanthidea.

This order of Zoantharia consists of a number of solitary or colonial Anemones that do not form a skeleton of horn or carbonate of lime, and are distinguished from the Actiniaria by the peculiar arrangement of their mesenteries.

Fam. 1. Zoanthidae.Sphenopus is a solitary coral and terminates aborally in a small sucker-like base, by which it may {405}be attached to foreign bodies. The genera Gemmaria and Isaurus include solitary forms.

In the majority of the species of Zoanthids, however, a basal encrusting stolon is formed, which may be thick and fleshy or membranous, or may consist of a plexus of bands from which several zooids rise and on which the new buds are formed.

The tentacles are numerous, simple, usually short, and arranged in one or two circles on the margin of the disc. Most Zoanthidae are encrusted with sand, shell fragments, or sponge spicules, but Zoanthus and Isaurus are naked. The foreign particles that form the incrustation are firmly attached to the ectoderm, and as a rule many of them sink down into the mesogloea to give additional support to the body-wall. It is the presence of so much incorporated sand that frequently gives these Zoantharia such a very brittle character. The stomodaeum usually exhibits a well-marked ventral siphonoglyph. The mesenteries consist of a pair of complete ventral directives, a pair of incomplete dorsal directives, while of the remaining protocnemes the lateral mesenteries which are first and second in the order of appearance are complete, the sixth is incomplete, whereas the fifth is complete in the Macrocneminae and incomplete in the Brachycneminae. Duerden[421] has found in specimens of three species that the arrangement of the mesenteries is "brachycnemic" (the sixth protocneme imperfect) on one side and "macrocnemic" (the sixth protocneme perfect) on the other. The metacnemes appear in the spaces between the sixth protocnemes and the ventral directives in unilateral pairs, of which one becomes complete and the other always remains incomplete (Fig. 163, 4, p. 368).

The Zoanthidae are usually dioecious, but hermaphroditism undoubtedly occurs in the genera Zoanthus and Isaurus. Little is known of their development, but a larval form discovered by Semper off the Cape of Good Hope, of cylindrical shape, with an opening at each end and distinguished by a longitudinal band of cilia running from one end to the other, is probably the larva of a Zoanthid. It is commonly known as Semper's larva. Other larvae provided with a ring of cilia have also been attributed to this group.

A great many Zoanthidae are epizoic in habit. Thus several {406}species of Epizoanthus form colonies on the shells of Gasteropods inhabited by hermit crabs. Parazoanthus tunicans is found on the stem of a Plumularia; Parazoanthus separatus, from Jamaica, is associated with a sponge. The base of the bundle of long spicules of the Sponge Hyalonema (p. 204) is almost invariably sheathed by a colony of Epizoanthus stellaris.

The only genera occurring within the British area are Epizoanthus (with six species), Parazoanthus (with four species), and Zoanthus sulcatus.

Of the species of Epizoanthus, E. incrustatus is fairly common, in depths of twenty to eighty fathoms on all our coasts, and is frequently commensal with different species of hermit crabs, while E. paguriphilus is found in much deeper water off the west coast of Ireland and is always commensal with hermit crabs. Parazoanthus anguicomus is found at depths of a hundred fathoms off the Shetlands and west of Ireland, and is usually associated with various species of Sponges.

Gerardia savalia is the largest "black coral" of the Mediterranean. The colony begins by encrusting the stem of one of the Gorgoniidae, but soon surpassing its support in growth, it forms a basal horny skeleton of its own and builds up very large branching colonies. A specimen in the British Museum,[422] from twenty fathoms off the island Negropont, is two metres high and two metres wide. The genus appears to be related anatomically to Parazoanthus.

fig177

Fig. 177.—Zoanthus macgillivrayi, a small colony. The tentacles are shown somewhat contracted by the preservative. Each zooid is about 25 mm. in length. (After Haddon.)

Fam. 2. Zaphrentidae.—This family of Palaeozoic corals is usually placed with the Turbinoliidae or in the separate group Tetracoralla. Recently Duerden[423] has given reasons, based on the method of increase of the septa in Lophophyllum, for believing that their affinities lie rather with the Zoanthidae than {407}with the Madreporaria. They are solitary turbinate corals, with numerous septa exhibiting a distinct bilateral symmetry in arrangement. Zaphrentis, Lophophyllum.

Order V. Antipathidea = Antipatharia.

The members of this order can readily be distinguished from all other Zoantharia by the presence of a horny axial skeleton (sclerobase) and the absence of any spicules of calcium carbonate. The skeleton is covered by a thin bark which consists of a number of simple, naked zooids united at their edges. The zooids bear six tentacles, or if there are more than six, six large prominent tentacles. In most genera there are but ten mesenteries, in others twelve. In Cladopathes only six mesenteries are found. The skeleton of the Antipathidea is simple in Stichopathes and Cirripathes, but in all other genera it is ramified. The ramification is usually profuse and irregular. The horny substance of which it is composed is free from any deposit or infiltration of lime. The surface of the younger branches is beset with numerous short spines, the number and arrangement of which are characters largely used in the determination of species. The basal parts of the main axis and the thicker branches are frequently bare, the zooids having died and become disintegrated. In these cases the spines wear away and the skeleton appears to be smooth. The presence of spines on some of the branches is, however, generally sufficient to enable the naturalist to distinguish a dried Antipathid from the axis of a Gorgonid, with which alone it might be confounded.

There are six complete mesenteries in each zooid, but as they bear no retractor muscles it is not certain that they represent the first six protocnemes of other Zoantharia. In a great many species the zooids are oval in shape, the longer diameter being parallel with the axis of the branch. The mouth and stomodaeum are compressed and at right angles to this diameter. It is usually assumed that the mesenteries attached to the angles of the stomodaeum are the directives, and that the remaining pair, which is axial in direction, corresponds with the first pair of protocnemes. The axial pair of mesenteries is frequently very well developed and alone bears the gonads. When other mesenteries are formed they always arise in bilateral pairs between the axial mesenteries {408}and the directives. The tentacles correspond with the intermesenteric chambers. In some genera there is a constriction of the zooid between the pairs of the tentacles on each side of the axial mesenteries and the directive tentacles. This gives them the appearance of a division into three zooids with two tentacles apiece, one with a mouth and two without a mouth; and as the mouthless parts alone bear the gonads on the single axial mesentery, they have been called the "gastrozooids" and "gonozooids" respectively. This must not be regarded, however, as a case of true dimorphism, as the cavities of the so-called gastrozooid and gonozooids are continuous.

The Antipatharia are widely distributed in nearly all the great seas of the world. Some species are found in shallow water in the tropics, but most of them occur in depths of fifty to five hundred fathoms. The genus Bathypathes is only found at enormous depths ranging from 1070 to 2900 fathoms. Specimens of Cirripathes spiralis, Antipathella gracilis, and another species have recently been obtained in deep water off the west coast of Ireland,[424] but these are the only Antipatharia known to occur within the British area.

The very simple structure of the Antipatharia is usually attributed to degeneration. On this view the Antipathidae with only six complete mesenteries are the most modified, whereas the Leiopathidae with twelve mesenteries are more closely related to the ancestral forms, and Gephyra dohrnii (see p. 382) is a link connecting the order with the Actiniaria.

There is no reason, however, for supposing that Gephyra is specially related to this order, and, as pointed out recently by Roule,[425] the simple structure of the zooids of the Antipathidea is more easily explained if they are regarded as primitive forms.

Gerardia (p. 406), from the Mediterranean, forms a horny axial skeleton like that of the Antipathidea, but this genus is probably a Zoanthid.

Fam. 1. Antipathidae.—In this family the zooids have six tentacles and six or ten mesenteries. It includes nearly all the familiar genera, such as Stichopathes, Cirripathes, Antipathes, Antipathella, Cladopathes, and Bathypathes. Schizopathes and {409}its allies occurring in deep water are the forms regarded by Brook as dimorphic.

fig178

Fig. 178.—A portion of a branch of Antipathes ternatensis, showing three zooids and the horny axis beset with thorn-like projections. (After Schultze.)

Fam. 2. Leiopathidae.—This family includes the single genus Leiopathes of the Mediterranean Sea. It is distinguished from the others by the presence of twelve mesenteries.

Fam. 3. Dendrobrachiidae.—This family also consists of a single genus, Dendrobrachia, from 400 fathoms in the South Atlantic. It is distinguished by having pinnate retractile tentacles.

Order VI. Cerianthidea.

This order contains the remarkable Sea-anemone called Cerianthus. Two of the species have been placed in separate genera, but they do not appear to be of more than sub-generic rank. Cerianthus has a long cylindrical body with a double crown of numerous long tentacles at the oral extremity and tapering to a blunt point or rounded at the aboral extremity.

There are numerous mesenteries, which increase in number by the addition of bilateral pairs, arising only in the ventral inter-mesenteric space throughout the greater part, if not the whole, of the life of the zooid. The right mesentery of each young pair is always more advanced than the left, so that the mesenteries have the appearance of arising alternately right and left. None of the mesenteries bear conspicuous bands of retractor muscles. The movements of the body are effected by a thick band of longitudinal fibres lying between the ectoderm and the mesogloea in the body-wall.

The absence or very slight development of muscles on the mesenteries renders it difficult to recognise the homologues of the protocnemes of other Zoantharia in the adult. From the evidence of embryology, however, it seems certain that the six dorsal pairs of mesenteries represent the protocnemes (Fig. 163, 3, p. 368) and the others are metacnemes.

{410}

The stomodaeum exhibits a single long deep siphonoglyph, which is probably dorsal in position.

There are two tentacles to each inter-mesenteric space, one being marginal and the other circumoral. The gonads are borne upon alternate mesenteries, and both ova and spermatozoa are produced by the same individual.

fig179

Fig. 179.—Cerianthus membranaceus. Colour pink, with tentacles annulated pink and brown. About 35 cm. in length. (After Andres.)

The ectoderm of Cerianthus is remarkable for the immense number of nematocysts and gland cells. The latter secrete a quantity of mucus which binds the threads of the discharged nematocysts into a sticky feltwork and this secures particles of sand and mud, the whole forming a long tube in which the animal freely moves. This tube is often of considerable thickness. It is tough and resistant, smooth inside but ragged and muddy outside. It is often many times the length of the animal's body.

The embryo of Cerianthus is set free before the completion of segmentation, and it gives rise to a floating pelagic larva known as Arachnactis. It has a variable number of tentacles and mesenteries according to its age, but when it reaches a size of {411}about 15 mm. in length it has developed characters which are sufficient to determine its position as a Cerianthid.

The genus Cerianthus appears to be widely distributed. C. membranaceus is the common species in the Mediterranean Sea, but a smaller species has been described from Naples under the name C. oligopodus by Cerfontaine. C. americanus occurs on the eastern coasts of North America. The British and North European species is C. lloydii, but another species, C. vogti, has been found at a depth of 498 fathoms in the North Sea. C. nobilis is a gigantic species supposed to be about 1 foot in length when complete, from Torres Straits.

C. bathymetricus of Moseley, placed by Andres in the genus Bathyanthus, is a species of small size (25 mm.), obtained by the "Challenger" from a depth of 2750 fathoms in the North Atlantic. It exhibits a remarkable prolongation of the stomodaeum into the coelenteron in the form of a sack which contained food. Moseley described a species of Cerianthus, 6 inches long, living on the coral reef at Zebu in the Philippines fully expanded in the tropical sunshine.

Several species of Arachnactis larvae have been described. Of these Arachnactis lloydii appears to be undoubtedly the larva of C. lloydii. The adult forms of Arachnactis albida from various stations in the Atlantic Ocean and of Arachnactis americana are not known. The larva of Cerianthus membranaceus has been called Dianthea nobilis, and is characterised by the great length of the column, by the general opacity of all parts of the body, and by the precocious appearance of the median marginal tentacle. A considerable number of remarkable pelagic larvae have been described by van Beneden[426] from the Atlantic Ocean, and provisionally assigned by him to five different genera. The adult forms of these larvae are not known, but they are probably members of this order.

{412}

CHAPTER XV

CTENOPHORA

The Ctenophora are spherical, lobed, thimble-shaped, or band-like animals, usually very transparent and gelatinous in structure. They are exclusively marine, and are found floating at or near the surface of the sea.

Although they are generally classified with the Coelenterata, they are regarded by some authors as having closer affinities with the Polyclad Turbellaria (cf. Vol. II. p. 7). They agree, however, with neither of these divisions in their essential characters, and the only way to indicate and emphasise their unique position is to place them in a separate Phylum.

They differ from all the Coelenterata in the absence of nematocysts, and in the presence in development of a definite mesoblast. The character from which they derive their name, Ctenophora, is the presence on the surface of bands of swimming plates. The plates are called the "combs" (κτείς, gen. κτενός = a comb) or "ctenophoral plates." They occur in all genera included in the Phylum except in Coeloplana (Fig. 183, p. 422).

Another peculiarity of all Ctenophora (except the Beroidae) is the presence, at some stage in the life-history, of two long and extremely contractile tentacles. There is also a well-developed sense-organ (statocyst) in the centre of the aboral area of the body.

The Ctenophora differ from the Turbellaria in the presence of the combs and of the two long tentacles, in the position and relative importance of the statocyst, and, with the exception of Coeloplana, in the general characters of the alimentary canal.

Shape.—Several of the Ctenophora are conical or spherical in shape, but exhibit at the pole where the mouth is situated {413}(Fig. 180, M) a slight conical projection, and at the opposite pole where the sense-organ is placed a slight depression (Ab). In others, the sides of the body are drawn out into a pair of wing-like lobes (Lobata), and the body is considerably flattened or compressed (Fig. 181). The Cestoidea have a long flattened ribbon- or band-shape (Fig. 182), and the Platyctenea (Fig. 183) are flattened in the oro-apical axis and exhibit a well-marked distinction between the dorsal and ventral surfaces. The shape of Beroe is that of a hollow cone or thimble.

fig180

Fig. 180.—Hormiphora plumosa. Ab, position of the aboral sense-organ; Ct, rib of ctenophoral plates; M, mouth; t, tentacle, with two kinds of pinnae. (After Chun.)

Ctenophoral plates.—In many Ctenophora eight lines can be traced, like the lines of longitude on a globe, from the area of the sense-organ to the base of the mouth-cone or hypostome. In the course of these lines are situated the ctenophoral plates. In some species they extend along the greater part of these lines of longitude, but in others they are more restricted. That part of the line that bears the plates is called the "rib" or "costa." These plates or combs form the principal organs of locomotion of the Ctenophores. They consist of a row of cilia fused at the base (cf. p. 141) to form the plate, but free at the extremity where they form the comb-like edge. They are alternately raised, by a rapid contractile action, and then slowly flattened down again. The plates are raised in succession from the aboral to the oral end of each rib, and the appearance given to the bands in the living animal is that of a series of waves travelling down the lines of longitude from the sensory area towards the mouth. The effect of these rhythmic movements of the combs is to {414}drive the animal slowly through the water with the oral cone forwards. In some Ctenophores the costæ are phosphorescent.[427]

Tentacles.—In all the Ctenophora, except the Beroidae and the adult stages of Lobata and Cestoidea, there is a single pair of tentacles. They are attached to the base of a blind funnel-shaped pit which opens to the exterior near the equator of the animal's body. The pits are on opposite sides of the body, and the plane which passes through them both vertically divides the body into approximately equal parts. It is called the "tentacular" or "transverse" plane (Fig. 180). The plane at right angles to this, which also passes through the mouth and statocyst, is called the "sagittal" plane.

The tentacles are solid, and in the Cydippidae, of considerable length. During life they are usually extended, and trail behind the animal as it progresses through the water. But they are extremely contractile, and when the animal is alarmed are suddenly withdrawn into the shelter of the tentacular pits. Each tentacle usually bears a row of short pinnae. The surfaces of the tentacles and of their pinnae are crowded with remarkable cells which carry little globules of an adhesive secretion, and are called the glue-cells or "colloblasts." These cells stick to any foreign body they touch, and may be drawn out some distance from the tentacle, but they remain attached to it by a long spiral thread which unwinds as the cell is pulled out. Although the colloblasts have the function of catching prey similar to that of the nematocysts of Coelenterata, they are true animal cells and are not therefore homologous with nematocysts, which are the cell products of the cnidoblasts.[428]

The Lobata and Cestoidea pass through a stage in development called the Cydippiform or Mertensia stage, when they possess a single pair of long tentacles similar to those described above. In the adult condition, however, these tentacles are absent, and their functions are performed by numerous small accessory tentacles or tentilla arranged in rows on definite lines along the body-wall.

Sense-organ.—At the aboral pole of the Ctenophore there is a hard granulated calcareous body, the "statolith." This is {415}supported by four tufts of fused cilia, and is usually covered by a dome of delicate protoplasmic texture, which is believed to be formed by a fusion of cilia. The dome enclosing the statolith is called the "statocyst."

Supporting the statocyst there is a circular or oval area of ciliated epithelium which is usually supposed, but on insufficient evidence, to be specially sensory in function. Extending from this area in the sagittal plane there are two strips of ciliated epithelium called the "polar fields."

The aboral sense-organ of the Ctenophora is one of the most characteristic organs of the Phylum. The aboral pole of the Medusae of Coelenterata is usually devoid of any special modification of the ectoderm of the bell, and in the Tiarid genus Stomatoca the little tassel at the aboral pole of the Medusa cannot in any sense be regarded as a homologue of the sense-organ of the Ctenophore. If the aboral sense-organ of the Ctenophora can be compared with that of any other group of animals, it would be with the statocyst of many of the Turbellaria, such as that of Convoluta, but it is far more satisfactory to regard it as an organ peculiar to the Ctenophora and as having no true relationship with any sense-organ found in other animals.

Alimentary Canal.—The mouth of the Cydippiform Ctenophores opens into a sac-like chamber called the "stomodaeum," flattened in the sagittal plane and stretching from the oral pole as far as the centre of the body. The stomodaeum passes into a chamber flattened in the transverse plane called the "infundibulum." From the infundibulum a narrow tube passes in the direction of the aboral pole called the "intestine," and from the extremity of this four short tubes pass to the sides of the polar fields at the place where these fields join the sensory area. Two, or, in some cases, all four of these tubes open to the exterior; but they do not appear to serve the purpose of ejecting the undigested portions of the food, which usually pass to the exterior by the mouth as in Coelenterata and Turbellaria.

From the lateral extremities of the infundibulum four pairs of tubes pass to the equatorial region of the body, where each one joins a longitudinal vessel which runs immediately beneath the epithelium supporting the ribs. These are called the longitudinal or "sub-costal" canals. From the infundibulum there also {416}passes a single pair of blind canals, the "paragastric canals," one on each side of the stomodaeum, to end in the oral cone.

In the Lobata the paragastric canals communicate with the longitudinal canals under the transverse costae,[429] and send long blind processes into the lobes. In the Cestoidea the arrangement of the canals is considerably modified in adaptation to the needs of the ribbon-like body. In the Beroidae the paragastric and longitudinal canals are in communication by a peripheral network of canals, and in the Platyctenea there is also a network of canals but without any definite longitudinal vessels.

Sexual Organs.—Most of the Ctenophora are undoubtedly hermaphrodite, but Willey was unable to find ova in some of his specimens of Ctenoplana that were producing spermatozoa. In the Cydippidea the ova are produced on one side of the longitudinal canal and the spermatozoa on the other. Each longitudinal canal therefore performs the functions of a hermaphrodite gland. When the sexual cells are ripe they escape into the infundibulum and are discharged by the mouth. In Ctenoplana there are definite and direct male genital ducts.

The ova are very small when discharged and undergo complete segmentation in the sea water. The development of the Cydippidea is really direct, but there is a stage passed through in which the tentacles are relatively very prominent and situated close to the aboral pole, and this stage is very different in appearance from the adult. In the Lobata and Cestoidea there is, however, a definite larval stage, of the general appearance of a Mertensia, and during this stage fertile eggs and spermatozoa are formed and set free.

Distribution.—Ctenophora are found at the surface of nearly all seas, and many of the genera have a cosmopolitan distribution. Some of the Lobata, the Cestoidea, and the Platyctenea are more commonly found in the warmer regions of the world. Pleurobrachia pileus, Bolina infundibulum, Beroe ovata, and B. cucumis occur off the British coast.

Most of the Ctenophora are from 5 to 20 mm. in diameter, but Beroe reaches the length of 90 mm., Eucharis multicornis {417}a height of 250 mm., and Cestus veneris has been found no less than 1½ metres from one extremity to the other.

Ctenophores usually go about in shoals, and in the case of Beroe cucumis and Eucharis multicornis the shoals may be of very great extent. Pleurobrachia pileus of the British coasts is often found at the end of the season (July) as a series of isolated individuals; but in June they occur in small shoals, swimming so close together that they will choke a tow-net in a very short space of time.

CLASS I. TENTACULATA

Ctenophora provided with a pair of tentacles in the larval stages only or in both larval and adult stages.

Order I. Cydippidea.

This order includes a number of spherical or oval Ctenophores, with a pair of tentacles retractile into deep tentacular pits in the adult stage.

Fam. 1. Mertensiidae.—The body is compressed in the transverse plane, and the ribs on the transverse areas are longer than those on the sagittal areas. The family includes the genus Euchlora, which occurs in the Mediterranean and in the northern part of the Atlantic Ocean. In Charistephane there are only two enormous ctenophoral plates in each of the longitudinal tracts. These plates are so broad that they almost meet laterally to form two continuous circlets round the body of the animal. This genus is found in the Mediterranean, but a few specimens have also been obtained in the Atlantic.

In Tinerfe the body is almost cylindrical, and there is a pair of kidney-shaped swellings at the sides of the aboral pole. It has a pale blue colour, and is found in the Guinea and south equatorial currents of the Atlantic Ocean.

The name Mertensia has been given to several forms that are undoubtedly the young stages of genera belonging to the Lobata, but Chun retains the name M. ovum for a species which is very abundant in the Arctic currents of the North Atlantic.

Fam. 2. Callianiridae.—Two or four wing-like processes, into {418}which the longitudinal canals extend, are found at the aboral pole. Callianira has two of these processes arranged in the transverse plane, and Lophoctenia has four. Callianira is found in the Mediterranean and in the Atlantic from the Arctic to the Antarctic waters.

Fam. 3. Pleurobrachiidae.—The body is almost spherical in form, and the eight ribs are equal in length.

This family includes the genus Pleurobrachia, in which the ribs extend for a considerable distance along the lines of longitude of the spherical body, but do not reach either the oral or the aboral areas. P. pileus is the commonest British Ctenophore, and may be found in shoals in May, June, and July at the surface of the sea or cast up on the sand as the tide ebbs. It is widely distributed in the North Atlantic waters. P. rhodopis of the Mediterranean has rather shorter ribs than P. pileus. Two new species have recently been described from the Malay Archipelago.[430] Hormiphora (Fig. 180, p. 413) differs from Pleurobrachia in having much shorter ribs, and in possessing two kinds of pinnae on the tentacles, those of the ordinary kind and others much larger and sometimes palmate in character. This genus has a world-wide distribution.

In Lampetia and Euplokamis the body is more cylindrical in shape than it is in the other genera, but the ribs and subjacent longitudinal canals extend up to the margin of the aboral field. Both these genera occur in the Mediterranean, but Lampetia is also found in the Malay Archipelago.

Order II. Lobata.

The body is considerably flattened in the transverse plane, and the sagittal areas are extended into the form of two wide peristomial lobes. The oral ends of the areas between the transverse and sagittal ribs are extended to form four flaps, called the "auricles." There are no tentacles nor tentacle-sheaths of the ordinary kind in the adult form; but numerous tentilla, similar in some respects to the pinnae of the tentacles of other Ctenophora, form a fringe round the margin of the auricles and the peristome. A single pair of long, filamentous, non-retractile tentacles arise from the sides of the peristomium in Eucharis {419}multicornis. These tentacles have no sheaths, and do not bear pinnae. They are probably not homologous with those of other Ctenophora.

The characters that separate the families of Lobata are chiefly those of varying size, shape, and position of the peristomial lobes and auricles. In the Lesueuriidae the peristomial lobes are rudimentary; in the other families they are moderately or very large. In the Bolinidae the auricles are short, but in most of the other families they are long and ribbon-like. In Eucharis they can be spirally twisted in repose.

The modifications of the external form seen in the Lobata are accompanied by some modifications of the internal structure. Among these, perhaps the most interesting is a communication between the transverse longitudinal and the paragastric canals, and the long convoluted tubes given off to the peristomial lobes by the sagittal longitudinal canals. Very little is known about the life-history and development of most of the Lobata, but Chun has shown that in Eucharis and Bolina there is a Cydippiform larval stage which produces ripe ova and spermatozoa. This is followed by a period of sterility, but when the adult characters are developed they become again sexually mature. To this series of sexual phenomena the name "Dissogony" is given.

fig181

Fig. 181.—Ocyroe crystallina. Ab, aboral sense-organ; au, auricle; Can, diverticulum from the paragastric canal passing into peristomial lobe; Ct, costae; M, mouth; Par, paragastric canal passing outwards to join one of the transverse subcostal canals; P.L, peristomial lobe; w, wart-like tubercles on the lobe. (After Mayer.)

The order contains only fifteen genera, but they are usually arranged in the following eight families:—

1. Lesueuriidae. Lesueuria.

2. Bolinidae. Bolina, Bolinopsis.

3. Deiopeidae. Deiopea.

4. Eurhamphaeidae. Eurhamphaea.

{420}5. Eucharidae. Eucharis.

6. Mnemiidae. Mnemia, Mnemiopsis.

7. Calymmidae. Calymma.

8. Ocyroidae. Ocyroe.

Most of these Ctenophores occur in the warm and tropical seas; but Bolina is found occasionally at Plymouth in the month of May, on the west coast of Ireland, and at other stations on the British coasts. Eucharis is regarded as one of the most beautiful of the Phylum. A swarm, some miles in length, of large specimens of E. multicornis was met by the Plankton Expedition in the south equatorial current of the Atlantic during the month of September.

Order III. Cestoidea.

In this order the body is so much compressed in the transverse plane and elongated in the sagittal plane that it assumes the shape of a long narrow band or ribbon. The tentacular sheaths are present but the tentacles are degenerate in the adult. The tentacular functions are performed by numerous tentilla situated in long grooves extending along the whole length of the oral side of the band-like body. The transverse ribs are reduced; the sagittal ribs extend along the whole of the aboral side.

Fam. Cestidae.—This is the only family of the order. Cestus veneris, the Venus's girdle of the Mediterranean Sea, is also found in the Atlantic Ocean, and specimens belonging to the same genus, but probably to a different species, occur as far north as the White Sea. Some of the larger specimens are considerably over 1 metre in length.

fig182

Fig. 182.—Cestus pectenalis. Ab, aboral sense-organ; Ct, the sagittal ribs; M, mouth. (After Bigelow.)

C. pectenalis was found in abundance off one of the Maldive Islands [431] and differs from C. veneris in having a large and {421}prominent orange patch at each end of the body. It is said to be extremely graceful in the water, moving with slow, ribbon-like undulations, and shining in the sunlight with a violet iridescence. Vexillum, from the Mediterranean Sea and Canary Islands, is rather more pointed at the extremities than Cestus, and differs from it in some important anatomical characters.

Order IV. Platyctenea.

This order has been constituted for two remarkable genera, in which the oro-apical axis is so much reduced that distinct dorsal and ventral surfaces can be distinguished.

There is a single pair of long milky-white tentacles capable of complete retraction into tentacular sheaths.

Fam. 1. Ctenoplanidae.Ctenoplana was discovered by Korotneff in 1886 floating with the Plankton off the coast of Sumatra. In 1896 Willey [432] discovered four specimens on a cuttle-bone floating off the coast of New Guinea. To these authors we are indebted for the only accounts of this animal that have been published.

When the Ctenoplana is creeping on the bottom of a dish or with its dorsal side downwards on the surface film of the water, it has the form of a flattened disc with a notch on each side. On the upper or dorsal surface eight short rows of ctenophoral plates may be seen, and in a position corresponding with the two notches in the margin of the body are situated the two sheaths from which the long pinnate tentacles protrude. In the exact centre of the dorsal surface is situated the statolith, supported by stiff processes from adjacent cells; and forming a circlet round the statolith there is a row of short ciliated tentacles. These tentacles, however, when examined carefully in the living animal, are found to be arranged in two sets of about nine in each, separated by narrow gaps on each side, the gaps corresponding in position with the axis through the tentacles.

When the animal is swimming it assumes a helmet-shape by depressing the sides of the body like a pair of flaps on the tentacular axis, and then the ctenophoral plates come into play and produce the progressive movements of the animals. The pinnate tentacles are opaque white in colour, and have peculiar serpentine {422}movements. Very little is known at present concerning many details of the internal anatomy, but there is one point of considerable theoretical interest—namely, the presence of definite male genital ducts.

Three of Dr. Willey's specimens were mottled with a green pigment, whereas his fourth specimen and Korotneff's only specimen were mottled with a red pigment. It has yet to be determined whether the differences which have been observed in the individual specimens are of specific value.

Fam. 2. Coeloplanidae.Coeloplana was originally discovered by Kowalevsky in the Red Sea, but has recently been found by Abbott [433] on the coast of Japan.

fig183

Fig. 183.—Coeloplana mitsukurii, floating at the surface of the sea with the dorsal side downwards. T, T, the tentacles expanded. (After Abbott.)

The Japanese species are found principally on encrusting Algae, Zostera, Melobesia, etc., which they resemble very closely in colour. The Red Sea species is, according to Kowalevsky, ciliated all over, but the Japanese species are ciliated only on the ventral surface. As in Ctenoplana, the body of Coeloplana is a flattened disc with a notch at each end of the tentacular axis, when creeping; but Coeloplana does not swim, nor at any time does it assume a helmet-shape. The tentacles are very long and of a chalky-white colour. They can be retracted into tentacle-sheaths. When the animal is excited it throws out the whole tentacle in a cloud of white filaments, "and to watch it at such a time, shooting out and retracting the tentacles, moving along the side of the aquarium like a battleship in action is truly a remarkable spectacle."[434] On the dorsal side of the body there is a series of processes which are called the dorsal tentacles. The statolith is very small, and is not surrounded by sensory processes as it is in Ctenoplana. There are no ctenophoral plates. The colours of the Japanese {423}species are scarlet or carmine red and dirty brown or brownish yellow. They are from 1 to 2 centimetres in diameter.

CLASS II. NUDA

Ctenophora without tentacles.

Fam. Beroidae.Beroe, the only genus of this family and class, differs from other Ctenophora in several important particulars. There are no tentacles, and the stomodaeum is so large that the body-form assumes that of a thimble with moderately thick walls. The infundibulum is small. The paragastric and longitudinal canals give rise to numerous ramifications which form a network distributed throughout the surface of the body. The statolith is unprotected by a dome, and the polar fields are bordered by a number of small branching papillae. The eight ribs extend for nearly the whole length of the body. Beroe is almost cosmopolitan, and is frequently found at the surface of the sea in great numbers. B. ovata is found off the Shetlands, Hebrides, and west coast of Ireland, but is rare on the east coast of the British Islands and in the English Channel. At Valencia it is common in August and September, and sometimes reaches the great size of 90 mm. in length by 50 mm. in breadth. It is usually of a pale pink colour.

Appendix to Ctenophora

Hydroctena salenskii has recently been discovered by Dawydoff[435] floating with the Plankton off the island Saparua in the Malay Archipelago. It is claimed to be a connecting link between the Ctenophora and the Medusae of the Hydrozoa.

In external features it is like one of the Narcomedusae, having a transparent jelly-like bell with a wide bell-mouth guarded by a velum (Fig. 184, V). There are only two simple but solid tentacles (t), provided with tentacle-sheaths, but inserted on opposite sides of the bell—not on the margin, but, as in the Ctenophore, at a level not far removed from the aboral pole. At the aboral pole there is a minute pore surrounded by a high ciliated epithelium bearing an orange pigment. This leads into {424}a short blind canal, which terminates in an ampulla bearing two statoliths supported by elastic processes from the ampullar epithelium.

The sub-umbrellar cavity extends for a distance of about one-half the height of the bell. The mouth (M), which opens into this cavity, leads into a wide cavity that gives off a short blind canal to the side of each tentacular sheath, and a straight tube that leads straight to the statocyst, where it also ends blindly. There are no radial canals and no ring canal at the margin of the umbrella. There are also no ctenophoral plates. In the absence of any information concerning the position of the genital glands, the character of the epithelium of the tentacles and the development, we are not justified in regarding Hydroctena either as a Ctenophore or as a connecting link between the Ctenophora and the Hydromedusae. It may be regarded simply as a Craspedote Medusa, probably related to the Narcomedusae, with a remarkable aberrant aboral sense-organ.

fig184

Fig. 184.—Hydroctena salenskii. ab, Aboral organ; M, manubrium; t, tentacle; V, velum. (After Dawydoff.)

ECHINODERMATA

BY

E. W. MacBRIDE, M.A., FRS.

Formerly Fellow of St. John's College
Professor of Zoology in McGill University, Montreal.

{427}

CHAPTER XVI

ECHINODERMATA—INTRODUCTION—CLASSIFICATION—ANATOMY OF A STARFISH—SYSTEMATIC ACCOUNT OF ASTEROIDEA

The name Echinodermata[436] means literally "spiny-skinned," and thus brings into prominence one very conspicuous feature of most of the animals belonging to this phylum. All, it is true, do not possess spines; but with one or two doubtful exceptions, all have calcareous plates embedded in the skin, and these plates, in many cases, push out projections which raise the skin into corresponding elevations, which are called the spines. The spines are, like the other plates, inside the skin, and to speak of an Echinoderm living in its shell, as we speak of a Snail, is a serious error. The shell of a Mollusc is fundamentally a secretion poured forth from the skin, and is thus entirely external to the real living parts; but the plates and spines of an Echinoderm may be compared to our own bones, which are embedded deeply in the flesh. Hence the name ossicle (little bone) is used to designate these organs.

Besides the possession of these spines, Echinoderms are characterised by having their organisation pervaded by a fundamental radial symmetry. The principal organs of the body are repeated and are arranged like the spokes of a wheel round a central axis instead of being, as, for example, in Chaetopoda, arranged behind one another in longitudinal series.

In addition to these striking peculiarities, Echinoderms possess a most interesting internal organisation, being in this respect almost exactly intermediate between the Coelenterata {428}and the higher Invertebrata. Like so many of the latter, the Echinodermata have an anus, that is, a second opening to the alimentary canal through which indigestible material is rejected; like them also, they have a body-cavity or coelom surrounding the alimentary canal—from the lining of which the genital cells are developed. On the other hand, there is no definite circulatory system, nor any specialised excretory organ, and the nervous system exhibits no concentration which could be called a brain, and is, moreover, in close connexion with the skin. In all these points the Echinodermata resemble the Coelenterata.

One of the most characteristic features of the internal anatomy of Echinodermata is the presence of a peculiar series of organs, known collectively as the water-vascular system or hydrocoel. This is really a special division of the coelom or body-cavity which takes on the form of a ring-shaped canal embracing the mouth, from which are given off long radial canals, usually five in number, running to the more peripheral parts of the body.[437] Each radial canal carries a double series of lateral branches, which push out the skin so as to appear as appendages of the body. These appendages are known as tentacles or tube-feet; they are both sensory and respiratory in function, and often in addition, as the name tube-foot indicates, assist in locomotion. As a general term for these appendages, to be applied in all cases without reference to their function, the name podium has been suggested and will be employed here. A system of canals, in many ways resembling the water-vascular system, is found in Brachiopoda, Gephyrea and Polyzoa, but the peculiarity of Echinodermata is the way in which it is kept filled with fluid. From the ring-canal in the interval (or interradius) between two radial canals, a vertical canal, termed the stone-canal, is given off, which communicates with the exterior by means of a sieve-like plate, the madreporite, pierced by fine canals. These canals and the stone-canal itself are lined with powerful cilia, which produce a strong inward current, and keep the water-vascular system tensely filled with sea water.

The phylum includes the familiar Starfish and Sea-urchins, which in sheltered spots are found between tide-marks; the {429}Brittle Stars and Sea-cucumbers, which can be dredged up from below low-water mark, and lastly the beautiful Feather-stars, of which there are comparatively few species still living, although huge beds of limestone are composed of the remains of fossil Feather-stars.

One species of Sea-cucumber (Synapta similis)[438] is said to enter brackish water in the mangrove swamps of the tropics; but, with this exception, the whole phylum is marine. A few species can endure partial exposure to the air when left bare by the receding tide, but the overwhelming majority are only found beneath low-water mark, and a considerable number live in the deepest recesses of the ocean.

Their distribution is, no doubt, partly determined by food, a number of species being strictly confined to the neighbourhood of the shore. On the other hand, since a very large number of species live on the layer of mud impregnated with animal remains which forms the superficial layer of the deposit covering the sea-floor, it is not surprising to learn that many have an exceedingly wide range, since this deposit is very widely distributed. Another equally important factor in determining distribution is wave-disturbance, and it is surprising to learn to what a depth this extends. Off the west coast of Ireland a large wave literally breaks on a submerged rock 15 fathoms beneath the surface. Speaking generally, it is useless to look for Echinoderms on an exposed coast, and the same species, which in the sheltered waters of the Clyde are exposed at low water, must be dredged up from 20 to 30 fathoms outside Plymouth Sound.

The ordinary collector is attracted to the group chiefly by the regularity and beauty of the patterns produced by the radial symmetry, but to the scientific zoologist they are interesting from many other points of view. Differing widely nevertheless from the higher Invertebrata in their symmetry when adult, they have as larvae a marked bilateral symmetry, and the secondary development of the radial symmetry constitutes one of the most remarkable life-histories known in the animal kingdom.

Then again, owing to the possession of ossicles, the Echinodermata are one of the few groups of Invertebrata of which abundant remains occur fossilised. In attempting, therefore, to {430}decipher the past history of life from the fossil record, it is necessary to have an exact and detailed knowledge of Echinoderm skeletons and their relation to the soft parts. Lastly, the internal organisation of Echinoderms throws valuable light on the origin of the complicated systems of organs found in the higher animals.

Echinodermata are divided into two great sub-phyla, which must have very early diverged from one another. These are:—

(1) Eleutherozoa,

(2) Pelmatozoa.[439]

The sub-phylum Pelmatozoa, to which the living Feather-stars (Crinoidea) and the majority of the known fossil species belong, is characterised by the possession of a fixing organ placed in the centre of the surface opposite the mouth—the aboral surface as it is called. Ordinarily this organ takes on the form of a jointed stalk, but in most modern species it is a little knob with a tuft of rooting processes, termed cirri. In the other sub-phylum, the Eleutherozoa, no such organ is found, and the animals wander about freely during their adult life, though for a brief period of their lar