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Superregnum: Eukaryota
Cladus: Unikonta
Cladus: Opisthokonta
Cladus: Holozoa
Regnum: Animalia
Subregnum: Eumetazoa
Cladus: Bilateria
Cladus: Nephrozoa
Superphylum: Deuterostomia
Phylum: Xenacoelomorpha
Subphylum: Acoelomorpha
Classis: Acoela
Familiae (21): ActinoposthiidaeAnaperidaeAntigonariidaeAntroposthiidaeChildiidaeConvolutidaeDakuidae – Diopisthoporidae – Hallangiidae – Haploposthiidae – HofsteniidaeIsodiametridaeMecynostomidae – Nadinidae – Otocelididae – Paratomellidae – Polycanthiidae – Proporidae – SagittiferidaeSolenofilomorphidaeTaurididae


Achatz, J.G. et al. (online first, 2012): The Acoela: on their kind and kinships, especially with nemertodermatids and xenoturbellids (Bilateria incertae sedis). Organisms Diversity & Evolution, abstract and pdf
Achatz, J.G.; Hooge, M.; Wallberg, A.; Jondelius, U.; Tyler, S. 2010: Systematic revision of acoels with 9+0 sperm ultrastructure (Convolutida) and the influence of sexual conflict on morphology. Journal of zoological systematics and evolutionary research, 48: 9–32. DOI: 10.1111/j.1439-0469.2009.00555.x
Egger, B. et al. 2009: To be or not to be a flatworm: the acoel controversy. PLoS ONE, 4(5): e5502. DOI: 10.1371/journal.pone.0005502
Faubel, A.; Kolasa, J. 1978: On the anatomy and morphology of a freshwater species of Acoela (Turbellaria): Limnoposthia polonica (Kolasa et Faubel, 1974). Bulletin de L'Academie Polonaise des Sciences serie des sciences biologiques, 26: 393–397.
Hooge, M.D.; Haye, P.A.; Tyler, S.; Litvaitis, M.K.; Kornfield, I. 2002: Molecular systematics of the Acoela (Acoelomorpha, Platyhelminthes) and its concordance with morphology. Molecular phylogenetics and evolution, 24: 333–342. DOI: 10.1016/S1055-7903(02)00236-1 PDF
Hooge, M.D. & Tyler, S. 2003. Two new acoels (Acoela, Platyhelminthes) from the central coast of California. Zootaxa 131: 1–14. Abstract & excerpt Reference page.
Kånneby, T.; Jondelius, U. 2013: Four new species of Acoela from Chile. Zootaxa 3736(5): 471–485. DOI: 10.11646/zootaxa.3736.5.3 Reference page.
Kolasa, J.; Faubel, A. 1974: A preliminary description of a freshwater Acoela (Turbellaria): Oligochoerus polonicus nov. spec. Boll. Zool., 41: 81–85.
Philippe, H. et al. 2011: Acoelomorph flatworms are deuterostomes related to Xenoturbella. Nature, 470: 255–258. DOI: 10.1038/nature09676
Ruiz-Trillo, I. et al. 1999: Acoel flatworms: earliest extant bilaterian metazoans, not members of Platyhelminthes. Science, 283: 1919–1923. DOI: 10.1126/science.283.5409.1919
Schockaert, E.R.; Hooge, M.; Sluys, R.; Schilling, S.; Tyler, S.; Artois, T. 2008: Global diversity of free living flatworms (Platyhelminthes, “Turbellaria”) in freshwater. Hydrobiologia, 595: 41–48. DOI: 10.1007/978-1-4020-8259-7_5
Zauchner, T., Salvenmoser, W. & Egger, B.. 2015. A cultivable acoel species from the Mediterranean, Aphanostoma pisae sp. nov. (Acoela, Acoelomorpha). Zootaxa 3941(3): 401–413. DOI: 10.11646/zootaxa.3941.3.6. Reference page.


Acoela in the World Register of Marine Species

Vernacular names
polski: wirki bezjelitowe

Acoela, or the acoels, is an order of small and simple invertebrates in the subphylum Acoelomorpha of phylum Xenacoelomorpha, a deep branching bilaterian group of animals, which resemble flatworms. Historically they were treated as an order of turbellarian flatworms.[1][2]

The etymology of "acoel" is from the Ancient Greek words ἀ (a), the alpha privative, expressing negation or absence, and κοιλία (koilía), meaning "cavity".[3][4] This refers to the fact that acoels have a structure lacking a fluid-filled body cavity.

Acoels are very small flattened worms, usually under 2 millimetres (0.079 in) in length, but some larger species, such as Symsagittifera roscoffensis, may reach up to 15 millimetres (0.59 in).[5] They are bilaterally symmetric and microscopic.[6]

They are found worldwide in marine and brackish waters, usually having a benthic lifestyle, although some species are epibionts.[7]

Members of the class Acoela lack a conventional gut, so that the mouth opens directly into the mesenchyme, i.e., the layer of tissue that fills the body.[8][9] Digestion is accomplished by means of a syncytium that forms a vacuole around ingested food. There are no epithelial cells lining the digestive vacuole, but there is sometimes a short pharynx leading from the mouth to the vacuole. All other bilateral animals (apart from tapeworms) have a gut lined with epithelial cells. As a result, the acoels appear to be solid-bodied.

As the basal lineage of bilateral animals, the Acoela provide interesting insights into early animal evolution and development.[10][11] The most thoroughly studied animal in this group is the species Isodiametra pulchra. Acoela used to be classified in the phylum Platyhelminthes. However, Acoela was separated from this phylum after molecular analyses showed that it had diverged before the three main bilaterian clades had formed.

The following sub-taxa are recognised in the order Acoela:[12]

Family Actinoposthiidae Hooge, 2001
Family Antigonariidae Dörjes, 1968
Family Antroposthiidae Faubel, 1976
Family Diopisthoporidae Westblad, 1940
Family Nadinidae Dörjes, 1968
Family Paratomellidae Dörjes, 1966
Family Taurididae Kostenko, 1989
Suborder Bursalia Jondelius et al., 2011
Infraorder Crucimusculata Jondelius et al., 2011
Family Dakuidae Hooge, 2003
Family Isodiametridae Hooge & Tyler, 2005
Family Otocelididae Westblad, 1948
Family Proporidae Graff, 1882
Superfamily Aberrantospermata Jondelius et al., 2011
Family Convolutidae Graff, 1905
Family Mecynostomidae Dörjes, 1968
Infraorder Prosopharyngida Jondelius et al., 2011
Family Hallangiidae Westblad, 1946
Family Hofsteniidae Bock, 1923
Family Solenofilomorphidae Dörjes, 1968

Wikimedia Commons has media related to Acoela.
Wikispecies has information related to Acoela.

Cannon, Johanna Taylor; Vellutini, Bruno Cossermelli; Smith, Julian; Ronquist, Fredrik; Jondelius, Ulf; Hejnol, Andreas (2016). "Xenacoelomorpha is the sister group to Nephrozoa". Nature. 530 (7588): 89–93. Bibcode:2016Natur.530...89C. doi:10.1038/nature16520. ISSN 0028-0836. PMID 26842059. S2CID 205247296.
Jimenez-Guri E, Paps J, Garcia-Fernandez J, Salo E (2006), "Hox and ParaHox genes in Nemertodermatida, a basal bilaterian clade", Int. J. Dev. Biol., 50 (8): 675–9, doi:10.1387/ijdb.062167ej, PMID 17051477.
Bailly, Anatole (1981-01-01). Abrégé du dictionnaire grec français. Paris: Hachette. ISBN 2010035283. OCLC 461974285.
Bailly, Anatole. "Greek-french dictionary online". Retrieved 2020-03-02.
Hooge, M. D.; Tyler, S. (2005). "New tools for resolving phylogenies: a systematic revision of the Convolutidae (Acoelomorpha, Acoela)". Journal of Zoological Systematics and Evolutionary Research. 43 (2): 100–113. doi:10.1111/j.1439-0469.2005.00301.x. ISSN 0947-5745.
Achatz, Johannes G.; Chiodin, Marta; Salvenmoser, Willi; Tyler, Seth; Martinez, Pedro (2013). "The Acoela: on their kind and kinships, especially with nemertodermatids and xenoturbellids (Bilateria incertae sedis)". Organisms, Diversity & Evolution. 13 (2): 267–286. doi:10.1007/s13127-012-0112-4. ISSN 1439-6092. PMC 3789126. PMID 24098090.
Ogunlana, M. V.; Hooge, M. D.; Tekle, Y. I.; Benayahu, Y.; Barneah, O.; Tyler, Seth (2005). "Waminoa brickneri n. sp. (Acoela: Acoelomorpha) associated with corals in the Red Sea". Zootaxa. 1008: 1–11. doi:10.11646/zootaxa.1008.1.1.
Boll, Piter Kehoma; Rossi, Ilana; Amaral, Silvana Vargas do; Oliveira, Simone Machado de; Müller, Eliara Solange; Lemos, Virginia Silva; Leal-Zanchet, Ana Maria (2013). "Platyhelminthes ou apenas semelhantes a Platyhelminthes? Relações filogenéticas dos principais grupos de turbelários". Neotropical Biology and Conservation (in Portuguese). 8 (1). doi:10.4013/nbc.2013.81.06. ISSN 2236-3777.
"The Platyhelminthes and the Acoela". Archived from the original on 2008-05-09. Retrieved 2009-03-21.
Hejnol A, Martindale MQ (2008), "Acoel development supports a simple planula-like urbilaterian", Phil. Trans. R. Soc. B, 363 (1496): 1493–1501, doi:10.1098/rstb.2007.2239, PMC 2614228, PMID 18192185
Hejnol A, Martindale MQ (2008), "Acoel development indicates the independent evolution of the bilaterian mouth and anus", Nature, 456 (7220): 382–386, Bibcode:2008Natur.456..382H, doi:10.1038/nature07309, PMID 18806777, S2CID 4403355
"WoRMS - World Register of Marine Species - Acoela". Retrieved 2020-02-08.

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