Acoela

Acoels are very small flattened worms, usually under {{convert|2|mm|in}} in length, but some larger species, such as Symsagittifera roscoffensis, may reach up to {{convert|15|mm|in}}.{{cite journal|last1=Hooge|first1=M. D.|last2=Tyler|first2=S.|title=New tools for resolving phylogenies: a systematic revision of the Convolutidae (Acoelomorpha, Acoela)|journal=Journal of Zoological Systematics and Evolutionary Research|volume=43|issue=2|year=2005|pages=100–113|issn=0947-5745|doi=10.1111/j.1439-0469.2005.00301.x}} They are bilaterally symmetric and microscopic.{{Cite journal|last1=Achatz|first1=Johannes G.|last2=Chiodin|first2=Marta|last3=Salvenmoser|first3=Willi|last4=Tyler|first4=Seth|last5=Martinez|first5=Pedro|date=2013|title=The Acoela: on their kind and kinships, especially with nemertodermatids and xenoturbellids (Bilateria incertae sedis)|journal=Organisms, Diversity & Evolution|volume=13|issue=2|pages=267–286|doi=10.1007/s13127-012-0112-4|issn=1439-6092|pmc=3789126|pmid=24098090}}

They are found worldwide in marine and brackish waters, usually having a benthic lifestyle, although some species are epibionts.{{cite journal|last1=Ogunlana|first1=M. V.|last2=Hooge|first2=M. D.|last3=Tekle|first3=Y. I.|last4=Benayahu|first4=Y.|last5=Barneah|first5=O.|last6=Tyler|first6=Seth|title=Waminoa brickneri n. sp. (Acoela: Acoelomorpha) associated with corals in the Red Sea|journal=Zootaxa|volume=1008|year=2005|pages=1–11|doi=10.11646/zootaxa.1008.1.1|doi-access=free}} Two species, Limonoposthia polonica and Oligochoerus limnophilus, live in freshwater.{{cite book | url=https://books.google.com/books?id=YHetDwAAQBAJ&dq=limonoposthia+polonica+oligochoerus+limnophilus&pg=PA92 | title=The Invertebrate Tree of Life | isbn=978-0-691-17025-1 | last1=Giribet | first1=Gonzalo | last2=Edgecombe | first2=Gregory D. | date=3 March 2020 | publisher=Princeton University Press }}

Species in the family Convolutidae often form endosymbiotic relationship with microalgae. In one of the genera, Waminoa, the algae are transmitted vertically from parents to offspring. In addition to Convolutidae, there appears to be a potential new and yet unnamed family of acoels that also live in relationships with microalgal endosymbionts.{{Cite journal |last1=Riewluang |first1=Siratee |last2=Wakeman |first2=Kevin C. |date=2023 |title=Biodiversity of symbiotic microalgae associated with meiofaunal marine acoels in Southern Japan |journal=PeerJ |volume=11 |pages=e16078 |doi=10.7717/peerj.16078 |doi-access=free |pmid=37814628|pmc=10560497 }}

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.{{cite journal|last1=Boll|first1=Piter Kehoma|last2=Rossi|first2=Ilana|last3=Amaral|first3=Silvana Vargas do|last4=Oliveira|first4=Simone Machado de|last5=Müller|first5=Eliara Solange|last6=Lemos|first6=Virginia Silva|last7=Leal-Zanchet|first7=Ana Maria|title=Platyhelminthes ou apenas semelhantes a Platyhelminthes? Relações filogenéticas dos principais grupos de turbelários|journal=Neotropical Biology and Conservation|volume=8|issue=1|year=2013|issn=2236-3777|doi=10.4013/nbc.2013.81.06|language=pt|doi-access=free}}{{cite web|url=http://devbio.umesci.maine.edu/styler/globalworming/platyhelm2003.html |title=The Platyhelminthes and the Acoela |access-date=2009-03-21 |url-status=dead |archive-url=https://web.archive.org/web/20080509144930/http://devbio.umesci.maine.edu/styler/globalworming/platyhelm2003.html |archive-date=2008-05-09 }} 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 in the families Diopisthoporidae, Hallangiidae, Hofsteniidae, and Solenofilomorphidae, and the genera Oligochoerus (Convolutidae) and Proporus (Proporidae), there is a short pharynx leading from the mouth to the vacuole.[The Acoela: on their kind and kinships, especially with nemertodermatids and xenoturbellids (Bilateria incertae sedis) The Acoela: on their kind and kinships, especially with nemertodermatids and xenoturbellids (Bilateria incertae sedis)] 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.{{citation |vauthors=Hejnol A, Martindale MQ | year = 2008 | title = Acoel development supports a simple planula-like urbilaterian | journal = Phil. Trans. R. Soc. B | volume = 363 | pages = 1493–1501 | doi = 10.1098/rstb.2007.2239 | pmid = 18192185 | issue = 1496 | pmc = 2614228 }}{{citation |vauthors=Hejnol A, Martindale MQ | year = 2008 | title = Acoel development indicates the independent evolution of the bilaterian mouth and anus | journal = Nature | doi = 10.1038/nature07309 | volume = 456 | pages = 382–386 | pmid = 18806777 | issue = 7220 | bibcode = 2008Natur.456..382H | s2cid = 4403355 }} 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, making flatworms as traditionally understood an evolutionary grade from which higher animals had evolved.

The following sub-taxa are recognised in the order Acoela:{{Cite web|url=http://www.marinespecies.org/aphia.php?p=taxdetails&id=2847|title=WoRMS - World Register of Marine Species - Acoela|website=www.marinespecies.org|access-date=2020-02-08}}

• 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

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{{Wikispecies|Acoela}}

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