Theetymology of "acoel" is from theAncient Greek wordsἀ (a), thealpha privative, expressing negation or absence, andκοιλία (koilía), meaning "cavity".[5][6] 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 asSymsagittifera roscoffensis, may reach up to 15 millimetres (0.59 in).[7] They are bilaterally symmetric and microscopic.[8]
They are found worldwide in marine and brackish waters, usually having abenthic lifestyle, although some species areepibionts.[9] Two species,Limnoposthia polonica andOligochoerus limnophilus, are known to live in freshwater.[10]
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 microalgalendosymbionts.[11]
Members of the class Acoela lack a conventionalgut, so that the mouth opens directly into themesenchyme, i.e., the layer of tissue that fills the body.[12][13] Digestion is accomplished by means of asyncytium that forms avacuole around ingested food. There are noepithelial 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 shortpharynx leading from the mouth to the vacuole.[14] All other bilateral animals (apart fromtapeworms) have a gut lined with epithelial cells. As a result, the acoels appear to be solid-bodied.
The Acoela provide interesting insights into early animal evolution and development.[15][16] The most thoroughly studied animal in this group is the speciesIsodiametra pulchra.
Acoela used to be classified in the phylumPlatyhelminthes.[citation needed] However, Acoela was separated from this phylum after molecular analyses showed that it had diverged before the three mainbilaterian clades had formed, making flatworms, as traditionally understood, anevolutionary grade rather than a monophyletic clade.[citation needed]
^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,PMID17051477.
^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.ISSN0947-5745.
^[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)]
