|Waminoa sp. on Plerogyra sp..|
In 2004 molecular studies demonstrated that they are a separate phylum, although their position in the tree of life is uncertain. In 2007 a paper dismissed the phylum as paraphyletic, with Acoela and Nemertodermatida as separate clades.
Most researchers believe them to be basal among the bilateria, slightly more derived than the Cnidaria. Recent results suggest that they (along with Xenoturbella) may lie near the base of the deuterostomes. An ongoing collaborative research project has "the researchers ... confident that they can reach an agreement about where acoels fit in evolutionary history".
Acoels are almost entirely marine, living between grains of sediment, swimming as plankton, or crawling on algae. Acoels have a statocyst, which presumably helps them orient to gravity. Their soft bodies make them difficult to classify.
References[change | change source]
- Baguñà, J.; Riutort, M. (2004). "Molecular phylogeny of the Platyhelminthes". Canadian Journal of Zoology 82 (2): 168. .
- Wallberg, A.; Curini-Galletti, M.; Ahmadzadeh, A.; Jondelius, U. (2007). "Dismissal of Acoelomorpha: Acoela and Nemertodermatida are separate early bilaterian clades". Zoologica Scripta 36 (5): 509. .
- Philippe H. et al. 2011. Acoelomorph flatworms are deuterostomes related to Xenoturbella. Nature 470 (7333): 255-258 
- Maxmen, A. (2011). "Evolution: A can of worms". Nature 470 (7333): 161–162. . .
- Petrov, A.; Hooge, M.; Tyler, S. (2006). "Comparative morphology of the bursal nozzles in acoels (Acoela, Acoelomorpha)". Journal of Morphology 267 (5): 634–648. . .