Acoela is a taxonomic order of very small invertebrate animals which resemble flatworms. This order is treated either as a group within the flatworms or as one of two classes within the phylum Acoelomorpha, in which case Acoela contains the majority of that phylum's species. Acoela comprises about 20 families. These animals have a simple nervous system and a saclike gut.
As the most primitive bilateral animals, the Acoela provide interesting insights into early animal evolution and development. The most thoroughly studied animal in this group is the species Isodiametra pulchra. Acoela was previously 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.
- ^ 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, PMID 17051477, doi:10.1387/ijdb.062167ej.
- ^ Hejnol A, Martindale MQ (2008), "Acoel development supports a simple planula-like urbilaterian", Phil. Trans. R. Soc. B 363 (1496): 1493–1501, PMC 2614228, PMID 18192185, doi:10.1098/rstb.2007.2239
- ^ Hejnol A, Martindale MQ (2008), "Acoel development indicates the independent evolution of the bilaterian mouth and anus", Nature 456 (7220): 382, PMID 18806777, doi:10.1038/nature07309