Acoela, an inverebrate, is an order of animals treated either as a group of flatworms or as one of the two classes of the phylum Acoelomorpha, containing the majority of that phylum's species. It contains about 20 families. Acoel flatworms have a simple nervous system and a saclike gut.
As the most primitive bilateral animals, Acoela provide interesting insights into early animal evolution and development. The best studied animal in this group is the European species Symsagittifera roscoffensis. Acoela was once classified in the phylum Phatyhelminthes. 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, doi:10.1387/ijdb.062167ej, PMID 17051477.
- ^ 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, doi:10.1038/nature07309, PMID 18806777