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 phylumAcoelomorpha, 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.