Acoela, or the acoels, is an order of small and simple invertebrates in the subphylum Acoelomorpha of phylum Xenacoelomorpha, a deep branching bilaterian group of animals, which resemble flatworms. Historically they were treated as an order of turbellarian flatworms. About 400 species are known, but probably many more not yet described. The etymology of "acoel" is from the Ancient Greek words, the alpha privative, expressing negation or absence, and , meaning "cavity". This refers to the fact that acoels have a structure lacking a fluid-filled body cavity.

Description

Acoels are very small flattened worms, usually under 2 mm in length, but some larger species, such as Symsagittifera roscoffensis, may reach up to 15 mm. They are bilaterally symmetric and microscopic. They are found worldwide in marine and brackish waters, usually having a benthic lifestyle, although some species are epibionts. Two species, Limonoposthia polonica and Oligochoerus limnophilus, live in freshwater. 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 microalgal endosymbionts. Members of the class Acoela lack a conventional gut, so that the mouth opens directly into the mesenchyme, i.e., the layer of tissue that fills the body. Digestion is accomplished by means of a syncytium that forms a vacuole around ingested food. There are no epithelial 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 short pharynx leading from the mouth to the vacuole. All other bilateral animals (apart from tapeworms) have a gut lined with epithelial cells. As a result, the acoels appear to be solid-bodied. As the basal lineage of 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 used to be 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, making flatworms as traditionally understood an evolutionary grade from which higher animals had evolved.

Taxonomy

The following sub-taxa are recognised in the order Acoela: