Acoels are very small, flattened worms, usually under in length, but some larger species, such as
Symsagittifera roscoffensis, may reach up to . 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,
Limnoposthia polonica and
Oligochoerus limnophilus, are known to 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. The Acoela provide interesting insights into early animal evolution and development. The most thoroughly studied animal in this group is the species
Isodiametra pulchra. ==Taxonomy==