Acoela
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. 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 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. 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 there is sometimes 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. Taxonomy. The following sub-taxa are recognised in the order Acoela: