Zoology: soft-bodied marine animals; with planula-like features; free-living, some live on the surface of other organisms; very small flatworms have no gut; digestion is by means of a syncytium that forms a vacuole around ingested food; no epithelial cells lining the digestive vacuole; solid-bodied, a-coel, no body cavity; entirely marine, living between grains of sediment, swimming as plankton, or crawling on algae; have a statocyst, which presumably helps them orient to gravity; soft bodies make them difficult to classify.
TaxonomyFormerly in
Platyhelminthes. In 2004 molecular studies demonstrated that they are a separate clade; basal among the
Bilateria, slightly more derived than the cnidaria. Recently they may lie near the base of the deuterostomes. Some favor the placement of Xenoturbella and
Acoelomorpha more basally among Metazoa.