Gnathostomulida is a relatively small group of marine and brackish worms. At present the group contains 94 described species, distributed among 25 genera. The group was first recorded from the Bay of Kiel in Germany, but appears to have a worldwide distribution with the greatest species diversity in tropical coralline sand. Gnathostomulids are vermiform animals measuring 300–3600 µm. The gnathostomulid body can be divided into a head and a trunk, and in some cases a small offset tail. The integument is totally covered with locomotory cilia that all arise from monociliated cells. The head may have different kinds of sensory structures. Caudally the forehead is limited by the mouth opening. The mouth leads to the muscular pharynx with its jaws. All gnathostomulids, except the genus Agnathiella, have a set of jaws and in some cases also an isolated basal plate. A very short esophagus extends from the pharynx into the gut. An anus is apparently missing but it has been shown that some species might have indications of a temporarily present anal pore. A coelom is absent and fluid wastes are excreted via monociliated protonephridia. All gnathostomulids are hermaphrodites.