In 1956 Ax described two turbellarian-like animals with cuticular hooks in the pharynx, Gnathostomula paradoxa and Gnathastomaria lutheri, and on the basis of these two species he established a new turbellarian order Gnathosiomulida. Later(1960,1961) he raised this group to the rank of a class. Beside the pharyngeal hooks, the structure of the epithelium — one single cilia per cell — is regarded as the most characteristic attribute of this group. G. paradoxa was found on the island of Sylt (intertidal zone, sand and gravel) and in the bay of Kiel (sub-littoral, 3.5-7 m, mostly pure sand, but also in sand with detritus and mud), G. luthsri on the French Mediterranean coast (Étang de Salses, Pyrénées Orientales, sandy beach, subsoil water). A further species of the genus Gnathostomula, G. maldivarum, is described by Gerlach (1958), found in sand, on the Maledive Islands.