Order Nemertodermatida Steinböck, 1931
Mouth ventral, without pharynx, gut with lumen. Nervous system epithelial, statocyst with more than one statolith. Without protonephridia. Common hermaphroditic gonad, sperm uniflagellate, male gonopore posterior or absent, usually without female ducts or accessory structures. Free-living and symbiotic, marine.
Nemertodermatidae Steinböck, 1930
This group has generated much interest regarding its phylogenetic position (Tyler and Rieger, 1977). According to Ax (1984) these animals are close relatives of the Acoela, differing most significantly in the number of statoliths (2) in the statocyst. The acoel Hofstenia is known sometimes to have multiple statoliths, but the acoel nervous system is not epithelial, nor are the sperm uniflagellate.