Described in 1930-31 by Steinböck who considered it the most primitive bilaterian, the turbellarian genus Nemertoderma is known for its role in platyhelminth phylogeny as much as for its muddled taxonomy. On the basis of material collected in the Mediterranean, Atlantic and Pacific Ocean since 1964 the known four genera and seven species (Nemertoderma bathycola Steinböck, 1930-31; N. westbladi Steinböck, 1938; N. psammicola Sterrer, 1970 (syn. N. rubra Faubel, 1976); Meara stichopi Westblad, 1949; Meara sp. Smith et al. 1994; Nemertinoides elongatus Riser, 1987; and Flagellophora apelti Faubel & Dörjes, 1978) are redescribed and one new genus with two new species (Ascoparia neglecta n. g. n. sp. and A. secunda n. sp.). Live observations on morphological variability, body size vs. reproductive state, statocyst structure and statolith variability, and sperm morphology and dimorphism are provided. The paper concludes with a discussion and diagnoses of the known taxa of Nemertodermatida, including the new family Ascopariidae.