Described in 1930-31 by Steinbock 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 Oceans since 1964 this paper re-diagnoses the known 4 genera and 7 species (Nemertoderma bathycola Steinbock, 1930-31; N. westbladi Steinbock, 1938; N. psammicola Sterrer, 1970 (syn. N. rubra Faubel, 1976); Meara stichopi Westblad, 1949; Meara sp. (see SMITH et al., 1994); Nemertinoides elongatus Riser, 1987; and Flagellophora apelti Faubel Be Dorjes, 1978), describes one new genus with 2 new species (Ascoparia neglecta n. g., n. sp. and A. secunda n. sp.), and provides observations from living material on morphological variability, body size vs. reproductive state, statocyst structure and statolith variability, and sperm morphology and dimorphism. The paper concludes with diagnoses for the known taxa of Nemertodermatida, including the new family Ascopariidae.