Contributions to a revision of the marine triclads of North America: the monotypic genera Nexilis, Nesion, and Foviella (Turbellaria: Tricladida)

Abstract

New interpretations of the anatomy and systematic positions of three North American marine triclads are given. Nexilis epichitonius is redescribed from the type material and it is claimed that the original interpretation of the copulatory apparatus was erroneous. A new account is given, the genus is redefined, and some curious similarities between this species and some triclads of Lake Baikal are discussed. A further assessment of the taxonomic affinities of Nesion arcticum is provided based on study of a new specimen that is better preserved than the types. The systematic position of Foviella affinis is discussed on the basis of studies of specimens from North America and Europe. The species cannot be a procerodid and it is here classified with the Uteriporidae. Probursa and Centrovarioplana are removed from the Uteriporidae and the family is redefined.