Description of Poseidonemertes caribensis sp.n., and Discussion of Other Taxa of Hoplonemertini Monostilifera with Divided Longitudinal Musculature in the Body Wall

A monostiliferous hop‐lonemertean, Poseidonemertes caribensis sp.n., is described from the Caribbean Sea, where it commonly occurs on marine plants in shallow water. Anatomical examination revealed a double layered longitudinal musculature in the body wall along the cerebral region, a feature known from only a few taxa of Hoplonemertini Monostilifera, i.e., species in the “ Amphiporus hastatus group” and in the genera Correanemertes, Paranemertes, Nemertes, Paramphiporus , and Poseidonemertes . The relationship between the longitudinal muscle layers, proboscis insertion, and head retractors is discussed, based on earlier descriptions and study of type specimens, and the various recognizable patterns are employed to further delineate some of the genera.