opisthobranch definition by American Heritage Dictionary
noun pl. o·pis·tho·branchs
Any of various marine gastropod mollusks of the subclass Opisthobranchia, characterized by gills, a shell that is reduced or absent, and two pairs of tentacles.
Origin: From New Latin Opisthobranchia, order name : Greek opistho-, behind; see opisthognathous + Greek brankhia, gills.