opisthobranch

Any of a group of marine gastropods characterized by gills located behind the heart, a shell that is reduced or absent, and two pairs of tentacles, and including the sea slugs and the sea butterflies.

A member of the Opisthobranchiata.

Having posterior gills; specifically, of or pertaining to the Opisthobranchiata.

Any of a large and diverse group of specialized complex marine gastropods previously united under Opisthobranchia within the Heterobranchia, characterized by two pairs of tentacles and a single gill behind and to the right of the heart.