Opisthohranchia

Opisthohranchia, a sub-order of Gastropoda (q.v.), including the sea-slugs (e.g. Doris, Eolis, Elysia), the Sea-hare (Aplysia). and the Bubble-shells (Bulla). The shell is usually absent; if present, it is internal and rudimentary. The principal character, and that to which the name is due, is that the gills or branchiae are at the hinder end of the body, and always behind the heart. They are all marine, and there are but few species known as fossils.