Opisthobranchia
[əʊpɪs'θəʊbrænkɪə]
Definition
(noun.) gastropods having the gills when present posterior to the heart and having no operculum: includes sea slugs; sea butterflies; sea hares.
Typist: Melba--From WordNet
Definition
(n. pl.) Alt. of Opisthobranchiata
Typist: Malcolm