(n)gastropods having the gills when present posterior to the heart and having no operculum: includes sea slugs; sea butterflies; sea hares, Syn.subclass Opisthobranchia
The Collaborative International Dictionary of English (GCIDE) v.0.53Collaborative International Dictionary (GCIDE)
{ ‖ } n. pl. [ NL., from Gr. 'o`pisqen behind + &unr_; gills. ] (Zool.) A division of gastropod Mollusca, in which the breathing organs are usually situated behind the heart. It includes the tectibranchs and nudibranchs. [ 1913 Webster ]