(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
{ ‖ } 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 ]