n. pl. o·pis·tho·branchs
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.
[From New Latin Opisthobranchia, former subclass name : Greek opistho-, behind; see OPISTHOGNATHOUS + Greek brankhia, gills.]
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