Opisthobranchia

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Noun1.Opisthobranchia - gastropods having the gills when present posterior to the heart and having no operculum: includes sea slugs; sea butterflies; sea hares
class Gasteropoda, class Gastropoda, Gasteropoda, Gastropoda - snails and slugs and their relatives
Nudibranchia, order Nudibranchia - comprising numerous marine gastropod mollusks lacking a shell in the adult state and usually having a body like a slug
class - (biology) a taxonomic group containing one or more orders