opisthobranchia

Opisthobranchia

O*pis`tho*bran"chi*a\, Opisthobranchiata \O*pis`tho*bran`chi*a"ta\, n. pl. [NL., from Gr. ? behind + ? gills.] (Zo["o]l.) A division of gastropod Mollusca, in which the breathing organs are usually situated behind the heart. It includes the tectibranchs and nudibranchs.
Webster's Revised Unabridged Dictionary, © 1996, 1998 MICRA, Inc.
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WordNet
opisthobranchia

noun
gastropods having the gills when present posterior to the heart and having no operculum: includes sea slugs; sea butterflies; sea hares 
WordNet® 3.0, © 2006 by Princeton University.
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