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From The Collaborative International Dictionary of English v.0.48:
Opisthobranchia \O*pis`tho*bran"chi*a\, Opisthobranchiata \O*pis`tho*bran`chi*a"ta\, n. pl. [NL., from Gr. 'o`pisqen behind + ? 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]