Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

Opisthobranchia
     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}]