opisthobranchiata

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]