The Collaborative International Dictionary of English v.0.48:

Prosobranchiata \Pros`o*bran`chi*a"ta\, n. pl. [NL., fr. Gr. ?
   forward, further + ? a gill.] (Zool.)
   The highest division, or subclass, of gastropod mollusks,
   including those that have the gills situated anteriorly, or
   forward of the heart, and the sexes separate.
   [1913 Webster]