Pygobranchia

from The Collaborative International Dictionary of English v.0.48

Pygobranchia \Py`go*bran"chi*a\, n. pl. [NL., fr. Gr. pugh` the
   rump + ? a gill.] (Zool.)
   A division of opisthobranchiate mollusks having the
   branchi[ae] in a wreath or group around the anal opening, as
   in the genus Doris.
   [1913 Webster]