polybranchia

from The Collaborative International Dictionary of English v.0.48

Polybranchia \Pol`y*bran"chi*a\, n. pl. [NL. See {Poly-}, and
   {Branchia}.] (Zool.)
   A division of Nudibranchiata including those which have
   numerous branchi[ae] on the back.
   [1913 Webster]