The Collaborative International Dictionary of English v.0.48:

Nudibranchiata \Nu`di*bran`chi*a"ta\, n. pl. [NL. See Nude,
   and Branchia.] (Zool.)
   A division of opisthobranchiate mollusks, having no shell
   except while very young. The gills are naked and situated
   upon the back or sides. See Ceratobranchia.
   [1913 Webster]