From WordNet (r) 3.0 (2006) [wn]:
nudibranch
n 1: any of various marine gastropods of the suborder
Nudibranchia having a shell-less and often beautifully
colored body [syn: sea slug, nudibranch]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Nudibranch \Nu"di*branch\, a. (Zool.)
Of or pertaining to the Nudibranchiata. -- n. One of the
Nudibranchiata.
[1913 Webster]