Ascidian

from WordNet (r) 3.0 (2006)

ascidian
    n 1: minute sedentary marine invertebrate having a saclike body
         with siphons through which water enters and leaves

from The Collaborative International Dictionary of English v.0.48

Ascidian \As*cid"i*an\, n. [Gr. ? bladder, pouch.] (Zool.)
   One of the Ascidioidea, or in a more general sense, one of
   the Tunicata. Also as an adj.
   [1913 Webster]