Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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