any minute marine invertebrate animal of the class Ascidiacea, such as the sea squirt, the adults of which are degenerate and sedentary: subphylum Tunicata (tunicates)
2. ascidian tadpole
adjective
3.
of, relating to, or belonging to the Ascidiacea
ascidian in American English
(əˈsɪdiən)
noun
any of a class (Ascidiacea) of fixed tunicates; sea squirt