\Tu`ni*ca"ta\, n. pl. [NL. See {Tunicate}.] (Zo["o]l.)
A grand division of the animal kingdom, intermediate, in some
respects, between the invertebrates and vertebrates, and by
some writers united with the latter. They were formerly
classed with acephalous mollusks. The body is usually covered
with a firm external tunic, consisting in part of cellulose,
and having two openings, one for the entrance and one for the
exit of water. The pharynx is usually dilated in the form of
a sac, pierced by several series of ciliated slits, and
serves as a gill.
Note: Most of the species when mature are firmly attached to
      foreign substances, but have free-swimming larv[ae]
      which are furnished with an elongated tail and somewhat
      resemble a tadpole. In this state the larva has a
      urochord and certain other structures resembling some
      embryonic vertebrates. See {Ascidian}, {Doliolum},
      {Salpa}, {Urochord}, and Illust. of {Social ascidian},
      under {Social}.