From The Collaborative International Dictionary of English v.0.48:

Tunicata \Tu`ni*ca"ta\, pr. n. pl. [NL. See Tunicate.] (Zool.)
   A grand division of the animal kingdom, intermediate, in some
   respects, between the invertebrates and vertebrates, and in
   modern classifications considered a subphylum of the
   vertebrates; called also urochordata. 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.
   [1913 Webster]

   Note: Most of the species when mature are firmly attached to
         foreign substances, but have free-swimming larvae 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.
         [1913 Webster] Tunicate
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