The Collaborative International Dictionary of English v.0.48:

Urochord \U"ro*chord\, n. [2d uro- + chord.] (Zool.)
   The central axis or cord in the tail of larval ascidians and
   of certain adult tunicates. [Written also urocord.]
   [1913 Webster] Urochorda

WordNet (r) 3.0 (2006):

urochord
    n 1: primitive marine animal having a saclike unsegmented body
         and a urochord that is conspicuous in the larva [syn:
         tunicate, urochordate, urochord]
    2: a notochord of a larval tunicate typically confined to the
       caudal region