urochord

from The Century Dictionary.

• noun The caudal chord of an ascidian or tunicate, likened to the notochord, chorda dorsalis, or dorsal chord of a vertebrate; the central axis of the appendage of certain adult tunicates, as an appendicularian, and the corresponding structure of embryonic or larval tunicates in general.
• noun Any member of the Urochorda.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) The central axis or cord in the tail of larval ascidians and of certain adult tunicates.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biology a notochord that is limited to the caudal region; especially in the Urochordata
• noun any marine organism of the Urochordata

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a notochord of a larval tunicate typically confined to the caudal region
• noun primitive marine animal having a saclike unsegmented body and a urochord that is conspicuous in the larva