Subphylum Urochordata (V & C fig. 7.160-7.161)

            Most Urochordata differ substantially from the other chordate subphyla. Examine a tunicate. These sessile animals collect food by filter feeding, using an expanded pharynx with gill slits. Tunicates have a motile larval stage (the "tadpole" larva) with a tail stiffened by a notochord, and a dorsal nerve tube that innervates the swimming muscles. Similar to these larvae are other Urochordates: larvaceans, which also have a tadpole larva, but which retain their tail, notochord, and nerve tube throughout life. See displays of tadpole larvae and larvaceans. Some zoologists regard the notochord and other swimming structures as having evolved as larval adaptations, and that these larvae were ancestral to the rest of the chordates through the evolutionary process of paedomorphosis (the achievement of sexual maturity in the larval or juvenile form). Others consider that adult tunicates simply lost the notochord and tail in taking up a sessile existence.