This gap was filled some years ago by the discovery of the lancelet—Amphioxus—and the young of the sea-squirt—Ascidia.
|sea squirt |
Any of various tunicates of the class Ascidiacea, having a transparent sac-shaped body with two siphons. One of the siphons is used to draw water (carrying oxygen and food particles) into the body, while the other expels it. Sea squirts are free-swimming as larvae but sessile as adults. Like other tunicates, sea squirts are chordates, but they have a notochord only in the larval stage.