The adult tunicate would seem to have few things in common with vertebrates. The animal is basically a single large filtering mechanism. When distubed some species eject water forcefully through the mouth, hence the name sea squirt. The relationship between the tunicates and the rest of the vertebrates is far more obvious in the larvae. The pharyngeal structure is also present, but much smaller than in the adult.
Urochordates are marine, sedentary animals and can be either solitary, or colonial.
There are about 30 species of these small, marine animals. The general body plan is similar to a larval tunicate. However in no other group are the defining features of the chordates so well defined with a strong dorsal, hollow nerve cord, and notochord. The earliest vertebrates were the Agnatha are represented by the living groups the Lampreys and Hag Fish, and the important extinct group the Ostracoderms. The Agnatha lack a jaw and mouth is simple.