The subphylum Urochordata, is also sometimes known as the Tunicata. These animals are commonly known as tunicates or sea squirts. Some pelagic tunicates are known as salps. There are two classes recognized within the subphylum: Thaliacea and Ascidiacea.

The body of an adult tunicate is quite simple. Essentially it is a sack with two siphons through which water enters and exits. Many tunicates have a larva that is free-swimming and exhibits all chordate characteristics, including a notochord. In many tunicates this "tadpole larva" eventually attaches to a hard surface and loses its tail, the ability to move, and most of its nervous system. The tunicate group knows as salps are entirely free-swimming, however.