That's because they have a spine. Sea squirts belong to the phylum Chordata, which includes all animals with a spinal chord, a supporting notochord (backbone), and gill slits at one point in their lives--everything from fish to humans. Tunicates have all these features as larvae, when they resemble tadpoles. Usually within 24 hours of planktonic life, the tunicate larvae will settle down on a hard surface and attach itself with adhesive organs. The tunicate then undergoes a transformation, rearranging its organs (absorbing its notochord, nerve cord and tail) and becoming a full grown sea squirt. Sea squirts possess both sex organs, but are unable physiologically to self-fertilize.
Tunicates actually "wear" tunics. They secrete the leathery sac--called a tunic--that protects the animal. There are two openings in the sac, called "siphons." Cilia on the pharynx move about to create a current and draw water in through the incurrent siphon. The water is then filtered through the mucus-coated pharynx, which traps food particles. Oxygen is drawn from the water as it passes through the gill clefts, and moves out through the excurrent siphon.
Sea squirts feed on the plankton and detritis they filter out of the the water. Sharks, skates, and other bottom-dwelling fish eat sea squirts. And many small organisms seek shelter and a flow of water inside tunicates.