That's because they have a spine. 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.
Sea squirts possess both sex organs, but are unable physiologically to self-fertilize.
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. Some sea squirts, like the sea vase and sea potato, are solitary animals. Others, like sea grapes, tend to group together.