The characteristics of tunicates are a bag-like body with two siphons, the branchial siphon and the atrial siphon. The animal takes in food and water through the mouth, or branchial siphon. It is then circulated through the pharynx, then the water passes out through the atrial siphon.
The tunicate's internal organs, including nervous system, muscles and blood vessels, are also protected by a test, or a tunic that can be vividly colored. Some tests are quite tough, while others are delicate and nearly translucent.
Some tunicates, such as sea squirts, are sessile. Sea squirts can live in colonies and share siphons that expel water and wastes. However, each sea squirt retains its own mouth. Each individual animal is called a zooid.
Other types of tunicates are free swimming. These include salps, doliolids and pyrosomes that swim in the ocean among plankton. They have an opening to take in water and food and an opening at the other end to expel wastes.
Pyrosomes differ from salps and doliolids because they are colony animals. Pyrosomes live in warm water and are bioluminescent. Salps and doliolids alternate between being solitary and living in colonies.
Tunicates are interesting because during their larval stage they have a notochord, which is a type of rudimentary spinal cord. They lose this during the adult stage.