Tunicates are the main organism that make up the Subphylum Urochordata. Tunicates are mostly sedentary animals that are widely distributed in all seas. These animals are named for their body covering, called the tunic, which is mostly comprised of a substance similar to cellulose. Some Tunicates are known as Sea Squirts because when they are irritated they will forcefully expel a stream of water. Their bodies are usually round or cylindrical and have two tubes. One brings in water and food and the other eliminates water and wastes.