ABSTRACT
The tunicates are found distributed in ocean waters from the polar regions to the tropics. While the thaliaceans are free-swimming, the sessile forms grow mainly on solid surfaces such as wharf piles, ship hulls, rocks, and the shells of various sea creatures. The subphylum Urochordata differs from other chordate subphyla in that the adult form has no notochord, nerve cord or tail. Ascidians occur as solitary or colonial species. Solitary species are individual animals with an inhalant siphon and an exhalent siphon, often with a thick leathery test that encloses the body of the animal. Colonial species are groups of small animals that are embedded in a gelatinous test as a colony. The pyrosomes are bioluminous colonial tunicates with a hollow cylindrical structure. The 70 species of larvaceans superficially resemble the tadpole larvae of amphibians, although the tail is at right angles to the body. Tunicates have a well-developed heart and circulatory system.
