The epidermis of Ascidians is covered by a characteristic tissue called the “tunic,” which is typically thick and fibrous but in some colonial species has the consistency of jelly. The tunic is composed primarily of proteins and carbohydrates as well as “tunicin,” a cellulose that forms sheets of parallel structural fibers. These sheets are often layered like plywood, with fibers oriented in different directions to produce a strong and tough outer layer. The tunic’s functions include support, protection, and attachment of the animal to the substratum. For additional protection and support, some species secrete calcareous plates or spicules within the tunic, while others incorporate sand or even their own feces. Unlike other animals with thick exoskeletons, ascidians do not molt as they grow. Instead the tunic grows along with the growing zoid.

Ascidians vary in size from a few millimeters to over a meter in length.

Sea squirts may have up to three general body regions, the thorax, abdomen, and post-abdomen. The abdomenal and post-abdomenal regions can form a long stalk, holding the thorax, with the siphons and pharynx, above/away from the surface to which the animal is attached. But in many species, the abdomen and post-abdomen are absent, and all the organs are contained within the thorax. (Ruppert et al 2004)