 ascidiacea (commonly known as the ascidians or sea squirts) is a class in the tunicata subphylum of sac-like marine invertebrate filter feeders. ascidians are characterized by a tough outer "tunic" made of the polysaccharide tunicin, as compared to other tunicates which are less rigid. ascidians are found all over the world, usually in shallow water with salinities over 2.5%. while members of the thaliacea and larvacea swim freely like plankton, sea squirts are sessile animals: they remain firmly attached to substratum, such as rocks and shells. there are 2,300 species of ascidians and three main types: solitary ascidians, social ascidians that form clumped communities by attaching at their bases, and compound ascidians that consist of many small individuals (each individual is called a zooid) forming colonies up to several meters in diameter. sea squirts feed by taking in water through the oral siphon. the water enters the mouth and pharynx, flows through mucus-covered gill slits (also called pharyngeal stigmata) into a water chamber called the atrium, then exits through the atrial siphon. ==anatomy== sea squirts are rounded or cylindrical animals ranging from about  in size. one end of the body is always firmly fixed to rock, coral, or some similar solid surface. the lower surface is pitted or ridged, and in some species has root-like extensions that help the animal grip onto the surface. the body wall is covered by a smooth thick tunic, which is often quite rigid. the tunic consists of a cellulose-like substance called tunicin along with proteins and calcium salts. unlike the shells of molluscs, the tunic is composed of living tissue, and often has its own blood supply. in some colonial species, the tunics of adjacent individuals are fused into a single structure. the upper surface of the animal, opposite to the part gripping the substratum, has two openings, or siphons. when removed from the water, the animal often violently expels wat
Animal_classes Ascidiacea Tunicates