Urochordata is subdivided into three orders, the Larvacea, the Ascidiacea and the Thailiacea. Members of the Larvacea are free-swimming and they retain the larval features in the adult stage. The ascidians or sea-squirts are mainly sedentary and often colonial. Most of the Thailiacea are large colonial pelagic forms, commonly known as salps. The ascidians (tunicates) seem to be the most characteristic representative of the urochordates. The body is enclosed in a tough tunic made of a polysaccharide resembling cellulose. This tunic is perforated by two external openings, one slightly higher than the other. The higher one is the mouth, the lower is the atrial opening, the atrium being a cavity which surrounds the pharynx. Into the atrium open the gill slits, the anus, and the genital openings. The pharyx is perforated by numerous clefts whose main function is food collection. Pharyngeal cilia create an inward current of water through the mouth; this water passes out into the atrim through the pharyngeal clefts and food particles are sifted out. They pass in a stream of mucus to the dorsal band along the pharynx whence they are carried back as a mucuous cord to the oesophagus and stomach. There is no notochord in the adult and the sole remnant of the nerve cord is a dorsal ganglion above the pharynx. These is a ventral heart which pumps blood alternately backward and forward. The larva is like a tadpole and is known as an appendicularia larva. It possesse a true notochord, a dorsal hollow nerve cord, visceral clefts in the pharynx, aventral heart, and a tail. None of the urochordates have limbs. On balance, it is clear that the urochordates possess the essential characteristic of chordates; their affinities seem to lie with the cephalochordates.
The cephalochordate Amphioxus is considered to be the basic chordate type. It possesses the notochord in its most complete state; the pharynx is perforated by visceral clefts; there is a dorsal hollow nerve cord; the blood flows forward ventrally and backward dorsally; there is a post –anal segmented tail. There are no limbs and there is no heart, though there are contractile bulbils at the bases of the pharyngeal bars. Its early development illustrates the basin principle of chordate embryology. Nevertheless, it has many primitive and also many specialized features.