The tunicates are marine, filter-feeding animals. The most prominent tunicates are the sea squirts (class Ascidiacea), which show affinities to other chordates only in the juvenile stage. Adult sea squirts are sessile (attached), globular or tubular animals, often with prominent incurrent and excurrent siphons; many kinds grow in colonies. Most of the body of the adult is occupied by a very large pharynx with numerous gill slits that act as a sieve for food. Water taken into the incurrent siphon enters the pharynx and passes out through the gill slits, leaving food particles trapped in the pharynx. A groove in the pharynx called the endostyle secretes mucus that traps the particles and conveys them into the digestive tract; the movement of the mucus is caused by the action of cilia. Water leaves the atrium, a sac surrounding the pharynx, by way of the excurrent siphon. Thus the gill slits in tunicates serve a feeding function, not a respiratory function.
The sea squirt larva is a free-swimming animal resembling a tadpole. The head, which will become the entire body of the adult, contains a rudimentary brain and sense organs, a small pharynx and digestive tract, and a ventral heart. Incurrent and excurrent openings are located at the top of the head. The tail is a muscular appendage that functions as a swimming organ. It contains a hollow nerve tube (connected to the brain), and a notochord that extends into the head and keeps the animal from telescoping when its muscles contract. When the larva is ready to undergo metamorphosis it attaches to an object head downward. The tail, notochord, and nerve cord degenerate, the pharynx enlarges, and the other organs shift in position; the incurrent and excurrent openings develop siphons.
There are two other classes of tunicates, both consisting of small planktonic animals. The salps (Thaliacea) metamorphose into barrel-shaped adults that swim by muscular contractions. The larvaceans (Larvacea) are neotenous, that is, they achieve sexual maturity and reproduce without losing the larval form. Many zoologists believe that tunicates of the sea squirt type were the first chordates and that the larval tail, with its notochord and nerve chord, was evolved as a means of dispersing their larvae. According to this theory, the later chordates, including the vertebrates, are descended from neotenous tunicates that, like the larvaceans, failed to assume the adult form.
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