tunicate

 

(tū'nəkĭt) , marine animal of the phylum Chordata, which also includes the vertebrates. The adult form of most tunicates (also called urochordates) shows no resemblance to vertebrate animals, but such a resemblance is evident in the larva. The most familiar tunicates are the sea squirts, or ascidians (class Ascidiacea). Adult sea squirts are sedentary, filter-feeding, cylindrical or globular animals, usually found attached to rocks, shells, pilings, or boat bottoms. The soft body is surrounded by a thick test, or tunic, often transparent or translucent and varying in consistency from gelatinous to leathery. The tunic (for which the tunicates are named) is secreted by the body wall of the adult animal. It is composed of cellulose, an almost unique occurrence of that material in the animal kingdom. Two siphons project from the animal's body; water enters the incurrent siphon at the top of the body and leaves the excurrent siphon at the side. Food particles are filtered from the water by the pharynx, which occupies most of the body, and are then passed into the digestive system. Some species reproduce by budding, resulting in the formation of colonies of sea squirts, joined at their bases by slender stalks or embedded in a slab of common tunic material. In addition, nearly all species reproduce sexually and are hermaphroditic. The free-swimming larva, called a tadpole, has a muscular tail and is similar in appearance to a frog tadpole. The larva has the characteristic chordate features also found in the embryos of vertebrates: a dorsal, hollow nerve cord; a stiffening rod, or notochord; and gill slits leading into the pharynx. The tadpole eventually settles and undergoes a drastic metamorphosis into the adult form. A common solitary sea squirt of both coasts of North America is the slender, yellow, transparent Ciona intestinalis, about 2 in. (5 cm) tall. The sea peach, Tethyum pyriforme, is a round, peach-colored sea squirt found from Maine north. Sea grapes are clusters of the greenish colonial squirt, Molgula manhattensis, common from Massachusetts south. Golden stars are colonies of various Botryllus species; the bright yellow individual animals are grouped in starlike clusters in a flat, encrusting, greenish tunic. Amaroucium species form colonies of minute animals embedded in a grayish, gristly tunic; chunks of such colonies, often washed ashore, are known as sea pork. There are two other groups of tunicates, both found in the plankton of open oceans. The salps (class Thaliacea) are barrel-shaped tunicates, open at both ends; they swim by muscular contractions that force water through the body. The larvaceans (class Larvacea) retain the larval form, with a tail and a notochord, as adults. A commonly held theory maintains that vertebrates evolved from animals like the larvaceans. Larvaceans have no tunics, but secrete gelatinous containers, called houses; these are used to filter food from the water and are continuously discarded and replaced. Tunicates are classified in the phylum Chordata, subphylum Urochordata.