Also found in: Dictionary, Thesaurus, Medical, Wikipedia.
a subphylum of animals of the phylum Chordata. Most tunicates have a saccular or urceolate body, measuring from 0.3 to 2.5 cm long (Appendicularia). Some are up to 50 cm long (certain Ascidiae), and others reach 30 m in length (colonies of Pyrosomata). The body is enclosed in a casing, or tunic, which is secreted by the ectoderm and has a gelatinous or cartilaginous consistency. The tunic is largely composed of tunicin, a substance similar to cellulose. Appendicularians have a notochord in both the larval and adult forms; all other tunicates have a notochord only in the larval stage. The mouth leads to a large pharynx, which takes in both food and respired air. The pharynx is perforated by gill slits that lead directly to the exterior (appendicularians) or lead to a special cavity, the cloaca. The posterior gut and the ducts of the sex glands (ascidians and Desmomyaria) also open into the cloaca. The cloacal pore leads from the cloaca to the exterior. The circulatory system consists of a heart and a more or less developed network of vessels. The central nervous system consists of a cerebral ganglion on the dorsal side of the body. A dorsal nerve trunk departs from the ganglion. The sense organs are poorly developed.
Tunicates are hermaphrodites. Reproduction may be sexual or asexual (budding). Alternation of sexual and asexual generations is common (for example, among Desmomyaria). Development is sometimes extremely complex and may be accompanied by regressive metamorphosis (for example, in Ascidians).
The Russian scientists A. O. Kovalevskii, who studied the development of ascidians, determined in 1866 that tunicates are chordates and that they are closely related to vertebrates and, especially, to Acrania. All tunicates are marine animals. They feed on small animals, algae, and organic remains suspended in the water. There are three classes: Appendicularia, Ascidiae, and Thaliacea. The last includes three subclasses—Pyrosomata, Desmomyaria, and Cyclomyaria. A different, taxonomy was previously accepted. There are more than 1,000 known species of tunicates.
REFERENCESOgnev, S. I. Zoologiia pozvonochnykh, 3rd ed. Moscow, 1941.
Beklemishev, V. N. Osnovy sravnitel’noi anatomii bespozvonochnykh, 3rd ed., vols. 1–2. Moscow, 1964.
Zhizn’zhivotnykh, vol. 2. Moscow, 1968.
A. V. IVANOV