Goodbody, Ivan Department of Zoology, University of the West Indies, Kingston, Jamaica.
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A class of Tunicata (Urochordata) which occurs as solitary zooids or, by a process of asexual budding, develops into colonies. Zooids of the class Ascidiacea vary in length from a few millimeters to 10 in. (25 cm). Individuals or colonies are invested by a protective covering, the tunic or test, made of polysaccharide material structurally close to cellulose. Beneath the test is the body wall or mantle. Each zooid has two apertures: inhalant (oral) and exhalant (atrial; see illustration). Water currents, created by cilia on the margins of stigmata in the pharyngeal wall, draw water into the branchial sac, where it is filtered and passed out through the exhalant aperture. The entrance to the branchial sac is guarded by tentacles which prevent large particles from entering. Filtration takes place on a mucous sheet secreted by the endostyle; the sheet is passed across the internal face of the branchial sac by ciliary or muscular action and is then rolled into a cord by the dorsal lamina (or by languets). Digestive enzymes are secreted into the stomach, and a pyloric gland, of unknown function, enters at the junction of stomach and intestine. Gonads are hermaphroditic, and may be situated in the loop of the intestine or in the mantle wall.
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