CONTRACTILE VACUOLE

contractile vacuole, in biology, a spherical space filled with liquid, which at intervals discharges into the medium; it is found in all fresh-water groups of protozoa, and some marine forms, also in the naked aquatic reproductive cells of algae and fungi. it is absent in states with a distinct cell-wall to resist excessive turgescence, such as would lead to the rupture of a naked cell, and we conclude that its chief function is to prevent such turgescence in unprotected naked cells. it fulfils also respiratory and renal functions, and is comparable, physiologically, to the contractile vesicle or bladder of rotifers and turbellarians. in many species it is part of a complex of canals or spaces in the protoplasm. see m. hartog, british association _reports_, and degen, _botanische zeitung_, vol. lxiii. abt. 1 (1905) (see also protozoa; protoplasm).