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 . I (1905) (see also PROTOZOA; PROTOPLASM) .