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 (q.v.), 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. Its function is to remove the water which must diffuse into un protected cells in a medium of lower osmotic pressure than them selves.