contractile vacuole A membrane-surrounded cavity in a cell that periodically expands, filling with water, and then suddenly contracts, expelling its contents to the cell's exterior. It is thus an organ of osmoregulation and excretion. Contractile vacuoles are common in freshwater sponges and typical of freshwater protoctists, such as Amoeba (which has one spherical vacuole) and Paramecium (in which a number of accessory vacuoles are attached to a main vacuole).

contractile vacuole An organelle which appears to function in the removal of excess water from the cells of protozoa. The contractile vacuole expands as it fills with water from the cytoplasm, and contracts as this water is emptied to the exterior.

contractile vacuole In many freshwater, unicellular organisms, a membranebound cavity that expands and contracts with a pulsating movement as it takes in water from the cell contents and expels it from the cell. Contractile vacuoles serve to regulate the water content of the cell.