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 protists, such as Amoeba (which has one spherical vacuole) and Paramecium (in which a number of accessory vacuoles are attached to a main vacuole).