In fresh water protozoans, the function of contractile vacuole is hydrostatic or osmoregulatory. Water in freshwater protozoan enters the organism by endosmosis and during feeding. The cytoplasm of a freshwater protozoan represents a medium denser than surrounding water and outermost plasmalemma acts as a semi-permeable membrane. Thus water continuously flows inside to dilute the cytoplasm. The vacuole periodically increases in volume (diastole) to get filled with water and contracts (systole) to discharge its water content to the surrounding environment. When freshwater protozoan is placed in marine water which is denser (hypertonic) than its cytoplasmic medium then the oragnism tends to lose water. Thus contractile vaculoes will no longer need to constantly pump out the excess water and hence will decrease in size.