Osmosis is constantly driving water into a protozoan because they are enclosed by a semipermeable membrane that can not keep the water out. The contractile vacuoles make it possible for a protozoan to get the water out. read more
Freshwater protozoan require contractile vacuole to regulate the amount of water in its body /cell because the liquids ,in the form of waste water etc ,present in protozoan are more concentrated than the surrounding freshwater water due to presence of different acid ,etc . read more