The fresh water Protozoa animals will face a problem with excess of wafer that enters into the body by Osmosis. The process by which water balance in the body is maintained is called Osmoregulation. In protozoans animal’s osmoregulation is carried on by contractile vacuoles.

In marine protozoans and parasitic protozoans contractile vacuole is absent. In Amoeba osmoregulation is carried out by contractile vacuoles. The contractile vacuole will absorb excess of water in the body Then it reaches the surface and ruptures. This sends out the excess water. Now-a-days "Duncan Wigg, EC. Bovee, T L Jahn' in-1967 worked on the contractile vacuole of Amoeba and came to the conclusion this contractile vacuole has no contraction capacity. It will rupture only to send out the excess water. So they called the contractile vacuole as Water pulsating vesicle'.

In Paramecium two contractile vacuoles are present. They are surrounded by 6 to 10 radiating canals. The two contractile vacuoles will work alternately and perform Osmoregulation.

Maintenance of constant internal environment is called homeostasis. This word is coined by W.B Cannon, in Protozoans homeostasis or Osmoregulation is carried on by contractile vacuoles.