Osmoregulation in Protozoans
Osmoregulation or water balance in protozoa is accomplished by contractile vacuoles. One to several contractile vacuoles may be present within the animal which may or may not have fixed sites in the cytoplasm and may have contributory canals or other vesicles opening into it.
These are water and ion regulating structures, acting as pumps to remove excess water from the cytoplasm. All freshwater protozoans have functioning systems of contractile vacuoles whereas, marine and parasitic forms have these less frequently.