Which organisms have contractile vacuoles and why?

Contractile vacuoles occur mainly in freshwater protists (e.g., Paramecium, Amoeba, many unicellular algae). They expel excess water that osmotically enters cells living in hypo-osmotic environments, preventing lysis. The organelle cyclically fills with water and then contracts to discharge it via a pore. Some soil protists also use them during transient hypo-osmotic stress. Function: osmoregulation and volume control in the absence of rigid cell walls or excretory systems.