Contractile vacuole

A membranous organelle that is very obvious, when present, is the contractile vacuole. It serves as a protective mechanism that prevents the cell from absorbing too much water and possibly exploding.  A contractile vacuole expels excess water periodically from the cytoplasm. One cycle takes several seconds, depending on the species. The mode of expulsion may vary among different amoeba species.
Such a mechanism is absent in marine species. Pelomyxa, a freshwater amoeboid, also lacks a contractile vacuole. Most amoebae have only one contractile vacuole, others have two or more.

Contractile vacuole Amoeba proteus
Amoeba proteus, a large naked amoebae with contractile vacuole (CV)
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