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.