Contractile vacuole

A membranous organelle that is very conspicuous, when present, is the contractile vacuole. It functions as an osmoregulatory mechanism that prevents the cell from taking up excessive water and potentially rupturing. The contractile vacuole periodically expels excess water from the cytoplasm; one cycle takes several seconds, depending on the species. The mode of expulsion varies among different amoebae.

This mechanism is absent in marine species, which live in an environment where osmotic influx of water is minimal. Pelomyxa, a freshwater amoeboid, also lacks a contractile vacuole. Most amoebae possess a single contractile vacuole, although some species have two or more.