contractile vacuole

Tiny organelle found in many single-celled freshwater organisms. It slowly fills with water, and then contracts, expelling the water from the cell.

Freshwater protozoa such as Amoeba absorb water by the process of osmosis, and this excess must be eliminated. The rate of vacuole contraction slows as the external salinity is increased, because the osmotic effect weakens; marine protozoa do not have a contractile vacuole.