Two contractile vacuoles occupy fixed position, one near each end of the body, their liquid contents have been alleged to possess ammonia, urea and uric acid by earlier workers. Probably they may partially help in respiration and excretion by eliminating nitrogenous wastes. But the recent workers maintain that their main function is osmoregulation i.e. keeping an osmotic equilibrium with the surrounding water by regulating the water contents of the body.

Thus, the contractile vacuoles function as hydrostatic organelle. An excess of water accumulates in the body because of continuous endosmosis, the concentration of cytoplasm being higher than that of the external medium. Small quantities of water also ingested with the food. The excess of water is got rid by means of the contractile vacuoles exactly as Amoeba. The contractile vacuoles contract and expand at regular intervals, assisted by the contractility of the myofibrils.

Water from cytoplasm is gathered by 6 to 10 radiating canals, which converge and discharge into each contractile vacuole. When the vacuoles have grown to its maximum size, it bursts and discharges to the exterior probably through an opening in the pellicle.