Contractile Vacuole

Contractile Vacuole, the structure in the Protozoa which is supposed to act as the excretory organ. In Amoeba (q.v.), for example, there is near the centre of the protoplasmic mass of which it consists, a clear round body which appears to alternately expand and contract; this is the contractile vacuole. In Amoeba it appears to be simply due to the gradual growth of a single drop of fluid; in other forms, as in the "Slipper Animalcule" (Paramecium), it is formed by the union of several droplets; these commence at first as mere lines radially arranged around a minute central cavity; the lines expand into pear-shaped bodies; these fuse at their thicker inner ends to a central vacuole; this continues to grow, the radial lines disappear, and the whole vacuole finally bursts; its contents are thus discharged. The function of the contractile vacuole is thus the expulsion of superfluous water; the soluble excretory products are doubtless dissolved in this water, while other insoluble innutritious particles have been observed to collect in the vacuole and are thus also got rid of. In some Protozoa, such as the "Bell Animalcule" (Vorticella), there is a special "vacuolar duct," into which the contractile vacuoles discharge.