The contractile vacuoles of this Paramecium caudatum, a ciliated protozoan measuring about 250 microns in length, can be seen opening and closing. The two contractile vacuoles, one at end end in this species, serve as pumps which control the amount of water in a protozoan, keeping it from bursting. The Paramecium is surrounded by thousands rod bacteria, one of its favorite foods. Recorded under Nomarski DIC illumination.