Paramecium caudatum, ciliated protozoans measuring about 250 microns in length, swimming in place in pond water. They move by beating their short, hairlike cilia which cover their bodies, but are only visible along their outer edges in microscopic views. Their contractile vacuoles can be seen opening and closing in a few of them. Contractile vacuoles serve as pumps which control the amount of water in a protozoan, keeping it from bursting. A number of circular food vacuoles are clearly evident. The green specks within some of them are bits of algae. Recorded under Nomarski DIC illumination.