Paramecium

Paramecium, genus of protozoa of the phylum Ciliophora, often called slipper animalcules because of their slipperlike shape. Paramecia are unicellular organisms usually less than 0.25 mm (0.01 in) in length and covered with minute hairlike projections called cilia. Cilia are used in locomotion and during feeding. When moving through the water, paramecia follow a spiral path while rotating on the long axis. When a paramecium encounters an obstacle, it exhibits the so-called avoidance reaction: It backs away at an angle and starts off in a new direction. Paramecia feed mostly on bacteria, which are driven into the gullet by the cilia. Two contractile vacuoles regulate osmotic pressure (see Osmosis) and also serve as excretory structures. A paramecium has a large nucleus called a macronucleus, without which it cannot survive, and one or two small nuclei called micronuclei, without which it cannot reproduce sexually. Reproduction is usually asexual by transverse binary fission, occasionally sexual by conjugation, and rarely by endomixis, a process involving total nuclear reorganization of individual organisms.

Paramecia abound in freshwater ponds throughout the world; one species lives in marine waters. They are easily cultivated in the laboratory by allowing vegetable matter to stand in water for a few days. The common species Paramecium caudatum is widely used in research.