paramecium parəmē´sĭəm [key], unicellular organism of the genus Paramecium, of the ciliate phylum Ciliophora found in freshwater throughout the world. Like other protozoans, paramecia, previously considered one-celled animals, are now customarily placed in kingdom Protista. The paramecium has a stiff outer covering that gives it a permanent slipper shape. It swims rapidly by coordinated wavelike beats of its many cilia—short, hairlike projections of the cell. A paramecium normally moves forward in a corkscrew fashion but is capable of reversing direction when it encounters adverse conditions. This trial-and-error behavior (backing up and then continuing forward in a slightly different direction until an optimum path is found) is conspicuous when the animal is observed through a microscope.

Paramecia and other ciliates are the most complex of all single-celled organisms. The paramecium has an external oral groove lined with cilia and leading to a mouth pore and gullet; food (typically smaller organisms, such as bacteria) is digested in food vacuoles. There are also an anal pore, two contractile vacuoles that regulate the water content of the cell, and two nuclei. The larger nucleus, or macronucleus, is thought to regulate most cell functions, while the smaller nucleus, or micronucleus, is involved in reproduction. Paramecia usually reproduce asexually by cell division but can also exchange genetic information via a process called conjugation, in which two individuals unite at the oral grooves and exchange micronuclei that serve as little packages of DNA, after which the cells divide, yielding daughter cells with DNA from each of the parents.

See A. Jurand and G. C. Selman, The Anatomy of Paramecium aurelia (1964).

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