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paramecium(parəmē`sĭəm), unicellular organism of the genus Paramecium, of the ciliate phylum CiliophoraCiliophora
, phylum in the kingdom Protista consisting of the ciliates, or ciliophores, complex freshwater or saltwater protozoans that swim by the coordinated beating of their cilia—short, hairlike structures that cover the cell surface.
..... Click the link for more information. found in freshwater throughout the world. Like other protozoansprotozoan
, informal term for the unicellular heterotrophs of the kingdom Protista. Protozoans comprise a large, diverse assortment of microscopic or near-microscopic organisms that live as single cells or in simple colonies and that show no differentiation into tissues.
..... Click the link for more information. , paramecia, previously considered one-celled animals, are now customarily placed in kingdom ProtistaProtista
, in the five-kingdom system of classification, a kingdom comprising a variety of unicellular and some simple multinuclear and multicellular eukaryotic organisms.
..... Click the link for more information. . 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).
a genus embracing the simplest organisms of the class Infusoria. The elongated oval body reaches a length of 0.3 mm. It has a thick external layer of cytoplasm, the pellicle, which consists of three membranes and is covered evenly with 10,000 to 15,000 cilia. The mouth is on the side, beneath the oral groove.
The species P. bursaria undergoes intracellular symbiosis with single-celled green algae, or zoochlorella. Two species, P. caudatum and P. aurelia, are widely used in laboratory experiments. Methods for reproducing paramecia are well developed.