paramecium
paramecium. (Image by Barfooz and Josh Grosse, GFDL)

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Paramecium

Paramecium

Paramecium are single celled eukaryotes , reminiscent of a football in shape, that belong to the group of microorganisms known as the Protozoa . The protozoan inhabits freshwater bodies such as ponds. The organism is useful as a teaching tool for light microscopy.

There are at least eight species of Paramecium. Two examples are Paramecium caudatum and Paramecium bursaria.

Paramecium are large enough to be visible to the unaided eye. However, the internal detail is resolved only by the use of a microscope . A student is best able to observe the complex internal organization of the organism by using what is termed the hanging drop technique. Here a drop of water is suspended upside-down on a cover slip that is positioned over a cavity on a microscope slide. The cover slip is sealed to prevent leakage.

Paramecium contain organized structures called vacuoles that are essentially a primitive mouth, stomach, and excretion system. As food enters the organism, it is stored in specialized vacuoles known as food vacuoles. These can circulate through the cytoplasm of the organism, in order to provide food where needed. Characteristic of eukaryotes, nuclear material is segregated by a nuclear membrane.

Another characteristic features of Paramecium is the socalled contractile vacuole. This vacuole is able to store water and then, by virtue of the compression of the side arms that radiate from the central vacuole, to expel the water out of the organism. In this way, the amount of water inside the paramecia can be controlled. The operation of the contractile vacuole is another feature that is visible by the light microscopic observation of living paramecia.

On the exterior lies a membrane that is called the pellicle. The pellicle is both stiff, to provide support and to maintain the shape of the organism, and is flexible, to allow some flexing of the surface. Also on the surface are hundreds of tiny hairs called cilia. The cilia wave back and forth, and act to sweep food particles (bacteria and smaller protozoa) towards the primitive mouth of the organism (the gullet). The cilia are also important in locomotion, acting analogous to the oars of a rowboat. The beating of the cilia is easily visible under light microscopic examination, especially if the movement of the organism has been retarded by the addition of a viscous compound such as glycerol to the sample.

See also Eukaryotes

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paramecium

paramecium (parəmē´sĬəm), 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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paramecium

paramecium Genus of freshwater, ciliated protozoans characterized by their ‘slipper’ shape, defined front and rear ends, an oral groove for feeding, food vacuoles for digestion, an anal pore for elimination, and two nuclei. Its stiff outer covering is studded with short, hair-like cilia. Order Holotricha; species include Paramecium bursaria and P. aurelia.

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paramecium

par·a·me·ci·um / ˌparəˈmēsēəm/ • n. Zool. a single-celled freshwater animal (genus Paramecium, phylum Ciliophora) that has a characteristic slipperlike shape and is covered with cilia.

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"paramecium." The Oxford Pocket Dictionary of Current English. 2009. Encyclopedia.com. 24 Sep. 2016 <http://www.encyclopedia.com>.

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"paramecium." The Oxford Pocket Dictionary of Current English. 2009. Retrieved September 24, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O999-paramecium.html

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