Paramecium,  Paramecium caudatum (highly magnified).John J. Leegenus of free-living protozoans of the holotrichous order Hymenostomatida. There are at least eight well-defined species; all can be cultivated easily in the laboratory. Although they vary in size, most Paramecium species are about the size of the period at the end of this sentence. The basic shape varies, depending on the species: P. caudatum is elongated and gracefully streamlined and P. bursaria resembles a footprint. The term paramecium is also used to refer to individual organisms in a Paramecium species.

These microscopic single-celled organisms are completely covered with fine hairlike filaments (cilia) that beat rhythmically to propel them and to direct bacteria and other food particles into their mouths. On the ventral surface an oral groove runs diagonally posterior to the mouth and gullet. Within the gullet, food particles are transformed into food vacuoles, and digestion takes place within each food vacuole; waste material is excreted through the anus.

A thin layer of clear, firm cytoplasm (ectoplasm) lies directly beneath the flexible body membrane (pellicle) and encloses the inner, more fluid portion of the cytoplasm (endoplasm), which contains granules, food vacuoles, and crystals of different sizes. Embedded in the ectoplasm are spindle-shaped bodies (trichocysts) that may be released by chemical, electrical, or mechanical means. Originally believed to be a defense reaction, they appear to be extruded as a reaction to injury or for use as an anchoring device.

A paramecium has two, occasionally three, contractile vacuoles located close to the surface near the ends of the cell. They function in regulating the water content within the cell and may also be considered excretory structures since the expelled water contains metabolic wastes.

Paramecia have two kinds of nuclei: a large ellipsoidal nucleus called a macronucleus and at least one small nucleus called a micronucleus. Both types of nuclei contain the full complement of genes that bear the hereditary information of the organism. The organism cannot survive without the macronucleus; it cannot reproduce without the micronucleus. The macronucleus is the centre of all metabolic activities of the organism. The micronucleus is a storage site for the genetic material of the organism. It gives rise to the macronucleus and is responsible for the genetic reorganization that occurs during conjugation (cross-fertilization).

Strictly speaking, the only type of reproduction in Paramecium is asexual binary fission in which a fully grown organism divides into two daughter cells. Paramecium also exhibits several types of sexual processes. Conjugation consists of the temporary union of two organisms and the exchange of micronuclear elements. Without the rejuvenating effects of conjugation, a paramecium ages and dies. Only opposite mating types, or genetically compatible organisms, can unite in conjugation. P. aurelia has 34 hereditary mating types that form 16 distinct mating groups, or syngens (now considered separate species by many authorities). Autogamy (self-fertilization) is a similar process that occurs in one animal. In cytogamy, another type of self-fertilization, two animals join together but do not undergo nuclear exchange.