Paramecium

Genus of protozoa. It is unicellular and has a large nucleus (meganucleus) and one or more small nuclei (micronuclei), and an arrangement of cilia (hairlike organs) by which it moves about.

There are also two contractile vacuoles which function alternately and act as osmotic organs; water is constantly absorbed by osmosis, and these vacuoles collect and forcibly expel it. Paramecium directs a current of water into its mouth by the movements of its cilia. The cilia also move the animal through the water.

Reproduction is by fission and by conjugation. In the former case, first the smaller nucleus, then the larger, become constricted and divide into two by oblique division, and the missing organs in either part are quickly regenerated. Frequent multiplication by fission diminishes the vitality of the organism and is succeeded by conjugation, when two individuals exchange part of the substance of their nuclear apparatus. When conditions become unfavourable the animals enclose themselves in cysts.

Classification Paramecium belongs to the subclass Hymenostomatia, class Oligohymenophora, phylum Ciliophora.

The cell also contains food vacuoles, formed by the particles of food collecting bubbles of water about them as they pass from the oral cavity into the endoplasm; the food passes in an elliptical path through the endoplasm and is gradually digested.