Paramecium

Paramecium is a genus of unicellular ciliate protozoa, commonly studied as a representative of the ciliate group. The shape resembles a grain of rice. The cell ranges from about 50 to 350 µm in length (more or less one tenth of a millimetre) and is covered with simple cilia, allowing the cell to move at speeds of approximately 12 body lengths per second. There is a deep oral groove containing inconspicuous tongue-like compound oral cilia (as found in other peniculids) used to draw food inside. In general, they feed on bacteria and other small cells, making them heterotrophs. Osmoregulation is carried out by a pair of contractile vacuoles, which actively expel water from the cell absorbed by osmosis from its surroundings. They are relatively large protists and can easily be seen with a medium-power microscope.

Paramecia are widespread in freshwater environments, and are especially common in scums. Recently, some new species of Paramecium have been discovered in the oceans.

Certain single-cell eukaryotes, such as Paramecium, are examples for exceptions to the universality of the genetic code: in their translation systems a few codons differ from the standard ones.