Paramecium is a well-known genus of ciliate protozoa, commonly studied as a representative of that group. The cells are slipper-shaped, and range from 50 to 300 μm in length, depending on species. Simple cilia covers the body, and there is a deep oral groove, containing inconspicuous compound oral cilia as found in other peniculids. Paramecium is widespread in freshwater environments, and is especially common in scums.
Paramecium was first described during the 1700s. The division into species is still in flux - for instance, P. aurelia has recently been divided into 14 species - but the following are representative: