Paramecium (pron. parruh-MEE-cee-uhm, IPA ˌpærəˈmiːsiəm) is a genus of unicellular Ciliate protozoa, commonly studied as a representative of the Ciliate group. Paramecia are widespread in freshwater, brackish and marine environments, and are often very abundant in stagnant basins and ponds. Because some species are readily cultivated and easily induced to conjugate and divide, Paramecium has been widely used in classrooms and laboratories to study biological processes. Its usefulness as a model organism has caused one ciliate researcher to characterize it as the "white rat" of the phylum Ciliophora.
Paramecia were among the first ciliates to be seen by microscopists, in the late 17th century. They were probably known to the Dutch pioneer of protozoology, Antoni van Leeuwenhoek, and were clearly described by his contemporary Christiaan Huygens in a letter of 1678. In 1718, the French mathematics teacher and microscopist Louis Joblot published a description and illustration of a microscopic "poisson" (fish), which he discovered in an infusion of oak bark in water. Joblot gave this creature the name "Chausson," or "Slipper," and the phrase "slipper animalcule" remained in use as a colloquial epithet for Paramecium, throughout the 18th and 19th centuries. The name "Paramecium"—constructed from the Greek παραμήκης (paramēkēs, "oblong") -- was coined in 1752 by the English microscopist John Hill, who applied the name generally to "Animalcules which have no visible limbs or tails, and are of an irregularly oblong figure." In 1773, O. F. Müller, the first researcher to place the genus within the Linnaean system of taxonomy, adopted the name Paramecium, but changed the spelling to Paramœcium. C. G. Ehrenberg, in a major study of the infusoria published in 1838, restored Hill's original spelling for the genus name, and most researchers have followed his lead.
Species of Paramecium range in size from 50 to 330 micrometres in length. Cells are typically ovoid, elongate, foot- or cigar-shaped. The body of the cell is enclosed by a stiff but elastic membrane (pellicle), uniformly covered with simple cilia, hairlike organelles which act like tiny oars to move the organism in one direction. Nearly all species have closely spaced spindle-shaped trichocysts embedded deeply in the cellular envelope (cortex) that surrounds the organism. Typically, an anal pore (cytoproct) is located on the ventral surface, in the posterior half of the cell. In all species, there is a deep oral groove running from the anterior of the cell to its midpoint. This is lined with inconspicuous cilia which beat continuously, drawing food inside the cell. Paramecia live mainly by heterotrophy, feeding on bacteria and other small organisms. A few species are mixotrophs, deriving some nutrients from endosymbiontic algae (chlorella) carried in the cytoplasm of the cell.