Paramecium

Paramecium (pronounced PARR-ə-MEE-s(ee-)əm or -see-əm; the plural "paramecia" is used only as a vernacular name) is a genus of single-celled, eukaryotic ciliates. These microorganisms thrive in freshwater, brackish, and ocean environments, and are especially common in stagnant ponds and basins. Because certain species are easy to grow in culture and can be readily induced to reproduce through conjugation and division, they have become standard tools in classrooms and laboratories for studying biological processes. They are frequently used as model organisms among ciliates and have been called the "lab rats" of the phylum Ciliophora.

Paramecia were among the first ciliates seen by early microscopists in the late 1600s. The Dutch protozoologist Antonie van Leeuwenhoek likely encountered them, and they were clearly described by his contemporary Christiaan Huygens in a 1678 letter. The earliest known illustration of a Paramecium species appeared anonymously in the *Philosophical Transactions of the Royal Society* in 1703. In 1718, French mathematics teacher and microscopist Louis Joblot described and illustrated a microscopic "poisson" (fish) he found in an oak bark infusion, naming it "Chausson" (slipper); the term "slipper animalcule" was used colloquially for Paramecium through the 18th and 19th centuries. The name "Paramecium," from the Greek *paramēkēs* meaning "oblong," was coined in 1752 by English microscopist John Hill for animalcules with no visible limbs or tails and an irregularly oblong shape. In 1773, O. F. Müller, the first to place the genus in Linnaean taxonomy, adopted the name but changed the spelling to *Paramæcium*. Johann Hermann altered it to *Paramœcium* in 1783. C. G. Ehrenberg, in a major 1838 study of infusoria, restored Hill's original spelling, which most researchers have since followed.

Paramecium species range from 0.06 mm to 0.3 mm in length. Their cells are typically ovoid, elongate, or shaped like a foot or cigar. The cell body is enclosed by a stiff but elastic pellicle, composed of an outer plasma membrane, a layer of flattened membrane-bound sacs called alveoli, and an inner epiplasm. The pellicle is textured with hexagonal or rectangular depressions, each perforated by a central aperture from which a single cilium projects. Between the alveolar sacs, most species have closely spaced, spindle-shaped trichocysts—explosive organelles that discharge thin, non-toxic filaments, often for defense. An anal pore (cytoproct) is usually located on the ventral surface in the posterior half of the cell. All species have a deep oral groove running from the anterior to the midpoint, lined with cilia that beat continuously to draw food into the cell. Paramecia are primarily heterotrophic, feeding on bacteria and other small organisms; a few species are mixotrophs, obtaining some nutrients from endosymbiotic algae (chlorella) carried in their cytoplasm. Osmoregulation is managed by contractile vacuoles that actively expel water to compensate for osmotic intake, with the number of vacuoles varying by species.

Paramecium moves by whip-like strokes of its cilia, which are arranged in tight rows around the body. Each cilium has a fast, stiff "effective stroke" followed by a slow "recovery stroke" where it curls loosely to one side and sweeps forward counter-clockwise. The cilia beat in coordinated waves across the "ciliary carpet," an effect compared to wind blowing across a field of grain. The organism spirals through water as it moves. When it hits an obstacle, it reverses the effective stroke and swims backward briefly—the avoidance reaction—before resuming forward progress, repeating the process until it passes the object. It has been calculated that a Paramecium uses over half its energy on locomotion, and this ciliary method is less than 1% efficient, though close to the theoretical maximum for an organism with such short cilia.

To gather food, Paramecium uses its cilia to sweep prey—bacteria, algae, and yeasts—along with water through the oral groove (vestibulum) and into the cell. Food then passes into the narrower buccal cavity (gullet), through a small opening called the cytostome (cell mouth), and into the interior. There, food particles are collected into food vacuoles, which periodically close off and release into the cytoplasm. These vacuoles circulate through the cell body via cytoplasmic streaming (cyclosis). As a vacuole moves, enzymes from the cytoplasm enter to digest its contents.