Flagellum

Flagellum, a vibratile filament of living matter associated with a cell, whether that be an isolated unit as in most flagellate Infusorians, or an element in a multicellular organism, as in the flagellate chambers of a sponge. A flagellate cell has in most cases a single flagellum, and two are not uncommon; cilia, however, usually occur in numbers. A cilium is simply bent and straightened again; a flagellum wriggles from side to side with a more complex undulatory contraction and extension. A rudder-like flagellum, driving the cell before it, as in spermatozoa and bacteria, has been called a pulsellum; while the commoner type, which draws the cell after it, as seen in many Infusorians, has been termed a tractellum. A flagellum is usually locomotor or food-wafting in function, and is an expression and outcome of marked activity in the cell to which it belongs. See CELL, CILIA, PROTOZOA.