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cytoplasmic processes of a cell, characteristic of all flagellates and many bacteria, zoospores, and spermatozoa of all plants and animals.
Earlier, flagella were considered in contraposition to cilia. Electron-microscope investigations have shown the structural resemblance of these processes, which differ from one another only in number: a cell usually has one or several flagella and many cilia (up to several thousand). For this reason the terms “flagella” and “cilia” are often used as synonyms. The lengths of flagella vary greatly from cell to cell, and their diameters are each approximately 0.2 microns. Each flagellum has a shaft covered with a plasmic membrane (a continuation of the cell membrane) and consisting of a homogeneous substance, of which there are nine double fibrils along the periphery and two single fibrils in the center (thickness 250-600 A) with an electronically denser edge and a less dense central area; for this reason the fibrils are called microtubules. At the base of the flagellum is the basal body, a homologue of the centriole. Several forms of movement are distinguished in flagella: rotary, including spiral, motion; undulating motion, with wavelike movement from the free end to the base of the flagellum; and paddle-stroke motion. A similarity between the protein of flagella and the proteins of muscle has been discovered.
M. E. ASPIZ