Flagellum

A flagellum (plural: flagella) is a long, slender projection from the cell body, which can be directly seen (or rendered visible after appropriate treatment) with the light or electron microscope. Its function is usually to propel a unicellular or small multicellular organism by beating with a whip-like motion. In larger animals, the flagella are often arranged en masse at the surface of a stationary cell anchored within an organ and serve to move fluids along mucous membranes, such as the lining of the trachea.

Three quite distinct types of flagella have so far been distinguished; bacterial, archaeal and eukaryotic.