An appendage used for movement or other functions. Bacterial flagella are driven by a bacterial flagellar motor; their construction differs from eukaryotic motile cilia.
A bacterial filament rotates as part of a motor-driven propeller, whereas a typical eukaryotic flagellum bends through microtubule-associated motor activity. The shared functional name therefore does not imply identical architecture or a single evolutionary origin for every appendage called a flagellum. [ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 16, Cilia and Flagella
A bacterial flagellum is a rotary swimming appendage comprising a filament, a flexible hook and an envelope-associated bacterial flagellar motor. Rotation of the helical filament converts motor torque into propulsion through a liquid. It differs structurally from a eukaryotic cilium, which bends through the action of dynein on microtubules. In the classic tethered-cell experiment, attachment through a flagellar filament lets the cell body rotate instead, making otherwise hard-to-see motor rotation visible by microscopy. [ref]
Flagellar rotation and the mechanism of bacterial motility by M. Silverman and M. Simon. Silverman and Simon, 1974, Flagellar rotation and the mechanism of bacterial motility
