The bacterium Proteus vulgaris (greatly magnified) showing flagella—© Lee D. Simon—Photo Researchers

Hairlike structure that acts mainly as an organelle of movement in the cells of many living organisms. Characteristic of the protozoan group Mastigophora, flagella also occur on the sex cells of algae, fungi (see fungus), mosses, and slime molds. Flagellar motion causes water currents necessary for respiration and circulation in sponges and cnidarians. Most motile bacteria move by means of flagella. The structures and patterns of movement of flagella in prokaryotes differ from those in eukaryotes. See also cilium.

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