flagellum
1. A thread-like organelle which usually functions in locomotion. Flagella are found on a range of eukaryotic cells, including those of certain protozoa and spermatozoa. Bacterial and eukaryotic flagella are very different both in structure and mode of operation and are not homologous. Bacterial flagella rotate; eukaryotic flagella undulate. The eukaryotic flagellum has the more complex structure, and there are two types: the whiplash type, which is smooth and whip-like, and the tinsel type, which bears numerous fine, hair-like projections along its length.

2. In insects, the antennal segments distal to the scape.

fla·gel·lum / ˌfləˈjeləm/ • n. (pl. -gel·la / -ˈjelə/ ) Biol. a slender threadlike structure, esp. a microscopic whiplike appendage that enables many protozoa, bacteria, spermatozoa, etc., to swim.DERIVATIVES: fla·gel·lar / fləˈjelər; ˈflajələr/ adj.

flagellum
1. (pl. flagella) (in prokaryotes) A long slender structure that protrudes from the cell surface of a bacterium. It rotates from its base and propels the bacterium along. Up to several micrometres in length, a flagellum is constructed of numerous subunits of the protein flagellin, while at the base a system of rings anchors the flagellum in the cell wall and plasma membrane. Surrounding these rings are paired motor proteins, which impart a rotary motion to the filament, and switch proteins, which can reverse the direction of rotation. Flagella may be attached singly or in groups, for example at the poles of the bacterial cell, or scattered over the cell surface.

2. (in eukaryotes) See undulipodium; tinsel flagellum.

flagellum (pl. flagella)
1. A thread-like organelle that usually functions in locomotion. Flagella are found on some bacteria and on a range of eukaryotic cells, including those of certain fungi, protozoa, and algae. Bacterial and eukaryotic flagella are very different both in structure and mode of operation. Bacterial flagella rotate; eukaryotic flagella undulate. The eukaryotic flagellum has the more complex structure, and there are two types: the whiplash type, which is smooth and whiplike, and the tinsel type, which bears numerous fine, hair-like projections along its length.

2. In certain liverworts, a slender, filamentous, runner-like stem which carries much-reduced leaves.

flagellum (flă-jel-ŭm) n. (pl. flagella) a fine long whiplike thread attached to certain types of cell (e.g. spermatozoa and flagellates). Flagella are responsible for the movement of the organisms to which they are attached.