Archaellum

Definition
noun, plural: archaella
A whip-like structure of filaments that extends outside the cell of many archaea
Supplement
Archaellum is a term used to refer to the whip-like structure on the cell surface of certain archaea and is similar to the flagellum in bacteria. It is previously thought of as homologous to bacterial flagellum based on gross morphology and behavior. Both the archaellum and the bacterial flagellum are made up of filaments that protrude on the cell surface. Both of them are involved in propelling the organism through rotation.¹
Bacterial flagella and archaella differ in many aspects. For one, the bacterial flagellar filaments rotate independent of one another whereas the archaeal flagellar filaments rotate as a single assembly. Another distinction is that the bacterial flagella grow by flagellin subunits added from the inside of the hollow filament up to the tip whereas the archaeal flagella grow with the addition of flagellin at the base. It is because the flagellin subunits cannot move up since the archaellar filament is too thin to allow it.²
Also called:

• archaeal flagellum

See also:

Reference(s):

¹ Cavalier-Smith, T. (1987). “The origin of eukaryotic and archaebacterial cells”. Ann. N. Y. Acad. Sci. 503 (1): 17–54.

² Ghosh, A. & Albers, S. V. (January 2011). “Assembly and function of the archaeal flagellum”. Biochem. Soc. Trans. 39 (1): 64–9.