What is Flagellum?
[[Image:Flagellum-beating.png|thumb|240px|Difference of beating pattern of flagellum and cilia. Flagellum is the one on the left, cilia are on the right.]] A '''flagellum''' (plural: ''flagella'') is a long, whip-like structure that helps some single celled organisms move. It is composed of [[microtubule]]s. They help [[propulsion|propel]] cells and [[organism]]s in a [[whip]]-like motion. The flagellum of [[eukaryote]]s usually moves with an “S” motion, and is surrounded by [[cell membrane]]. Flagella are structurally almost identical with the much smaller [[Cilia]]. So much so that it has been proposed [[protist]]s bearing either should be unified in the [[Phylum]] ''Undulipodia''.<ref>Margulis L & Dolan M.F. 2002. ''Early life: evolution on the Precambrian Earth''. 2nd ed, Jones & Bartlett, Boston. p89</ref> Previously, Margulis had proposed that the [[Ciliate]]s alone should be placed in a Phylum ''Ciliophora''.<ref>Margulis L. Schwartz K.V. & Dolan M. 1999. ''Diversity of life: the illustrated guide to the five kingdoms''. Jones & Bartlett, Boston, p94. In this work the authors propose 19 phyla for the Protista, and call this 'Kingdom' the 'Protoctista', a term which is unfortunately almost unpronounceable.</ref> Admittedly, the Protista is a collection of disparate single-celled forms, but while a more sophisticated taxonomy is in flux (changing), Protista is still a useful term. Cilia and flagella are cell [[organelle]]s, specialised units which carry out well-defined functions, like [[mitochondria]] and [[plastid]]s. It is fairly clear now that all or most of these organelles have their origin in once-independent [[prokaryote]]s ([[bacteria]] or [[archaea]]), and that the [[eukaryote]] cell is a 'community of micro-organisms' working together in 'a marriage of convenience'.<ref>Margulis L. and McMenamin 1990. Marriage of convenience. ''The Sciences'' '''30''', 31-36.</ref> == Types == Three types of flagella have so far been distinguished; bacterial, archaeal and eukaryotic. The main differences among