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undulipodium (pl. undulipodia) A slender flexible outgrowth of a eukaryote cell used for locomotion or propelling fluids over the surface of the cell. The term ‘undulipodium’ is used to designate a eukaryotic ‘flagellum’ or a cilium (which have the same structure), to emphasize the distinction between these structures and the flagellum of a bacterium. Many protoctists and sperm cells swim by means of undulipodia, and various organisms use them to establish feeding currents, or to clear debris from epithelial surfaces. All undulipodia have a shaft, about 0.25 μm in diameter, consisting of a longitudinal array of microtubules, the axoneme, which is surrounded by an extension of the cell's plasma membrane. The axoneme has two singlet microtubules running down the middle and nine doublet microtubules around the periphery, giving a characteristic 9 + 2 array. At its base the axoneme connects with a basal body (or kinetosome). This is a short cylinder that, like a centriole, consists of nine triplet microtubules; it organizes assembly of the axoneme microtubules and is part of a complex array of fibres and microtubules forming a root structure within the cell. Cilia are shorter than flagella, and move by a whiplike power stroke followed by a recovery stroke in the opposite direction. Flagella generate successive waves that pass from the base to the tail. In both cases, flexing of the shaft is produced by a sliding motion of the microtubule doublets relative to each other. This involves the successive formation and breakage of molecular bridges between adjacent doublets. The bridges are composed of a protein, dynein, and their formation requires energy in the form of ATP.