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An undulipodium or 9+2 organelle is an intracellular projection of a eukaryotic cell containing a microtubule array. Both eukaryotic flagella and eukaryotic motile cilia are considered undulipodia.
Flagella use a whip-like action to create movement of the whole cell, such as the movement of sperm in the reproductive tract, and also create water movement as in the choanocytes of sponges.
Motile (or secondary) cilia are more numerous, with multiple cilia per cell, move in a wave-like action, and are responsible for movement in organisms such as ciliates and Platyhelminthes, but also move extracellular substances in animals, such as the ciliary escalator found in the respiratory tract of mammals and the corona[clarification needed] of rotifers.
Undulipodia are an extension of the cell membrane containing both cytoplasm and a regular arrangement of microtubules known as an axoneme. At the base of the extension lies a structure called the kinetosome or basal body which is attached via motor proteins to the microtubules. The kinetosome mediates movement through a chemical reaction, causing the microtubules to slide against one another and the whole structure to bend.
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