Undulipodium

An undulipodium (a Greek word meaning "swinging foot") or a 9+2 organelle is a motile filamentous extracellular projection of eukaryotic cells. It is basically synonymous to flagella and cilia of the eukaryotic cells. In fact the name was coined to differentiate from the homologous structures present in prokaryotic cells. It is structurally a complex of microtubules along with motor proteins. The usage of the term is most vocally supported by Lynn Margulis, especially in support of her endosymbiotic theory. The eukaryotic cilia are structurally identical to eukaryotic flagella, although distinctions are sometimes made according to function and/or length.

Undulipodia 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 of rotifers.

Primary cilia function as sensory antennae, but are not undulipodia as primary cilia do not have the rotary movement mechanism found in motile cilia.

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