An undulipodium or 9+2 organelle is an intracellular projection of a eukaryotic cell containing a microtubule array. Both flagella and cilia are considered undulipodia and are similar structurally, however each differs functionally. Flagella use a whip-like action to create movement of the whole cell, such as the movement of sperm in the reproductive tract. Cilia primarily use a waving action to move substances across the cell, such as the ciliary esculator found in the respiratory tract. Cilia may also function as sensory organs.
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 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.