Undulipodium

An undulipodium or 9+2 organelle is an extracellular 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, and also create water movement as in the choanocytes of sponges. Cilia are more numerous, move in a waving 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 and the corona of rotifers. 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 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.