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.

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.