Undulipodium

An undulipodium or undulopodium (Greek: "swinging foot"; plural undulipodia) is a motile filamentous extension of eukaryotic cells, composed of a membrane protrusion held by a cytoskeletal structure called the axoneme. It is divided into cilia and flagella – which are differing terms for structurally identical organelles used on different types of cells, but are distinguished according to function and/or length, and usually corresponds to different waveforms of the organelles beating motion. The Gene Ontology database does not make a distinction between the two, referring to most undulipodia as "motile cilium", and to that in the sperm as sperm flagellum.

The name was coined to differentiate from the analogous structures present in prokaryotic cells, although "flagellum" would be a misnomer for the prokaryotic structure as they function more like propellers or corkscrews and, thus, do not "whip".

The usage of the term was early supported by Lynn Margulis, especially in support of endosymbiotic theory.