An undulipodium or undulopodium (Greek: "swinging foot"; plural undulipodia), or a 9+2 organelle is a motile filamentous extracellular projection of eukaryotic cells. It is basically synonymous to flagella and cilia which are differing terms for similar molecular structures used on different types of cells, and usually correspond to different waveforms.
The name was coined to differentiate from the analogous structures present in prokaryotic cells.
The usage of the term was early supported by Lynn Margulis, especially in support of endosymbiotic theory. The eukaryotic cilia are structurally identical to eukaryotic flagella, although distinctions are sometimes made according to function and/or length. 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.
In the 1980s, biologists such as Margulis advocated the use of the name "undulipodium", because of the apparent structural and functional differences between the cilia and flagella of prokaryotic and eukaryotic cells. They argued that the name "flagellum" should be restricted to prokaryotic organelles, such as bacterial flagella and spirochaete axial filaments. A distinction was drawn between "primary" cilia which function as sensory antennae, and ordinary cilia: it was argued that these were not undulipodia as they lacked a rotary movement mechanism.
However, the term "undulipodium" is not generally endorsed by biologists, who argue that the original purpose of the name does not sufficiently differentiate the cilia and flagella of eukaryotic from those of prokaryotic cells. For example, the early concept was the trivial homology of the flagella of flagellates and the pseudopodia of amoebae. The consensus terminology is to use "cilium" and "flagellum" for all purposes.
Synonyms: motile cilia, microtubule-based flagellum, motile primary cilia, motile primary cilium, motile secondary cilium, nodal cilium