An undulipodium or undulopodium (Greek: "swinging foot"; plural undulipodia), or a 9+2 organelle is a motile filamentous extracellular projection of eukaryotic cells.
[5] 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.
[7] 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.