A flagellum (plural: flagella) is a tail-like structure that projects from the cell boy of certain prokaryotic and eukaryotic cells, and it functions in locomotion. An example of a eukaryotic flagellated cell is the sperm cell, which uses its flagellum to propel itself through the female reproductive tract. An example of a flagellated bacterium is the ulcer-causing Helicobacter pylori, which uses its multiple flagella to propel iteself through the mucus lining to reach the stomach epithelium. Prokaryotic and eukaryotic flagella have some notable differences, such as protein composition, structure, and mechanism of propulsion. Flagella are structurally identical to eukaryotic cilia, although distinctions are sometimes made according to function and/or length. Flagella are cellular structures, not organelles.