Cilia and flagella are structures found in various prokaryotes as well in some eukaryotic cells. They play defense, nutrition and movement roles for the cell. In eukaryotic cells of protists and animals they originate from centrioles that migrate in the direction of the plasma membrane and differentiate into structures projected outside the cell. Every cilium or flagellum is made of nine peripheral pairs of microtubules and one central pair all covered by membrane. (In bacteria, flagella are form of a protein named flagellin and there can also be fimbria made of pilin.)