Single-celled algae and animal sperm cells are widely separated in evolution but both swim in the same way, by waving their protruding hairs, called cilia or flagella. Motion is driven by molecular motors, complex assemblies of proteins that exert a force when changing shape. The motor proteins are connected to the cell's internal skeleton of microtubules; the moving force from the motor causes microtubules to slide, moving the flagella and propelling the cell.