Cilium. Coloured transmission electron micrograph (TEM) of a cross-section through a protozoan cilium. Cilia are thread-like structures attached to the surface of some cells. They can make cells motile, as in this case. Another type helps to keep air passages in the lungs clean. Each cilium contains two central microtubules (red rings) surrounded by nine pairs of microtubules arranged like a wheel. This is known as the 9+2 arrangement. Pairs of parallel microtubules slide over each other causing the cilia to move in a beating motion. Magnification: unknown.