Cilia are minute, hair-like processes on the surface of protozoans or on metazoan cells. Each cilium contains a peripheral circle of nine doubled microtubules arranged around two single microtubules. Each microtubule is composed of tubulin proteins. The mechanism of ciliary movement is not completely understood. The fluxes of $\mathrm{Ca}^{2+}$ across the membrane are not responsible for controlling the organised beating of cilia.