CILIUM

plural Cilia, short eyelashlike filament that is numerous on tissue cells of most animals and provides the means for locomotion of protozoans of the phylum Ciliophora. Cilia may be fused in longitudinal rows to form membranes, in shorter transverse rows to form membranelles, or in tufts to form cirri. Capable of beating in unison, cilia move mammalian ova through oviducts, generate water currents to carry food and oxygen past the gills of clams, carry food through the digestive systems of snails, circulate cerebrospinal fluid of animals, and clean debris from the respiratory systems of mammals. In modified form, cilia trigger the discharge of stinging devices in jellyfish and give rise to the light-sensitive rods of the mammalian retina. A cilium, like a flagellum (q.v.), is composed of a central core (the axoneme), which contains two central fibres that are surrounded by an outer ring of nine double fibres. The fibres are covered by a membrane continuous with the cell membrane; movement is controlled by the basal body (or kinetostome) that is located just inside the cell surface at the base of the cilium. Beneath the surface of some cells there is a network of fibrous rootlets or microtubular bundles that may provide support for the epithelium or coordinate ciliary beating in protozoans.