Tiny hair-like projections on the surface of some cells, which are thus called 'ciliated'. Coordinated beating of the cilia moves mucus and mucus-like substances (such as the cumulus mass) on the surface of ciliated cells in the direction of the cilial beat. An individual cilium has the same basic sub-microscopic structure as the tail (the flagellum) of a sperm cell (a spermatozoon). See also immotile cilia syndrome. Plural: cilia.