Cilia

Cilia (Lat., 'eyelashes'), hair-like lashes borne by cells. They are mobile modifications of the living matter of the cell, and exhibit alternate bending and straightening. Their occurrence is very wide—e.g. on the active stages of many unicellular plants; in great perfection on the ciliated Infusoria; on the free-swimming embryos of sponges, Cœlenterates, worms, echinoderms, and molluscs; on the outer surface of many lower animals such as simple worms; on the lining of the alimentary cavity, and in most of the tubular organs of Invertebrates; and more restrictedly, though not less markedly, in some regions (such as trachea) of higher forms. They are absent throughout the Arthropods (with one possible exception), a fact probably to be associated with the predominance of Clitin (q.v.). In many cases, normal, pathological, and artificial cilia may sink down into less motile amoeboid processes. A single lash with an undulating movement is distinguished as a flagellum; and Ray Lankester emphasises the useful distinction between a flagellum such as that of a bacterium, or the tail of a spermatozoon, which acts 'like a tadpole's tail' and drives the cell before it, and the motive process of a flagellate Infusorian which carries the cell behind it. See CELL.