(L. cilium, an eye-lash), in Anatomy, the name given to minute hair-like organs found in various animals on the surfaces of certain tissues, and requiring the aid of lenses to discover their form. They were first observed on the bodies of infusorial animals, and appear to be their organs of motion. They are most conspicuous on the Ciliograde Medusaria, and on the gills of various mollusca. On the external surface of infusoria they were first noticed by Leeuwenhoek; but they have since been found not only in the invertebrata and cold-blooded vertebrata, but Parkinje and Valentin have demonstrated cilia to exist on the moist tissues of the higher types of animals—and they not only exist on the external surfaces of animals inhabiting water, but also in the alimentary system, the organs of respiration, and those of reproduction. Their office seems to be to produce motion in the fluids on such surfaces. Their form is generally that of elongated cones, but sometimes they are flattened filaments varying in length even in the same animal, from $\frac{1}{3}$th to $\frac{1}{8}$th of an inch. They are usually colourless and transparent, sometimes slightly coloured, and exist in a state of perpetual motion, but which is seldom rapid. See Zoology.