Cilia (Lat. cilium, an eyelash), minute, hairlike, constantly moving organs on the surface of animal and vegetable tissues. They are abundantly found in all the individuals belonging to the class of microscopical animalcules, the most minute and the lowest in organization of all created beings. Examined by the microscope, these remarkably delicate organs are seen to be in incessant vibratory action, and hence they are usually described as vibratile cilia. In this lowly class of animals the cilia appear to perform three very important functions: 1, their vibration causes a vortex in the water, by means of which particles of food in the phytivorous species, and smaller animalcules in the carnivorous tribes, are brought unresisting to the mouth; thus are they the fruitful agents for procuring food; 2, they constitute the sole organs of locomotion; and 3, they are the respiratory organs. In consequence of the general minuteness of animalcules, it is very difficult to discern the precise kind of motion incidental to the vibratile cilia; the superior size, however, of the members of the class rotifera has left nothing to be regretted on this account.

If the water containing specimens of the common wheel animalcule (rotifer vulgaris) be slightly poisoned, sufficiently to interfere with their respiration, the cilia will be seen to move so very slowly that their precise action may be readily perceived. It will then be seen that the down stroke is a very rapid one - so rapid that, in a healthy vigorous state, it cannot be seen, the up strokes being alone visible; and placed on a rounded tubercle, as they are in R. vulgaris, their combined action gives the appearance of a wheel revolving, and hence the common name of the class; the up stroke is singularly slow. Striking the arm down quickly, and drawing it back slowly, is the best approximation to the true action of the vibratile cilia. - The excellence of modern microscopes, and the improved methods of conducting the examination of difficult objects, have thrown a new light on this interesting subject. We now know that these remarkable organs are not restricted to the animal kingdom, but are extensively developed in the lower plants, where they appear to perform two of the functions witnessed in the animalcules, viz., locomotion and respiration. They are extensively developed in the common but beautiful fresh-water alga, rohox glooata, in all the species of conferva, closteria, etc.

In all theso locomotive plants, by the application and careful management of the achromatic illuminator, the cilia while the plants are in motion become distinctly visible, but they are no longer to be seen when the plant is at rest. - Passing from the lowest to the higher grades of animal life, even among vertebrates vibratile cilia are constantly found, but differing in their mode of arrangement. In animalcules these delicate hair-like processes appear to be attached directly to the tissues of which they form a part, but in mollusks we find the unmistakable evidence of the development of epithelium as the protecting medium of certain organs. Thus the oyster, mussel (mytilus edulis), and all the bivalves, whether of the fresh waters or marine, have their respiratory organs, the gills, covered frith epithelium, and in this particular situation it is always ciliated. So far as is at present known, these organs are constantly to be found in connection with the respiratory function; but in animals as high as the vertebrates, especially the mammalia and man, they are found associated with other functions.

The epithelium which coats the base of the frog's tongue is ciliated, and if removed swims freely through a film of water by this agency; in the batrachians, however, it is highly probable that the tongue is an important organ, adjunctive to respiration, and to these organs vibratile cilia belong through all the classes from animalcules up to man. In the warmblooded mammalia and in man, these organs are found in connection with the epithelium covering the mucous membrane of the trachea, and throughout the distribution of the bronchi; so far they are respiratory. They are also found on the epithelium of the ear, the nose, and of the Fallopian tubes. The most remarkable part of their history remains to be told: they have been seen in active vibration many hours after death in cats, dogs, rabbits, pigeons, etc. In man there has been no diminished action apparent GO hours after death, and in a tortoise they have been observed in rapid motion three weeks after death, when all the rest of the body was in a putrid condition.

It must be quite apparent, therefore, that they possess and maintain motion altogether irrespective of vitality, for they continue to move energetically when all around them is not only dead, but decomposing; and from this latter fact it should appear that their true place is among the vegetative organs of the body. The exact phenomena of their motion are yet to be discovered. (See Animalcules.)