CILIA,T (in anatomy, Fr. Ci/s; Germ. Wimperhaare.) This term is used to desig nate a peculiar sort of moving organs, re sembling small hairs, which are visible with the microscope in many animals. These organs are found on parts of the body which are habitually in contact with water or other more or less fluid matters, and produce motion in these fluids, impelling them along the surface of the parts. The currents or other motions thus produced serve various purposes in the economy of the animals in which they occur. In other circumstances the cilia serve as organs of locomotion, some aquatic animals propelling themselves through the water by their means.
Cilia have now been ascertained to exist in a great many invertebrated and in all brated animals, except Fishes ;§ having been very recently discovered by Purkinje and lentin on the respiratory and uterine mucous membranes of Mammalia, Birds, and Reptiles. The terms " vibratory motion" and " ciliary motion" have been employed to express the appearance produced by the moving cilia ; the latter is here preferred, but it is used to express the whole phenomenon as well as the mere motion of the cilia.
A considerable space has been allotted to the present article, more perhaps than its re lative importance may seem to demand, chiefly for the reason that, with one exception, no attempt has been hitherto made to collect and describe under appropriate heads, the facts known on the subject. The exception alluded to is a work by Purkinje and Valentin,* which appeared while this article was in progress, and which contains not only an account of their own discovery, but a history of all preceding obser vations. But the manner of treating the sub ject in the work alluded to is for the most part so different from that which is here followed, that its publication has not seemed to vvarrant any material abridgement of the following article, which, on the contrary, it has increased by affording much new and important matter, as will be acknowledged in its proper place. Another ground on which indulgence may be claimed for details which are, perhaps, greater than may seem commensurate with the impor tance of the subject, is that many of the facts are here described for the first time, and it was felt desirable to state them in their full extent, which could not be done intelligibly without considerable length of description.
The article is divided into two parts ; the first comprehends the particular facts, or an account of the phenomena as they occur in the different tribes of animals considered in Zoo logical order, with the history of their dis covery; the second part consists of general deductions from the first, and also treats of the structure and mode of action of the cilia in general. This method has been adopted as appearing on the whole best suited to the pre sent state of knowledge on the subject.
J. AI Alea Al • 1. Cilia exist very extensively in the different tribes of Infusory Animalcules ; indeed they constitute the principal organs of motion in these small animals. When a drop of water containing Infusoria is brought under the microscope, these creatures are seen swim ming rapidly through 'it in various directions; and as they move along, small particles of foreign matter which happen to lie near their path are thrown into agitation, obviously in dicating the existence of currents in the neigh bouring water. When the animals remain steady in one place, these currents become much more distinct, setting in particular di rections, and causing the small particles to run in a stream to and from the animal. If the magnifying power be sufficiently strong, small transparent filaments will be distinguished, projecting from the surface of the animalcules and moving in a very rapid manner. These are the cilia ; they serve like fins or paddles to carry on the animal in its progression through the water, and when it is stationary, they impel the water in a current along the surface, which is beset with them. They may be often most distinctly seen when their motion be comes languid or impeded, as is the case when the water round the animal is diminished by evaporation to such a degree as not to afford scope for their full and rapid play.