2. The Respiratory Systeme—Cilia have been detected in the lining membrane of the air-passages of reptiles, birds, and Mammalia, in the gills of the larvae of the and on those of the Mollusca and .Aneelida. Those on the external surface of the Infusoria, Polypes, and Medusce must also be regarded as belonging to the respiratory system.
3. Alimentary System.—They are found in the mouth, throat, and gullet of Reptiles, in the entire alimentary canal of Mollusea, in the stomach of the Asteries, &c.
4. Reproductive System.—Ciliary movements have been observed in the mucous membrane of the Fallopian tubes, in the uterus and vagina of Mannactler, and in the oviduct of Birds and Reptiles. A peculiar ciliary movement has been observed in the embryo of many animals. This movement occurs while the embryo is in the ovum, the Cilia producing a current in a certain direction along its surface, or causing the whole embryo to move, in an opposite direction. In many instances when the embryo has escaped the egg it moves about by means of Cilia in the same way as occurs in the naked gernmules of the Sponge.
There can be little doubt that the functions performed by the Cilia in these various parts of the body of animals are important ; at the same time their absence in a great number of cases, in organs which perform the same functions as those which possess them, must lead to some hesitation before pronouncing a decided opinion with regard -to their use. Where they are situated on the external surface of the bodies of animals they seem to be the active organs of movement. Where respiration is carried on by means of the external surface, as in the Infusoria and Polypifera, the Cilia assist this process by removing the used water and bringing fresh currents to the surface containing the matter to be oxygenated. They may undoubtedly perform the same office when seated on internal respiratory mem branes. On the surface of the reprodnetiva organs of the higher animals they may also assist in bringing the unimpregnated ovum in contact with the fertilising cells of the male fluid. The movement in the embryo has probably the same object in view as that on the respiratory membranes, the bringing the surface in contact with currents of oxygenated water.
In cbming to the conclusion that the motions of fluids on the surfaces of membranes are produced by Cilia where these organs exist, Dr. Sharpey observes, "The currents cease when the motion of the cilia stops, they are strong and rapid when it is brisk, and feeble when it languishes; and though there are modifying circumstances or perhaps exceptions, yet in general the magnitude and velocity of the current seem to be proportioned to the size and activity of the cilia. It is true that while doubts remained as to the existence of
cilia in several well-marked instances where the water unequivocally received its motion from the surface over which it flowed, and inde pendently of any visible contractions of the animal tissue, there was always considerable room to doubt, whether, even in the cases where cilia were manifest, the effect of these organs was wholly mechanical, and whether the motion of the water was not rather due to some peculiar impulsive power in the tissue differing from mechanical action. Bnt more extended observation has almost wholly removed these exceptions, while it has considerably increased the number of conforming instancee, insomuch that there seems at present no necessity for having recourse to any other explanation of the motion of the fluids than that it is produced by the action of the cilia, and that their action is the result of muscular contractility, a known property of animal tissues." There are however some remarkable exceptional cases. Currents are observed in the Sponge, in the stem and branches of the Sertularifr, but no Cilia. There are also a number of remarkable cases of the movements of fluids In cells In the vegetable kingdom, which cannot be ascribed to the existence of Cilia, as those seen in the cells of Chara, Valisneria, the hairs of Tradeacantia, &c.
When first discovered the Cilia were supposed to be confined to the organs of animal bodies. In 1843 Meyer and Thuret announced that they bad discovered Cilia on the spores of several species of Coesferree. They were subsequently discovered on the surface of several species of unicellular plants, as also on the spermatozoids, which occur in the Ferns and many of the lower forms of Cryptogamia.
For further information consult the article 'Cilia,' by Dr. Sharpey, in the Cyclopmdia of Anatomy and Physiology,' to which we are much indebted in drawing up this article ; also the paper of Purkinje and Valentin, entitled Commentatio Physiologica de Phaenoineno Motus Vibratorii continui,' &c., translated in the 'Dublin Journal of Medical and Chemical Science' for May, 1835, and in 'Edinburgh New Philosophical Journal,' vol. xis. ; also ' On Unicellular Plants and Animals, 'Microscopical Journal,' vol. i.