CILIA, in Anatomy, small moving organs found on the surface of the tissues of most animals, resembling hairs, and requiring the use of the microscope to be distinctly observed. They are mostly found on tissues which are in contact with water, or which produce fluid secretions. They are constantly in a state of active movement, and impart to the fluid with which they are in contact the same motion. This is called vibratory or ciliary motion. The best time for observing Cilia with the microscope is when their movement begins to slacken. Their figure is generally that of slender conical or some flattened filaments, which are broad at the base or root and gradually taper to the point Their size differs greatly on different parts of the same animal. " The largest I have measured," says Dr. Sharpey, " are those on the point or angle of the branchial in the Bruccinunt undutunt ; they are at least EL3 of an inch long. I have not attempted to determine the exact size of the smallest, but Purkinjo and Valentin state it at 0.000075 of an inch, while they make the largest they have met with only 0.000903 of an inch, which is considerably less than I have found them ; but they had no oppor tunity of examining marine animals, in which generally speaking the largest cilia are met with. In the sea-mussel the darker coloured cilia are about of an inch long, the others considerably less." The substance of the Cilia is for the most part transparent and colour less, in some however a slight colouring may be observed. They assume also various forms, and Ehrenberg has described compound Cilia in the Infusoria. In the Ciliogrado Medium the Cilia consist of rows of broad flattened organs, each of which is made up of several simple filaments joined together by a connecting membrane through out their whole length. In most cases the Cilia are arranged in regular order. On the gills of the mussel they are placed in straight rows ; in many of the Infusoria they are arranged in circles or spiral lines. In some instances they are erect, but in others they are placed at right angles to the surface on which they are seated.
The movement of the Cilia is not very rapid, and may be easily observed with a lens of A inch focus. Their most obvious movement is of a fanning lashing kind, the Cilium being bent in one direction and returning to it again. In addition to this movement Professor Quekett detected another in the Cilia of the gill-rays of the common mussel. This consists of a alight movement of the Cilia on themselves, each Cilium turning on its own axis through the space of a quarter of a circle, with a movement like that of the Conflicting of an oar in rowing. This observation of Mr. Quekett'e is of importance, as it explains how it is that the Cilia are capable of propelling bodies over their points which could not be effected by the first-observed up and down movement When the surface of an organ is examined on which are seated a large number of Cilia, a wave-like motion in the whole is observed, which arises from the regularity with which each Cilium is affected with the movement The Cilia were first observed as present on the external surface of the bodies of infusory animalcules. Leeuwenhock seems to be one of
the earliest observers who describes' the presence of the Cilia in animalcules. In his 'Continuatio Aramorum Nature' he describes in many places the nature' of the Cilia in the common polygnstrie animalcules as well as in the wheel-animalcules. Ile also pointed out the probable use of these organs, for ho says, " Moreover it is necessary that these animals, and in general all such as are fixed and cannot change their place, should be provided with an apparatus for stirring up motion in the water, by which motion they obtain any matters that float in the water for their nourishment and growth, and for covering their bodies." Since the period that Leeuwenhoek wrote they have been observed in almost every species of Infusoria, and seem to be the active organs by means of which these animals move from place to place and carry their food into their stomachs. In the Polypes they are found in great numhera covering the surface of the tentacula by which these animals obtain their food. Although they have not been observed on the full-grown Sponges they have been described by Dr. Grant as existing upon the ova of these animals before they become fixed. Though not abundant in the Acaleplar, they have been seen by Dr. Grant and others in the Berrie pilaus and other Meduscr. Dr. Sharpey has observed them in the various forms of the Echin,odermata, and also in the Annclida. In the Mollusca they are very abundant, and one of the best means of examining these organs is afforded by the common mussel. Till within a recent period it was supposed that Cilia were confined to the Invertebrate classes of animals, or at least the observation of their existence to any extent in the Vertebrata was very limited. One of the earliest obser vations of their presence in Vertebrate animals was by Steinbuck, German anatomist, who found them upon the gills of the Salamander. Within the last few years Purkinje and Valentin have devoted much attention to the subject, and have found that Cilia exist very generally on the moist surfaces of the membranes of all the higher animals. The systems of organs on the surface of which Cilia have been detected are as follows : 1. The Surface of the Body.—In this situation Cilia have been detected in the Infusoria, Polypi, Medusw, Actinia, Echinodermata, and in the larvae of the Batrachian Reptiles.