As the cilia have the appearance of moving in waves round the disc (fia-. 64), each wave may be thus analysed. Fbrom a dozen to twenty cilia are concerned in the production of each apparent wave, the highest point of which is formed by a cilium extended to its full length, and the lowest point between every two waves by one folded down com pletely upon itself, the intervening space being completed by others in every degree of extension, so as to present something of the outline of a cone.
As, how-ever, the persistence of each cilium in any one of these positions is only of the shortest possible duration, and each takes up in regular succession the action of the ad joining one, so that cilium, which by being completely folded up formed the lowest be tween any two waves, now in its turn, by its complete extension, forms the highest point of a wave ; and thus, while the cilia are alter nately bending and unbending themselves each in regular succession after the other, the waves only travel onwards, whilst the cilia never change their position in this direction, having, in fact, no lateral motion. When the waves travel very rapidly, they appear smooth on one side and fringed on the other. The whole of the ciliary motions are so evidently under the control of the animal, as to leave no doubt on this point. The whole fringe of cilia may be instantly set in motion, and as instantaneously stopped, and their action regu lated to every degree of rapidity. Sometimes one or two only of the waves are seen conti nuing their action, while the remainder are at rest, or isolated cilia may be observed slowly bending and unbending themselves, or pro jectino. entirely at rest. The body is generally pointebd towards one extremity of the oval, and at this part may be observed a bundle of cilia longer than the rest, and moving very rapidly. Their vibrations were in several instances counted very evenly at 230-a minute, continuing in action when all the others were folded up. These Dr. Farm thinks may be respiratory whilst the others are chiefly loco motive. Dr. Farre thinks there can be little doubt that this explanation of the action of the cilia in the gemmules is applicable like wise to those of the tentacula of the adult animal, and not only in the Hylodaetylus, but throughout the class generally; for he ob served that the tentacular cilia are infinitely more numerous when at rest than they appear to be when in motion, and also that they vi brate, not in the direction of the plane of' the arms, but at right angles to it, and with the same hook-like form as in the gemmules. In
this way the apparent travelling of the cilia up one side of the arm and down the other, as the eye is seduced to follow the waves which they seem to produce, is at once ex plained.
It would be impossible to account for the variety of motions which the gemmules are capable of executing, were it not obvious how complete is their control over the action of the cilia, which are their sole locomotive organs. They generally swim with the convex part forwards, and with the greatest rapidity. Sometimes they simply rotate upon their axis or they tumble over and over, or, selecting a fixed point, they whirl round it in rapid cir cles, carrying every loose particle after them. Others creep along the bottom of the watch glass upon one end with a waddling gait; but generally, after a few hours, all motion ceases, and they are found to have attached themselves to the bottom of the glass. At the expiration of forty-eight hours the rudi ments of a cell were observed extending be yond the margin of the body, but at this stage the animals invariably perished, and Dr. Farre had no opportunity of witnessing their further metamorphosis.* Reproduction by ova. — In the genus Pedicel lina Van Beneden discovered in most of the individuals he examined, situated immediately above the stomach, some rounded opaque cor puscles of a lactescent appearance (fig. 65, k), which seem to be attached to that viscus ; this he considers to be the ovary, containing ova in various stages of development. In the same situation he perceived an organ that he looks upon as being the testis his opinion being founded on the fact that When a mature specimen of the animal is placed between two plates of glass, and gently compressed so as to rupture its parietes and cause the escape of the viscera, spermatozoa are discoverable in the one and ova in the other. The sper matozoa exhibit considerable vivacity in their movements, have a disc-like body and a cau dal filament, and are proportionately of large size • around them !nay be seen multitudes of iree cellules without caudal appendages, which are apparently young sperrnatozoa.