(1125). The whole of the ciliary movements are so evidently under the control of the animal as to leave not the slightest doubt in the mind of the observer upon this point. The whole fringe of cilia may be instantly set in motion and as instantaneously stopped, or their action regulated to every degree of rapidity. Sometimes one or two only of the waves are seen continuing their action, whilst the remainder are at rest; or isolated cilia may be observed slowly bending and unbending themselves, while the others are quiescent. It is by the constant succession of these movements that the eye is seduced to follow the waves which they seem to produce, and thus the apparent rotation of the wheels is easily understood.

* Phil. Trans, for 1837.

(1126). M. Dujardin's explanation of this phenomenon is based upon the fact, that if equal and parallel lines placed at equal distances from each other become bent at regular intervals, so as to overlap the neighbouring lines, they produce dark intersections somewhat resembling the teeth of a saw, instead of a uniform surface (fig. 226.) In this manner the vibratile cilia, being arranged parallel to each other and separated by similar interspaces, would equally intercept the light, so that none would appear more conspicuous than others; but if, in consequence of a general movement propagated along such a row of cilia, some of them, by being momentarily bent down, are placed in juxtaposition with the neighbouring cilia, the light being more intercepted, a darker or more obscure line will be the consequence. It is easy, then, to conceive that, when all the cilia thus bend themselves in regular succession, numerous intersections of this kind will occur, apparently progressively advancing in the direction of the propagation of the movement; consequently, if each of these intersections whilst in motion preserves the same form, as being formed by the same number of equal lines the inclination of which is similar as respects each other, it will give to the eye the appearance of a solid body of a definite shape, such as the teeth of a saw, or of a wheel in uniform movement.

In this way it is easy to understand how the circular rows of cilia in the Rotifera produce the appearance of a dentated wheel in motion*.

Fig. 226.

* To render intelligible the production of this wheel-like appearance by ciliary movement, we annex M. Dujardin's figure representing the position of a row of cilia at a given moment. In this, it is to be supposed that the straight cilia, which are parallel and equidistant from each other, are susceptible of successive oscillations, like the cilium a b, the first of the series, - each capable of describing by a uniform movement the angle bac, of which the apex is at the point of attachment, by changing its position from the perpendicular, a b, till it attains the position a c, and towards the orifice of the shell, it will, of course, push before it the wheels, so as to evert the tegumentary membrane connecting them with the shell, by unrolling it like the finger of a glove, and thus causing the rotatory organs to protrude at the pleasure of the animal.

(1127). Such being, as we conceive, the nature of the ciliary motion, we will proceed to examine the uses to which it is made subservient in the class of animals under consideration. A very slight examination of one of these creatures under the microscope will show that the cilia answer a double purpose: if the Rotifer fixes itself to some stationaiy object by means of the anal forceps, the ciliary action, by producing currents in the water all directed towards the oral orifice, ensures a copious supply of food, by hurrying to the mouth whatever minute aliment may be brought within the range of the vortex thus caused; or, on the other hand, if the animal disengages itself from the substance to which it held by its curious anchor, the wheels, acting upon the principle of the paddles of a steamboat, carry it rapidly along with an equable and gliding movement.

(1128). The whole ciliary apparatus, when not in use, is retracted within the orifice of the shell, and lodged in a kind of sheath formed for it by the inversion of the tegumentary membrane. The muscular fasciculi by which this is effected are very conspicuous; they arise from the lining membrane of the shell, and run in distinct fasciculi in a longitudinal direction, to be inserted into the lobules whereon the cilia are arranged (fig. 230, m m).

(1129). But, besides these retractor muscles, other fasciculi of muscular fibres are distinctly seen to run transversely, crossing the former at right angles: these are, most probably, the agents provided for the extrusion of the wheel-like apparatus; for, arising, as they do, from the inner membrane of the hard integument, they will, by their contraction, compress the fluid in which the viscera float, and, forcing it outward then returning with the same rapidity of motion to its first condition, A b, repeating continually similar movements in both directions. Now, as the other cilia of the series only commence this movement one after the other, each being in advance of the preceding one on the left hand by a fourteenth part of the space occupied by the entire wave, and the same distance from that which succeeds it on the right hand, at every fourteenth interval the cilia present themselves in the same state of flexure, and a row of cilia in motion presents, for the instant, the appearance represented in the figure, in which, at spaces of from fourteen to fifteen cilia, there is a shaded intersection, which advances with a uniform movement from left to right as each cilium successively assumes the position of that which follows it on the right-hand side.

Suppose, now, the duration of each oscillation divided into fourteen instants, a given cilium will occupy successively the positions A b, or a o, a n, A m, A I, a k, a i, xh, ac, in the space b a c, during the first half of the oscillation, the movement taking place from left to right. The other positions during the second half of the oscillation, the movement being from right to left, are, A g, a f, a e, Ad, Ac, a b, a a, - the position a a' being the same as A b, or A o, constituting the limit of the second half of that oscillation and the commencement of a new one.