EXPLANATION OF WHEEL-LIKE MOVEMENT. 



435 



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. 



(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 neigh- 

 bouring lines, they produce dark intersections somewhat resembling the 

 teeth of a saw, instead of a uniform surface (fig. 226). In this manner 



Fig. 226. 



onmll i kyf edciaorunliiJty/ edcittonrolJaiJi>y/edci a, 



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 Eotifera produce the 

 appearance of a dentated wheel in motion*. 



* 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 B A c, of which the apex is at the point of attachment, bj 

 changing its position from the perpendicular, A B, till it attains the position A c, and 



2r2 



