400 GENERAL HISTORY OF THE INFUSORIA. 



ample may be adduced as that of the least complicated rotary organ among 

 Rotifera. 



It is in these aberrant forms alone that the ciliated apparatus can be strictly 

 called " polytrochous f in them, also, the wheel-like motion is completely 

 absent. This peculiar motion, on the contrary, is most e\ident where the 

 wreath is a simple circle, as in Conochilus and Actinurus, or where, as in 

 Rotifer and Philodina, it is peculiarly involuted, although continuous. Where, 

 on the contrary, it is interrupted by a notch at any point, or is sinuated, or 

 complicated by supplementary processes, as in Hydatina (XL. 1), Diglenaj 

 many Notommata (XXXVII. 29, 32), Synchceta, &c., the illusion of com- 

 plete revolutions vanishes. 



Formerly, the belief existed that an actual whiiiing of the ciliated cephahc 

 appendages took place, and that the little animals moved along, by the aid of 

 these wheels, after the manner of a steamer with its paddle-wheels. Such 

 an opinion is no longer entertained ; and various explanations of the apparent 

 rotary motion are now offered in its stead. Dutrochet attributed it to the 

 undulation of a delicate membrane fi^inging the head of the Rotatoria. 

 Faraday explains it by supposing the distinct cilia to become visible by 

 slowly returning to an erect state, after having previously been suddenly 

 bent. Ehrenberg assumed the existence of four muscles at the base of 

 each eilium, — each muscle acting in its own dii'eetion, and so producing a 

 revolution around the fixed point of attachment or base of the eilium. In 

 this way, each eilium would be alternately nearer to or more remote from 

 the eye, and more or less visible. 



Another explanation has been offered by Dujardin. He says — *' The vibra- 

 tile cilia being arranged parallel and at equal distances, will equally refi-act or 

 intercept the light, and none will be more visible than the rest ; but if, by a 

 movement propagated along the row of ciha, some, momentarily inclined, are 

 brought into juxtaposition with adjoining ciUa, the light will be more inter- 

 cepted, and a band more or less dark will be the result. It can be imagined, 

 therefore, that if the cilia come to be inclined one after another, a series of 

 juxtapositions, or of apparent intersections will be produced in the direction 

 of the general movement. Further, if each of the intersections preserve the 

 same form, as if produced by a number of equal lines, and are equally in- 

 clined to each other, an appearance of a solid body of a definite foim, like the 

 teeth of a saw or the spokes of a wheel, moving uniformly, presents itself to 

 the eye." 



The action of the trochal disk is under the control of the animal. The 

 cihary movement can be arrested at will or exercised ^ith varying rapidity ; 

 or the whole organ may be retracted, partially or entirely, within the body. 

 When completely withdrawn, the ciliary wreath can frequently be detected 

 at the fore part of the animal, oftentimes deep within the trunk, and gene- 

 rally in the form of a striated cyhnder at the bottom of a funnel-like canal. 

 In complete retraction the anterior extremity of the body is involuted, or 

 doubled inwards, and supports, as it were, the ciliated wreath within, whilst 

 the contractility of the integument at the margin closes the entrance pretty 

 accurately, giving a more or less conical outline to the fore part of the ani- 

 mal (XXXYII. 19 ; XXXIX. 17). In complete retraction of the trochal 

 disk, the antenna-like processes which may be seated on it are also with- 

 drawn ; but at other times, when the inversion is incomplete, these processes 

 continue to project from the head, and in the process of evolution are always 

 the first to appear, as if intended to test the safety of unfolding the delicate 

 ciliary wreath. 



The inversion of the ciliary apparatus and appendages is effected by strong 



