120 ROTIFERA. 



conceive what could be the nature of the connection between such 

 appendages and the body of the animal. The apparent rotation 

 has, however, been long proved to be an optical delusion, and to 

 be produced by the progressive undulations of the cilia placed in 

 the neighbourhood of the mouth. 



(157.) With respect to the agents employed in producing the 

 ciliary movement in the rotifera, we are as much in ignorance as we 

 are concerning the cause of the same phenomenon in the polygastrica. 

 Ehrenberg describes the cilia as arising from a series of lobes as re- 

 presented in Notommata clavulata (Jig. 51 a); these he regards as 

 being muscular, and capable of producing by their contractions the 

 rapid vibrations of the fibrillse attached to them. We confess, 

 however, that such lobes, even was their existence constant, seem 

 very clumsy instruments for effecting the purpose assigned to them, 

 and it is not easy to conceive how the rapid and consecutive undu- 

 lations to which the appearance of rotation is due can be produced 

 by organs of this description. 



The observations of Dr. Arthur Farre* concerning the ciliary 

 movements visible upon the gemnmles of some of the Bryozoa appear 

 best calculated to throw light upon the nature of the action of these 

 wonderful appendages, and to explain the cause of the apparent 

 rotatory motion of the so-called wheels of the rotifera. The very 

 accurate observer alluded to remarks that under high powers, the 

 cilia have the appearance of moving in waves, in the production of 

 each of which from a dozen to twenty cilia are concerned, the 

 highest point of each wave being formed by a cilium extended to 

 its full length, and the lowest point between every two waves by 

 one folded down completely upon itself, the intervening space be- 

 ing completed by others in every degree of extension, so as to pre- 

 sent something of the outline of a cone. As the persistence of 

 each cilium in any one of these positions is of the shortest possible 

 duration, and each takes up in regular succession the action of the 

 adjoining one, that cilium which, by being completely folded up, 

 formed the lowest point between any two waves, in its turn by its 

 complete extension forms the highest point of a wave ; and thus, 

 while the cilia are alternately bending and unbending themselves, 

 each in regular succession after the other, the waves only tra- 

 vel onward, whilst the cilia never change their position in this di- 

 rection, having, in fact, no lateral motion. 



The whole of the ciliary movements are so evidently under the 

 control of the animal as to leave not the slightest doubt in the 



* Phil. Trans, for 1837. 



