230 EOTIFERA. 



spin round, and yet preserve their connexion with 

 the animal, they could not conceive. Better instru- 

 ments, however, and closer observation, have solved 

 the difficulty. Instead of being real revolutions of 

 wheels, it is now clearly established that the ap- 

 parent rotations are merely an optical illusion, 

 similar to that by which, when the tide is rolling in 



FlG. 176. SKELETON WHEEL BEARER. 



upon the beach, the waves appear to the eye to move 

 rapidly forward, while, as is well known, they merely 

 rise and fall in constant succession. The true ex- 

 planation of the once mysterious phenomenon is as 

 follows : Examined under high powers, the cilia 

 have the appearance of moving in waves, in the 

 production of which from a dozen to twenty cilia 

 are concerned, the highest point of each wave being 

 formed by a ciliuin extended to its full length, while 

 the lowest point between every two waves is occu- 

 pied by one folded down upon itself, the intervening 

 space being filled by others in every intermediate 

 degree of extension. As the continuance of each 

 cilium in any one of these positions is of the shortest 

 possible duration, and each takes up in regular sue- 



