WHEEL-BEARERS. 



197 



ceaseless regularity cf a complex piece of machinery. The 

 early observers supposed that the wheels really did rotate, 

 though they found it impossible to imagine how any part 

 of a living animal could do so. And no beholder can 

 wonder at their supposition, for with all our knowledge of 

 how the phenomena is produced, it is almost impossible, 

 while looking at it, to persuade ourselves that there is not 

 an actual rotation of the parts. The explanation of the 

 appearance has been already given : * it is a rotatory pro- 

 gression of waves caused by the rhythmical bending and 

 straightening of cilia, in themselves stationary. The effect 

 of this movement is to produce circular vortices in the 

 surrounding water, which are made conspicuously mani- 

 fest when any minute particles of solid matter are held 

 in suspension ; as when a little carmine or indigo is 

 mixed with the water. In this case, the coloured atoms 

 are caught, and involved in the rapid rotation, passing off 

 after many gyrations, in a continuous thick stream from 

 a point between the two wheels, like the dense black 

 cloud of smoke that streams away behind a steamer's 

 f aim el. 



The objects of the ciliary rotation are principally two. 

 When the animal is stationary, adhering to foreign sub- 

 stances by the extremity of its foot, the vortices bring 

 ever fresh particles of water to be respired, and whatever 

 atoms fit for food may be floating in the vicinity; the 

 whirls leading to a central point, at which is placed the 

 entrance to the stomach. On the other hand, when the 

 animal relinquishes its foot-hold and commits itself to the 

 '^pen water, the cilia act like a steamer's paddle-wheels, 



* Sec page 4, supra. 



