WHEEL-BEARERS. 
197 
ceaseless regularity of 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 bo. And no beholdor 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 
funnel. 
The objects of the ciliary rotation arc principally two. 
When the animal is stationary, adhering to foi-eign 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 
open water, the cilia act like a steamer’s paddle-wheels, 
* Sec page 4, supra. 
