WHEEL^BEARERS 263 



hold, the whole force of the ciliary stroke is spent on the 

 water, and currents are the result; but as soon as this 

 hold is broken, the force acts on the animal (= boat), 

 which is thus rowed rapidly forward. 



The use of the cilia in this latter case is obvious. 

 They enable the little animal to rove about at its way- 

 ward will; and doubtless motion is as pleasant and neces- 

 sary to it as to the fish in the sea or the bird in the air. 

 But what is the object of their vigorous rotation, when 

 the animal chooses to maintain a firm hold with its foot? 

 What is the use of rowing a boat, if you do not choose 

 to let go the painter? 



To solve this enigma, let us search up our little 

 Brachion once more; he will not roam long before he 

 settles soberly again. Yes, here I have him moored. 

 Now, mark carefully the vortices, which are so vigor- 

 ously circling around the animal's front, and you will 

 perceive that the movement is not a strictly circular one, 

 but that each whirlpool has an outlet close to the cilia; 

 for the accumulated and condensed particles of pigment, 

 after many rotations, pass off in a united stream between 

 the two crowns, and go away horizontally in a line from 

 the ventral side of the front. That is to say, each vortex 

 pours off its accumulation at a point on the inner side of 

 the ciliary circle, and the two streams, uniting, pass off 

 from the lip of the shell, to be drawn in again, however, 

 by and by, when the centrifugal force is exhausted. 



Now this stream passes immediately over the mouth, 

 which is an opening in the flesh of the front, forming a 

 deep cleft on the ventral side, the lips of which, as also 

 the whole interior of the tube of which it is the orifice, 

 are richly covered with cilia. A certain portion of the 



