WHEEL-BEAEEES. 265 



and its foot is the rope. As long as this last maintains 

 its 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 forwards. 



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 

 necessary to it as to the fish in the sea or the bird in 

 the aii\ But what is the object of their vigorous rota- 

 tion, 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 Brach- 

 ion once more ; he will not roam long before he settle so- 

 berly again. Yes, here I have him moored. Now, mark 

 carefully the vortices, which are so vigorously circling 

 around the animal's fi'ont, 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 vor- 

 tex 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 ia again, 

 however, by and by, when the centrifugal force is ex- 

 hausted. 



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 iaterior of the tube of which it is the orifice, 



12 



