262 EVENINGS AT THE MICROSCOPE 



continues to whirl round with a rapidity which increases 

 as it approaches the centre of rotation. 



But the Brachionus suddenly lets go its foothold, and 

 a surprising change takes place. No more currents are 

 made in. the water, but the animal itself glides swiftly 

 away head foremost with an even course, revolving on 

 its axis as it goes. What is the immediate cause of its 

 movement? The ciliary action which before produced 

 vertical currents. 



In order to explain this, let me suggest to yon a 

 homely comparison. Suppose you see a boat on a still 

 lake, and in it a man pulling a pair of oars. He pulls 

 vigorously, but the boat does not move an inch, and you 

 perceive that she is fast moored; a rope holds her to a 

 post on the bank. But does the man's rowing produce 

 no effect? Oh, yes; the successive strokes of his oars 

 upon the water have communicated motion to the fluid, 

 and a strong current is made on each side of the boat, in 

 a direction opposite to that in which he strives to row 

 her forward, the force of which is felt to a distance pro- 

 portionate to the vigor and continuance of his pulling. 

 The reason of this is that the boat is fixed, and all the 

 force of the impact is spent on the water. 



But now another man approaches the post and unties 

 the rope. Instantly the boat glides ahead, and continues 

 to do so, urged by the repeated strokes of the oars, whose 

 effect on the water in making currents is now slight and 

 imperceptible. The reason of this is that the water is 

 now the fixed body (or nearly such), and the force of the 

 impact is mainly spent on the movable boat. 



The Brachionus is the boat, its cilia are the oars, and 

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



