GLIMPSES OF NATURE. 



sponge. It is this ciliary movement which, acting like so 

 many oars, drives the animalcule hither and thither by 

 rotary movement. But, all the same, it is not the wheels 

 themselves which move. This is a mistake in observa- 

 tion commonly made by young observers. They fancy 

 the wheels themselves rotate ; whereas this appearance 



Fig. 27. Rotifer : Male. 



Fig. 28. Rotifer : Female. 



a, The ciliated wheel-discs ; b, the jaws ; c, the stomach ; d, water-vessels ; 

 e, the contractile water-sac ; f, the pincer-like tail ; g, the " eye." 



is produced by the cilia, each bending rapidly in its 

 turn, and so quickly straightening and bending again, 

 that the wheel appears itself to be in action. The 

 wheels, as Leeuwenhoek tells us, can be withdrawn 

 or protruded from the body, but it is their cilia which 

 alone are active. The optical illusion here is similar to 

 hat whereby, when the stalks of wheat or corn bend 



