188 THE ^MECHANICAL FUNCTIONS. 



denly inflated, and branching forth into star-like 

 radiations, or assuming various grotesque shapes, 

 while other parts will, in like manner, be as 

 quickly contracted. Thus the whole figure may, 

 in an instant, be completely changed, by meta- 

 morphoses as rapid as they are irregular and 

 capricious. 



The Volvox globator (Fig. 79) is found in pro- 

 digious numbers at the surface of many stagnant 

 pools. Its figure is perfectly spherical ; and its 

 movements consist in a continual and rapid 

 rotation round its axis, frequently remaining all 

 the while in the same spot. Another species, 

 the Volvox conflictor, moves by turning alter- 

 nately to the right and to the left. 



The progressive movements of infusory ani- 

 malcules are of two kinds, the one consisting in 

 a smooth and equable gliding through the fluid, 

 produced apparently by the vibrations of cilia, 

 which are set on various parts of the body, and 

 often seem to cover the whole surface : the other, 

 more rapid and energetic, when the animalcule 

 darts forward in a particular direction, as if in 

 pursuit of prey, and proceeds by sudden and 

 irregular starts, like a vivacious insect or fish. 

 The voluntary nature of their motions is evident 

 from the dexterity they display in avoiding ob- 

 stacles, while swimming together in myriads in 

 a single drop. 



The great agent in the movements of the 

 animal frame being the muscular fibre, it was 





