170 THE MECHANICAL FUNCTIONS. 



mass of soft gelly, highly irritable and contractile 

 in every part ; at one time wholly shrunk into a 

 ball, at another stretched out into a lengthened 

 ribbon : and again, at another moment, perhaps, we 

 find it doubled upon itself like a leech. If we 



78 79 



watch its motions for any time, we see some parts 

 shooting out, as if suddenly 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 metamorphoses as rapid as they are irregular 

 and capricious. 



The Volvox glohator (Fig. 79) is found in prodi- 

 gious numbers at the surface of many stagnant 

 pools. Its figure is perfectly spherical ; and its 

 movements consist in a continual and rapid rota- 

 tion round its axis, frequently remaining all the 

 while in the same spot. Another species, the Vol- 

 vox conjiictor, moves by turning alternately to the 

 right and to the left.* 



The progressive movements of infusory animal- 

 cules are of two kinds, the one consisting in a 



* The later observations of Ehrenberg have led him to the conclu- 

 sion that the globular mass, hitherto considered as a single animal- 

 cule, is in reality an aggregate of a multitude of more minute 

 individuals constituting a particular species of monad, and which 

 are retained within a vesicular envelope. This enveloping membrane, 



