DESMIDS, DIATOMS, AND FRESH-WATER ALG^. 99 

 3. V6LV0X. 



A small sphere continually in movement, rolling 

 through the water in a graceful manner, its surface 

 studded with green points. Undera low power itseems 

 like a hollow globe, and the cause of the motion 

 is a mystery; but the -^-inch objective, when the V61- 

 vox moves slowly or rests, shows that each of thfe 

 green points that together stud the surface, bears two. 

 fine cilia or little hairs continually vibrating and lash- 

 ing the water. It is from their vibrations that the. 

 Volvox receives its rolling- motion. The deep-green 

 balls often seen within the larger globe are young 

 plants in different stages of development. When ma- 

 tured the mother-globe is ruptured, and the young 

 plants then float out and roll away through the water, 

 revolving there as they are often seen to do even be- 

 fore leaving the parent, and there leading an independ- 

 ent existence. 



The water in some localities is, in June, so filled 

 with these rolling globes that it is colored green by 

 them, and when the collecting-bottle is held against 

 the light they become visible to a sharp eye as small 

 pale-green spheres. The diameter of a full grown 

 plant is about one-fiftieth of an inch. V. globdtor. 



They occur everywhere but are not common any- 

 where. In those localities where they usually exist, 

 they seem to have periods of profusion and of de- 

 ficiency. In a small and shallow but permanent pond 

 where several years ago I found them literally and 

 actually by the million, I have never since been able 

 to discover a single specimen. This is not an instance 

 of periodic deficiency, but of complete absence. Still, 

 the rule seems to hold good for this country. In Eng- 



