The Algae 



455 



the filamentous algae as a source of food for small fish and minute 

 water animals. 



G. M. Smith 



Fig. 276. Plankton algae: A, Chlamydomonas ; B, Paniorina; C, Ccelastrum; D, Pedi- 

 aslrum; E, Pleodorina ; F, GlceotcBnium ; G, Selcnastrwn , 

 J, Sorastrum; K, Crucigenia; and Z, Nephrocytium. 



H, Scenedesmus ; I, Trochiscia; 



The importance of the algae. Both green and blue-green algae 

 are generally considered a nuisance in ponds and streams, and 

 they are commonly thought to have no economic importance; 

 but the fact is that these pond scums are the primary food supply 

 of all the water animals. They bear the same relation to aquatic 

 animal life that the herbaceous plants bear to animal life on the 

 land. Nearly all the water animals, from minute insects and 

 crustaceans to the largest fishes, ultimately depend upon them 

 for their supply of food. For, like the land plants, these small 

 water plants manufacture food, and the animals that live in the 

 water must feed either on them or on other animals that get their 

 living from the plants. Without the algae the fish would soon 

 disappear from our waters, because their primary food supply 

 would be cut off. A decrease in the number of fish in a lake 

 frequently follows the draining of its swampy margins, for the 



