FIELD BOOK OF PONDS AND STREAMS 



Plant-feeding nymphs of the mayfly Callibcetis (Fig, 158) 

 dart through the open water. Many of the population have 

 taken to the bottom and when a netful of mud is scooped up 

 or a handful drained through one's fingers, its inhabitants be- 

 gin to appear — mayfly nymphs, little red worms (Tubifex) 

 (Fig. 114), and dozens of other pale worms (Nais) (Fig. iii), 

 and midge larvas, each one squirming enough to be finally 

 discovered. They all find food in this mud mainly from 

 the diatoms with which it is saturated. 



Fig. 20. — Typical plants and animals of plank- 

 ton society. A. Plants — diatoms: i, Fragillaria; 2, 

 Asterionella; — desmids: 3, Cosmariuni; 4, Stauras- 

 trum. B. Animals — protozoans: ^, Arcella; — rotifers: 

 6, Notholca; — crustaceans: 7, Cyclops. 



Most small ponds are so shallow that water-lilies and 

 pondweeds grow even in the deepest places. The surface 

 waters of the larger ponds and lakes are occupied by a float- 

 ing population known as plankton, made up of millions of 

 microscopic plants and animals (Fig. 20). The plant cells 

 are mainly diatoms and desmids, so translucent and minute 

 that thousands of them would go unseen in a glass of drink- 

 ing water. There are a few kinds of one-celled animals or 

 protozoans, many rotifers, and swarms of diatom-eating 



16 



