CHAPTER XVI 



THE FRESH-WATER CLAM: A STUDY OF RIVER 



FAUNAS 



The main divisions of tlie animal kingdom originated along 

 the seashore. Fronr the seashore animals have passed to the 

 bottom of the deep sea, over the surface of the seas, and into 

 the estuaries that open into the seas. Certain forms have 

 penetrated farther and farther up the estuaries and have 

 become, eventually, permanent inhabitants of fresh water. 



The habitats offered by fresh water are of several sorts. 

 First there are the flowing streams (Figs. 214, 21.5). Near the 

 mouths of streams the current is slow, while in the upper 

 courses it is rapid. Then there are the permanent bodies of 

 fresh water, such as ponds and lakes. Finally there are the 

 intermittent streams and ponds in which animals are subjected 

 alternately to water and drought. In the present chapter 

 we have to consider the animals that live in rivers and some of 

 the adaptations they show to a fluviatile life. In the first 

 place the number of different kinds of animals and the abun- 

 dance of individuals is less in swiftly flowing rivers than in 

 lakes, for the swiftly flowing river is a difficult habitat in 

 which to live. Free-swimming organisms tend to be swept 

 away unless they are powerful enough swimmers to stem the 

 current. The minute organisms that swarm at home on the 

 surface of lakes are found in rivers merely as victims of floods, 

 and are being swept away to their destruction. The animals 



22.3 



