THE FRESH WATERS 149 



THE WEB OF LIFE IN THE FRESH 

 WATERS 



Nowhere do we find better examples of the 

 web of life than in the fresh waters, meaning 

 by the web of life the linkages between living 

 creatures, binding them together. Here are 

 some examples. 



The eggs of the common salt-water mussel 

 are wafted out into the sea and develop into 

 free-swimming larvae, which eventually settle 

 down ; but the eggs of the fresh-water mussel 

 are retained inside the shell and develop in a 

 special brood-chamber, the cavity of the basket- 

 work-like outer gill. They develop into tiny 

 pinhead-like larvae, called Glochidia, each with 

 two valves toothed at the margin. The eggs 

 are produced about midsummer in Britain, but 

 the Glochidia are not allowed to escape till early 

 in the following year. They are not allowed 

 to escape unless a fish, such as a minnow, comes 

 swimming slowly past. Then the mother- 

 mussel allows some of her offspring to escape, 

 and they come crowding out, like boys set free 

 from school, clapping their valves in the water 

 and exuding delicate gluey threads. Some of 

 them are lucky enough to get attached to the 



