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 re- 

 tained 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 larva?, called Glochidia, each with two 

 valves toothed at the margin. The eggs are pro- 

 duced 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 



