94 FRESH-WATER ANIMALS 



bird, irritated by the pincers, swells up and forms 

 a capsule within which the young Anodonta 

 completes its development. When it has attained 

 the adult form it leaves its host, drops down to the 

 bottom of the stream, and henceforth leads the life 

 of the adult, half buried in the mud at the bottom 

 of the stream, along which it slowly ploughs its 

 way by means of its powerful muscular foot. 



No better illustration could possibly be given of 

 the special modifications acquired by fresh-water 

 animals as regards their life-history. Marine 

 bivalves, such as the oyster or cockle, all give rise 

 to small ciliated embryos, adapted to a free 

 swimming existence. So also does Anodonta, but 

 the embryos are not set free in the streams, where 

 as we have seen they would be entirely unable to 

 hold their own, but are retained within the gills of 

 the parent until they have attained sufficient size 

 and strength to look after themselves. The second 

 phase in their life-history, during which they are 

 attached to fish or birds, has probably been acquired 

 rather as a means of ensuring wider distribution 

 than as a precaution against the strength of the 

 currents. The adult Anodonta leads a very 

 sedentary life, while through the stickleback or 

 bird, new colonies may be started miles further up 

 stream, or even in other streams some distance off. 



Among fish very numerous examples are met 

 with of marine animals that have taken partly or 

 completely to fresh-water life. In many cases, as 

 in the salmon, the fish live normally in the sea, and 

 run up rivers merely for the sake of laying eggs in 

 places where they are exposed to less danger than 



