io6 THE WONDER OF LIFE 



Inter -Relations. — There are many good instances 

 among freshwater animals of the way in which the Uf e of 

 one creature becomes wrapped up with that of another. 

 We shall afterwards refer to the extraordinary fact that 

 the continuance of the race of freshwater mussels depends 

 on the presence of minnows and other small fishes, while 

 on the other hand, the continuance of the freshwater fish 

 known as the bitterUng {RJiodeus amarus) depends on the 

 presence of freshwater mussels. The young stages of the 

 hver-fluke of the sheep are spent within the small fresh- 

 water snail [Lymnceus truncatulus), and the larvae of the 

 formidable guinea-worm of man are found inside certain 

 species of water-flea or Cyclops. Some tropical freshwater 

 fishes feed greedily on the aquatic larvae of mosquitoes and 

 thus help to lessen malaria which is due to a microscopic 

 animal temporarily parasitic in the insects. There are 

 endless nutritive chains of great practical importance. 

 Thus the voracious cormorants so often shot down on the 

 shores of the estuary, where they certainly engulf many 

 fishes, are not to be dismissed so summarily, for in certain 

 locaUties they keep down the eels and crabs which destroy 

 the fry of valuable species. Some freshwater fishes feed 

 on crustaceans and insect-larvse, which feed on minute 

 organisms, which, again, depend on decaying organic 

 matter. The insectivorous bladderwort {Utricularia) 

 catches small animals in its neat traps and these are said 

 to be utihzed by the water-spider. As we shall see, some 

 caddis-worms spread nets for the ' dwarf-plankton,' and 

 the green freshwater Hydra owes its colour and its success 

 to having entered into partnership with very miaute 

 Algae which Uve within the cells of its inner or endodermic 

 layer. 



