THE FRESH WATERS 151 



develop in the gill-chamber, and the larval fishes 

 spend some time there before they find their 

 way out. 



Another linkage of the fresh-water mussel is 

 in connection with pearls, for some of the pearls 

 are due to the larval stages of parasitic worms 

 allied to the liver-flukes the adults of which 

 live inside aquatic birds. When the microscopic 

 parasite settles down in the skin-fold or mantle 

 of the mussel, it is smothered in layer after 

 layer of translucent lime, mingled with a little 

 organic matter, and the result is a pearl. 



The pied-wagtail, so often seen curtseying 

 on the stones by the side of the stream, is linked 

 to successful sheep-farming, for it is < very fond 

 of the little water-snail (Limncea truncatula] 

 which harbours the juvenile stages of liver-fluke, 

 which often causes fatal liver-rot in sheep. 



Some authorities say that the decline of 

 Greece was partly due to the introduction of 

 malaria. If this be so, we may link the decline 

 of Greece to the mosquito which harbours and 

 spreads the microscopic animal that causes 

 malaria in man. The malaria-organism, Plas- 

 modium by name, is imbibed along with the 

 blood when the mosquito bites a malaria patient ; 

 it goes through complicated changes within the 



