THE FRESH WATERS 151 



The eggs 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 micro- 

 scopic 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 (LimrKza 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 pa- 

 tient; it goes through complicated changes 



