THE FRESH WATERS Ist 
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 (Limnea 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 inman. 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 
