THE HAUNTS OF LIFE 109 



is undoubtedly severe elimination, but there are some 

 tough creatures which do not necessarily die even when 

 encased in ice. Provided that they can form small cavities 

 around themselves, they may last till the thaw comes ; 

 thus a leech has been known to survive forty-eight hours 

 in a block of ice. The worst case is when the ice is thick 

 on a shallow pond, for then there is risk of suffocation ; 

 oxygen becomes scarce ; sulphuretted hydrogen and 

 ammonia accumulate ; the fishes come eagerly to holes in 

 the ice ; and there is often great mortality. We are 

 impressed, however, by life's toughness as well as by its 

 fragility ; thus the water-snail, Limnceus stagnalis, may 

 be seen creeping quite actively on the under surface of the 

 ice. Leeches and eels are also notable for their powers 

 of resistance. We are familiar with the contrast between 

 the crowded and busy lif e of the pond and loch in summer 

 and the clear deserted appearance in winter, but the fact is 

 that the water is seldom so empty as it looks. There is 

 plentiful life in some of the Alpine lakes which are frozen 

 most of the year ; and in the depth of winter in Britain 

 and similar countries there may be abundant representa- 

 tion of ' water-fleas', rotifers, bear-animalcules, infusorians, 

 amoebae, and other small animals. 



Vital Economy of the Freshwaters. The population 

 of a freshwater basin may be divided into producers, 

 consumers, and middlemen. The raw materials consist of 

 air, water, and salts, which the producers, the green plants, 

 work up, with the help of the sunlight, into complex 

 carbon compounds. These are utilized by the consumers, 

 the animals, who dissipate the stores of energy which the 

 plants have accumulated. The middlemen are in great 

 part the Bacteria, which often make vegetable products 



