CRUSTACEA AND MOLLUSCS 229 



pond. The little creatures have returned as if by 

 magic, and fill the water once more. Other aquatic 

 animals were unable to sustain the drought, but they are 

 there again as soon as the water covers the bottom ; 

 the interval seems to have had no effect on their life. 



These minute animals, which fill the ponds and 

 seas in such numbers that every cast with a fine net 

 brings up a mass of their bodies from the bottom, are 

 called water-fleas. They belong to the gill-breathing 

 articulates, the Crustacea. They have a shell, and 

 grow by casting the skin, like all other articulates. 



The small crustaceans play an important part in the 

 life of a pond, as most of the fishes and a good many 

 other animals could not exist without them. These 

 build up their frames with the flesh of the little 

 crustaceans. The water-fleas form a tooth in the great 

 cog-wheel of the earth's round. It turns on and on ; 

 substances are formed from stone and water ; from these 

 is built up the living matter that appears first of all in 

 the tiniest organisms ; from these are made larger and 

 larger animals which will decay in turn, until the turn of 

 the wheel is complete, and the stone and water are there 

 once more to provide material for the fresh start. 



In order to understand better this "economy of 

 nature," we must begin a little further back. 



We know that the mass of the earth, with all the 

 rocks, the water, and the air that surrounds our planets, 

 consists of elements, of which more than seventy are 

 known. These can combine in so many different ways 

 as to produce the immense variety of forms of matter 

 that we see every day. Such elements are, for instance, 



