like the tapeworm, it is constantly bathed in a pre-digested 

 food supply, the digestive system of the parasite goes to 

 pieces. When the constant watch and ward against ever- 

 present enemies is relaxed, the nervous system follows suit. 

 And so it happens to all the tissues of the body when they 

 cease to do work. 



But degeneration is not the only price v/hich the parasite 

 pays for its inactivity. Its life is entirely dependent upon the 

 hf e of its host. If the host dies the parasite, too, must cease to 

 exist, being unable, through degeneration, to support itself. 

 The entire energy of the parasite then is devoted to the 

 problem of perpetuating its kind. With what success this 

 problem is solved many of the diseases of animals and men 

 bear witness. 



The solution of this one difficulty of the parasite's life 

 often brings about a surprising complication in its life his- 

 tory. It is forced to resort to elaborate methods to secure 

 for its descendants the same effortless existence at the ex- 

 pense of the chosen host. 



Take, for instance, the liver-fluke, a parasitic worm., 

 about an inch and a half in length, v/hich lives within the 

 liver of the sheep. Its numerous eggs are laid in the bile 

 duct and are carried thence through the digestive system of 

 the sheep to the grass. If, by chance, the eggs meet v/ith 

 water they develop into tiny swimming embryos. These 

 embryos mature into a form quite unlike the parent liver- 

 fluke, and, in their turn, give rise to another generation of 

 embryos, which can develop only if they chance to meet with 

 a water-snail. They penetrate the respiratory cavity and 

 here within the protecting mantle, degenerate, lose all their 

 organs and become mere egg-sacs. 



From their eggs develop f orm.s unlike any of those vv^hich 

 bore their way into the liver of the snail. Here several gen- 

 nerations of the same form may be produced until at the warm 

 season of the year they give rise to new forms which leave 

 the snail and swim about in the water. If the water dries 



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