272 DARWINISM AND THE PROBLEMS OF LIFE 



tape-worm or the echinorhyncus^ and in the nematodes 

 it is at least very much simplified, and is devoid of the 

 subsidiary glands such as the liver and other appendages. 

 The locomotive organs of the parasites are equally 

 degenerate, and are replaced by clinging apparatus. 

 There is bound to be such an apparatus in external 

 parasites, otherwise the unwelcome guest could easily 

 be rubbed off when they are not concealed from their 

 host by a thick coat of hair. And clinging organs 

 are necessary in intestinal parasites, as they could 

 not stand against the pressure of the fluid food 

 if they were not attached, and would be forced out at 

 the anus. It is only the maw-worms and other thread- 

 like worms that can maintain themselves in the gut 

 owing to their shape alone. As they are long, at the 

 end thin, and round, the chyle runs past without bearing 

 them along with it. 



Being cut off from the outer world, the parasite has 

 no need of sense-organs, and is usually without them. 

 Its respiratory organs are less altered, and hence it is 

 that the gill-breathing Crustacea only attach themselves 

 to aquatic animals, and the air-breathing tracheates 

 only, as a rule, to land-animals. The ancestors of the 

 intestinal worms breathed through the skin, and this 

 has been preserved in their descendants, who can do 

 so because they are constantly bathed in the fluids 

 of their host that contain oxygen. It is due to this way 

 of breathing that they are found both on land and 



1 These are round worms, often of a considerable size ; their snout is 

 equipped with spines with which they fasten themselves to the wall 

 of the gut. They are generally found in fishes and aquatic birds, 

 more rarely in mammals, and very rarely in man. 



