270 DARWINISM AND THE PROBLEMS OF LIFE 



forms amongst the parasitic crustaceans, some of which 

 are entirely like the free animals, and differ from them, 

 perhaps, only in having longer claws, with which they 

 attach themselves for a time to other animals and derive 

 their food from them. But the longer the animals live 

 on their hosts the more profoundly are they modified ; 

 the legs, which are no longer necessary, degenerate 

 more and more, the sense organs disappear, even the 

 alimentary canal may atrophy, and the animals feed, as 

 the sacculina does, in plant fashion, by means of roots 

 passing into their host. The structure is also entirely 

 changed by the enormous development of the sexual 

 organs, which are of great importance in every parasite. 

 Of the tracheates there are lice and fleas, which 

 ordinary folk call insects. Here again there are animals 

 that have been most curiously modified. We have an 

 instance in the Pentastomum tanioides. This animal, 

 which looks very much like a tape-worm, as its name 

 suggests, and has very little of the characteristics of a 

 spider, to which it really belongs, lives though rarely 

 in the nasal cavities of the dog. The eggs pass through 

 the nostrils to the ground, and when a hare or rabbit 

 takes them into its stomach with the grass, larvae issue 

 from them, pass through the stomach into the liver of 

 the ruminants, and cover themselves with a membrane, 

 inside which they cast their skin several times in the 

 manner of the articulates. When they have grown 

 bigger, they break the capsules, and disperse through 

 the various passages of the liver. They then bury 

 themselves in capsules once more, and if their host is 

 devoured by a dog or a fox they develop into sexually 



