WORMS AND CCELENTERATA 271 



mature animals inside it. They are often so numerous 

 in the hare as to cause its death. They are less 

 dangerous to man, who also may be infected with them, 

 the eggs being received on the hand from the dog's 

 sniffing and being passed to the mouth. 



This pentastomum is a thorough parasite, and changes 

 its host ; this, as we shall see presently, is characteristic 

 of the majority of intestinal worms, which are, so to say, 

 the most complete parasites. We can, however, find 

 transitional forms even amongst these, leading gradually 

 to them from their free-living relatives. Amongst the 

 nematodes many species are still perfectly free ; among 

 the platodes the tape-worms descend from the suctorial 

 worms, and these are closely related to the turbellarians, 

 small, flat, dark or light worms that we find in every 

 pond. 



Parasites are, therefore, animals that have adapted 

 themselves to living on other animals. This kind of 

 life is clearly a very safe one. Living in the warm 

 interior of the host, the parasite is almost entirely 

 sheltered from climatic troubles, and has nothing to 

 fear from direct enemies. Finally, it riots in an abund- 

 ance of food, and this is often brought to him already 

 digested. This is the case with the parasites of the 

 alimentary system, which are surrounded with a 

 constant stream of nourishment, and have often even 

 lost their own alimentary canal, as the food can pass 

 directly through the wall of their body, without having 

 to undergo further changes within the body of the 

 parasite. 



Thus we find no alimentary system either in the 



