14 PARASITES IX GENERAL 



parasites since inhabitants of the mouth and nasal cavities and 

 such worms and mites as burrow just under the surface of the 

 skin might be placed in either category. 



Effects of Parasitism on Parasites. — The effect of parasit- 

 ism is fell by both parasite and host. There is a sort of mutual 

 adaptation between the two which is developed in proportion 

 to the time that the relationship of host and parasite has existed. 

 It is obviously to the disadvantage of internal parasites to cause 

 the death of their host, for in so doing they destroy themselves. 

 It is likewise to the disadvantage of external parasites, not so 

 much to cause the death of their host, as to produce such pain 

 or irritation as to lead to their own destruction at the hands of 

 the irritated host. It is interesting to note, for instance, that 

 insects which depend to a large extent on man for food have 

 less painful bites than do insects which only occasionally or ac- 

 cidentally bite human beings. Together with a softening down 

 of the effects of the parasite on the host, there is a concomitant 

 increase in the tolerance of the host to the parasite. It is a well- 

 established fact that a disease introduced into a place where it 

 is not endemic, i.e., does not normally exist, is more destructive 

 than in places where it is endemic. The variations in suscepti- 

 bility to parasites are directly connected with the subject of 

 immunity, which will l)e discussed later. An organism and the 

 parasites which are particularly adapted to live with it maj-, in 

 a way, be looked upon as a sort of compound organism. Those 

 parasites which live part of their life in vertebrate animals and 

 part in other parasites of these animals, as lice, ticks and biting 

 flies, are absolutely dependent for their existence on the relation- 

 ships of the vertebrates and their parasites, and form a sort of 

 third party to the association. 



Aside from the toning down of their effects on the host, para- 

 sites are often very highly modified in structure to meet the de- 

 mands of their particular environment. As a group, parasites 

 have little need for sense organs and seldom have them as highly 

 developed as do related free-living animals. Fixed parasites do 

 not need, and do not have, well-developed organs of locomotion, 

 if, indeed, they po.ssess any at all. Intestinal pai'asites do not need 

 highly organized digestive tracts, and the tapeworms and spiny- 

 headed worms have lost this portion of their anatomy completely. 

 On the other hand, parasites must be specialized, often to a very 



