302 BIOLOGY AND HUMAN LIFE 



From the point of view of the host there are also degrees. In 

 some cases the parasite helps himself to a small quantity of 

 material that is hardly missed ; in other cases the amount of 

 damage done may be serious or even fatal. It is in these cases 

 that we come to the problem of how diseases are caused. 



The European cuckoo will lay her eggs in the nests of strange birds, 

 thus getting from other organisms at least two direct benefits — the work 

 of building a shelter for the young and the work of keeping eggs warm 

 during incubation. There is also the feeding of the young by the strange 

 foster mother. This is a case of getting services from another organism 

 without giving anything in return. It is in this sense that we use the 

 word parasitism in connection with higher animals, and especially in con- 

 nection with human affairs. 



233. Injury done by parasites. The simplest relation between 

 a parasite and its host is that in which the parasite takes for its 

 own use materials accumulated or produced by the host. The 

 tapeworm (see Fig. 156) lives in the intestine of a human being 

 or some other mammal and absorbs digested food through its 

 own body wall. It rarely deprives the host of enough nourish- 

 ment to cause him serious injury. Very rarely is an oak tree 

 sufficiently injured by the mistletoe to show bad effects, and the 

 flea or the mosquito takes so little blood that we rarely miss it.^ 

 The digestive tract of every animal that has one is likely to be 

 inhabited by various species of protozoa, bacteria, and minute 

 worms ; in most cases the food which these inhabitants with- 

 draw is not sufficient in amount to cause any privation to the 

 host. The hookworm, however, attaches itself to the lining of 

 the intestine and feeds upon the rich materials manufactured 

 by its host. This is a serious privation, and those who suffer 

 from hookworm disease show loss of energy and initiative, in 

 some cases to a fatal degree (see page 334). 



Many parasites, possibly all, give off peculiar substances that 

 result from their metabolism, just as do our own tissues. In 



^The bites or stings of insects are often more serious than the removal of 

 blood, as we shall see later (Chap. XXXI). 



