148 ELEMENTARY ZOOLOGY. 



Good illustrations of such parasites are the tape-worms 

 (see page 189). There is a tape-worm which infests the 

 trout of Yellowstone Lake. The larvae enter the tissues 

 of the trout and by their ravages weaken and kill the host. 

 The dead fish are eaten by pelicans. The worms develop 

 to the adult sexual condition in the digestive canal of the 

 second host and the eggs or young embryos escape into 

 the water w r ith the excreta, and from there are taken up 

 by other trout. Thus the infection spreads. It is clear 

 that the passage from one host to another is helped, and 

 possibly arose, through the carnivorous habits of some 

 animals. 



Parasitism is an adaptation to a very special mode of 

 life in which the common nutritive difficulties are reduced 

 to a minimum ; the great activities of the parasite cluster 

 about reproduction. 



162. Practical Exercises. Find other illustrations of parasites 

 that must pass from one host to another. What are the means of 

 accomplishing it? Is one of the hosts always carnivorous? Are 

 there any vertebrates which are parasites? Enumerate the common 

 parasites, transient and permanent, which are known to infest man, 

 and find to what groups of animals they belong. Make a special 

 report on the most common parasites of man, as tape-worm, trichina, 

 etc. What other hosts are demanded to complete the life cycle? 

 In what way can we prevent the entrance of these parasites into the 

 human system? What is known of the cause of malaria? Of 

 yellow fever? How are these parasites brought to man? Examine 

 the mouth parts of the mosquito (Fig. 103). To what kind of feeding 

 are they adapted? Would you call the mosquito a parasite ? Make 

 a study of the transient parasites? Are any other transient parasites 

 responsible for the transmission of internal parasites dangerous to 

 man or to the domestic animals? 



163. The Origin of Adaptations. One of the most in- 

 teresting questions in the theory of biology is this : How 

 have these adjustments, which we see between organisms 



