RELATIONS AND SURROUNDINGS. 147 



tage of the parasite not to kill the host outright, especially 

 if the parasite should be a permanent one. In transient 

 parasites the life of the individual host is of no conse- 

 quence after the end of the period of parasitism, and 

 hence the entire destruction of the host 's body may take 

 place just as in the case of preying species. 



Parasites are almost exclusively invertebrates. The 

 worms and arthropods furnish the most numerous in- 

 stances. Some of the Protozoa are internal parasites, 

 sometimes being parasitic within the cells. 



161. The Adaptations of Parasites. Very profound 

 modifications occur in the structure of parasites, which 

 are the outcome of, and in part the adaptation to, their 

 special and dependent mode of life. There is usually 

 a degeneration of the organs of digestion, of motion, and 

 of sensation, since the parasite depends on the host for 

 the performance of these functions. The reasons why 

 useless or unused organs degenerate are not fully known. 

 It is known that lack of use causes an organ to deteriorate 

 during the life of an individual ; but whether this kind of 

 deterioration can be passed directly on to the next genera- 

 tion is not known. Some naturalists claim that it may 

 be; others, that it is not. 



The reproductive organs become very complicated and 

 the reproductive elements are produced in great abun- 

 dance. This great reproductive power is an adaptation 

 to the difficulties in finding the proper host. It is often 

 the case that a part of the life is spent in one host and the 

 rest in a host of an entirely different species. In order 

 to insure that a single individual will get from one host 

 into the other, many offspring must be produced. Most 

 of them never succeed, and hence die. 



