helminthes among animals and the Fungi among plants is an 

 indication that parasitic adaptations have been much easier of 

 accomplishment in these groups than in others. 



The direct structural adaptations of parasites to their hosts 

 are exceedingly various. Fungus parasites among plants commonly 

 exhibit absorptive or haustorial filaments by which they set up a 

 more intimate association with the cellular materials of the host, 

 while animal parasites frequently exhibit special organs of attach- 

 ment and a protecting cuticle for the surface of the body. Such 

 adaptations, however, are of minor significance in comparison with 

 others which ensure the production of a large number of young and 

 the transfer of the parasite in one stage or another to its proper 

 host. 



To all organisms the risks of life, or constructively the mechan- 

 ism of survival, is of prime importance. But it is especially sig- 

 nificant in parasites where there is added to ordinary risks the 

 isolation of the parasite when established in the body of the host. 

 Access on the part of the offspring to the body of a new host must 

 in this case become more than a matter of mere chance. Parasites 

 accordingly exhibit various adaptations for increasing the number 

 of young, and make use of peculiar associations on the part of the 

 host to ensure continuity from one generation to another. It is 

 also on this account that parasites commonly exhibit an intricate 

 life-history, involving considerable differences in successive 

 generations, and alternate primary and secondary hosts. 



Parasites usually also give indications in their structure of 

 degenerative changes when compared with free-living members 

 of the groups to which they belong. These changes are adapta- 

 tions in the sense that they make way for other adaptations of 

 more positive nature, of importance in the economy of the organism. 

 Such modifications include among animals the reduction of the 

 nervous system, especially the organs of special sense, and of the 

 muscular and digestive systems; among fungous plants degenera- 

 tion of the normal thallus form. 



There is doubtless a more fundamental relation existing be- 

 tween parasites and their hosts than is implied by adaptations of 

 a structural or mechanical nature. The fact that certain parasites 

 attach themselves to particular hosts, and to no others, or the 

 specificity of the organisms, has in all probability a chemical 

 basis in the composition of their respective tissues. This kind of 

 relation has been more thoroughly studied in the case of the 

 pathogenic bacteria and is becoming more and more important in 

 practical methods of serum therapy. 



