(ECO LOG Y. 203 



it is the result of chance, in which the nature of the hydroid 

 polyp is in no way concerned. The case must be consid- 

 ered quite otherwise, if the polyp in question were able to 

 live only upon the crab, and perished, if in any other place. 

 Such a dependent condition usually occurs only when the 

 mode of nutrition is also dependent upon the place of 

 abode ; when the host not only serves for a dwelling-place, 

 but also furnishes the dweller with food; when, conse- 

 quently, the dweller lives at the expense of the host. 



Degeneration Caused by Parasitism. --The degree 

 to which a parasite has become dependent upon its host 

 varies in the different species ; it is determined by the extent 

 to which the parasite has adapted itself to the organization 

 of its host. Therefore it is necessary in speaking of para- 

 sitism to consider the cliangcs of form tvJiicli the parasitic 

 mode of life lias caused in t/ic structure of animals. These 

 concern most immediately the organs of locomotion and 

 nutrition. Since a parasite needs to fix itself as firmly as 

 possible to the host, the locomotor apparatus gradually dis- 

 appears, or at least becomes less highly developed. An 

 apparatus for fixation to the host becomes necessary ; para- 

 sites of the widest diversity are provided with hooks, 

 claspers, sucking-disks, etc. The blood, or tissue-sap, or 

 liquid food of the host furnishes nourishment to the para- 

 site : these are substances in solution which scarcely need 

 digestion. Usually, therefore, the digestive canal is sim- 

 plified or quite disappears ; among the parasites there are 

 gutless worms as well as gutless crustacca. The mode of 

 life of a parasite is also simplified, since it is no longer 

 compelled to seek for food ; in all parasites the nervous 

 system and sense-organs undergo a high degree of degen- 

 eration ; the former becomes limited usually to the most in- 

 dispensable portion, the latter may entirely disappear. 



Modification of the Sexual Apparatus by Parasitism. 

 -The sexual apparatus, on the contrary, undergoes a strong 

 development. While it becomes easier for the parasite to 

 maintain itself, the existence of the species is more preca- 

 rious. If a man dies, then most of his parasites die with 



