(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 tlic changes of form ivJdcJi the parasitic 

 mode of life has caused in the 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 Crustacea. 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 it?elf, the existence of the species is more preca- 

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



