1G8 GENERAL PRINCIPLES OF ZOOLOGY. 



sedentary animals which, when opportunity offers, attach themselves to a 

 stone, a plant, or another animal ; in such cases the term parasitism is a 

 misnomer, because it cannot be called a dependent condition. If a 

 hydroid fasten itself upon the back of a crab instead of on a stone, it is the 

 result of chance, in which the nature of the hydroid is in no way con- 

 cerned. The case would be different if the polyp 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 depend- 

 ent upon the place of abode ; when the host not only serves for a dwell- 

 ing-place, but also furnishes the dweller with food ; when, consequently, 

 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 parasitism to consider the changes of form 

 which the parasitic mode of life has caused in the structure of 

 animals. These concern most immediately the organs of locomo- 

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

 possible to the host, the locomotor apparatus more or less com- 

 pletely disappears and an apparatus for fixation to the host 

 becomes necessary; parasites of different groups are provided with 

 hooks, claspers, sucking-discs, etc. The blood, tissue-fluids, or 

 liquid food of the host furnishes nourishment to the parasite: 

 these are substances in solution which scarcely need digestion. 

 Usually, therefore, the digestive canal is simplified or quite dis- 

 appears; among the parasites there are gutless worms as well as 

 gutless Crustacea. The mode of life of a parasite is also simpli- 

 fied, since it is no longer compelled to seek its food ; in all parasites 

 the nervous system and sense-organs undergo a high degree of 

 degeneration; the former becomes limited usually to the most 

 indispensable portion; the latter, except those of touch, may 

 entirely disappear. 



Modification of the Sexual Apparatus by Parasitism. The 

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

 ment. While it becomes easier for the parasite to maintain itself, 

 the existence of the species is more precarious. If a man die, 

 then most of his parasites die with him, especially those which 

 exist in the interior of his body. In order that a parasitic species 

 may not become extinct in a short time, it is necessary that the 

 eggs be introduced into a new host. Since this transmission is 

 attended with difficulties, the parasites must produce an enormous 

 number of eggs. The eggs, too, are distinguished by great resist- 



