(ECOLOGY 



157 



oe 



a parasite needs to fix itself as firmly as possible to the host, the locomotor 

 apparatus more or less completely disappears and an apparatus for 

 fixation becomes necessary; parasites of different groups are provided 

 with hooks, sucking-discs, etc. The fluids of the host furnish nourish- 

 ment to the parasite: these are substances in solution which scarcely need 

 digestion. Usually, therefore, the digestive canal is simplified or dis- 

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

 Crustacea. Very frequently the intestinal parasites live without oxygen; 

 they are anaerobic (p. 92). The mode of 

 life of a parasite is also simplified, since it 

 is no longer compelled to seek its food; the 

 nervous system and sense-organs undergo 

 great 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 in the interior of his body. 

 In order that a parasitic species may not 

 become extinct, it is necessary that the eggs 

 be introduced into a new host. Since this 

 is attended with difficulties, the parasites 

 must produce an enormous number of eggs. 

 The eggs, too, are distinguished by great 

 resisting power and well-developed protec- 

 tive organs, such as strong shells, etc.; the 

 eggs of Ascarids continue to develop for 

 some time in alcohol, being protected by 

 their impermeable shell. 



Ectoparasites and Entoparasites. 

 All the above-mentioned phenomena are 

 more conspicuous in the case of parasites which live inside of 



112. 



Fir.. IT.}. 



FIG. 112. Ta-nia muni (after 

 Leuckart). 



FlG. 113. Pentastonnim 

 tirnioides female (after Li-u. - 

 kart). //., hooks right ami left 

 of mouth; or, unpaired ovary, 

 I.:, i .rliing into two oviduct-, 

 which unite into the unpaired 

 vagina (;,;); the latter receives 

 the outlets of two rcceptacula 

 semiiiis (rs), and winds around 

 the digestive tract (rf); n; ceso- 

 phagus. 



other animals, entoparasites, than in the case of the dwellers upon the 

 skin or other superficial organs, the ectoparasites. In case of ento- 

 parasites the transforming influence of parasitism is so considerable 



