CECOLOGY 



157 



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 

 \\dth 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 "wath 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, sach 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 

 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 



Fig. 



l-IG. 113. 



Ta:nid nana (after 



Fig. 1X2.- 

 Leuckart). 



Fig. 113. — Pentastomum 

 twnioides female (after Leuc- 

 kart). h, hooks right and left 

 of mouth; oi', unpaired ovarj', 

 branching into two o\'iducts, 

 which unite into the unpaired 

 vagina {va)\ the latter receives 

 the outlets of two receptacula 

 seminis (rs), and winds around 

 the digestive tract {d) ; a:, oeso- 

 phagus. 



