204 



GENERAL PRINCIPLES OF ZOOLOGY, 



rs- 



him, namely, those which exist in the interior of his 

 body. In order that a certain parasitic species may not 

 become extinct in a short time, it is necessary that the 

 eggs be introduced into a new host. Since this transmis- 

 sion is attended with difficulties, the parasites must pro- 

 duce an enormous number of eggs. The eggs, too, are dis- 

 tinguished by great resisting power and well-developed 

 protective organs, such as strong shells, etc. ; thus it is 

 known, for example, that 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 

 e the case of the dwellers upon the 

 skin or other superficial organs, 

 the ectoparasites. In case of en- 

 toparasites the transforming in- 

 fluence of parasitism is so con- 

 siderable that representatives of 

 the most diverse animal groups 

 take on a remarkable similarity 

 of appearance and structure. 

 Pentastomum tcenioides (Fig. 1 08), 

 for example, belongs in the 

 same class with the spiders, the 

 AracJioidea, but in external ap- 

 pearance it is entirely unlike 

 them, resembling the flat worms 



FIG. 107. FIG. 108. (Fig. 107). Hence for a long 

 Fl &)'~ T "* ia na " a ' (AfterLeuc " time all entoparasites, on account 



FIG. 108. Pentastomum ttenioides. f f-1 1f ; r cirri ila rifir wprp nnifprl 

 Female. (After Leuckart.) A, Ot "ICir Similarity, WC 



hook right and left of mouth ; ov, ,' n fn o ct'ncrlp tM/stprrmtir oroun 

 unpaired ovary, branching into two HltO a Single Systematic group 



,"f; te the nt0 iae e r u under the name of ' ' Helminthesr 

 comprising crustaceans, worms, 

 and spiders, as well as animals 

 of entirely different groups of the animal kingdom. Only 



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