2O4 



GENERAL PRINCIPLES OF ZOOLOGY. 



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 

 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 timioides (Fig. 1 08), 

 for example, belongs in the 

 same class with the spiders, the 

 Arachoidea, but in external ap- 

 pearance it is entirely unlike 

 them, resembling the flat worms 

 (Fig. 107). Hence for a long 

 time all entoparasites, on account 

 of their similarity, were united 

 into a single systematic group 

 under the name of " Helminthes" 

 comprising crustaceans, worms, 

 and spiders, as well as animals 

 of entirely different groups of the animal kingdom. Only 



107 



FIG. 108. 



FIG. 107. Tienia. itanti 



kart.) 

 FIG. 108. Pentastomum ttenioides. 



F'emale. (After Leuckart.) /;, 

 hook right and left of mouth ; ov, 

 unpaired ovary, branching into two 

 oviducts, which unite into the un- 

 paired vagina (7>a) ; the latter re- 

 ceives the outlets of two recr/>tacula 

 sf minis ('\rl, and winds around the 

 digestive tract (d) ; oe. oesophagus. 



