(ECOLOGY. 



169 



ing 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, entopara- 

 sites, than in the case of the dwellers 

 upon the skin or other superficial 

 organs, the ectoparasites. In case of 

 entoparasites the transforming influ- 

 ence of parasitism is so considerable 

 that representatives of the most diverse 

 animal groups take on a remarkable 

 similarity of ajjpearance and structure. 

 Pentastommn tienivides (fig. 112), for 

 example, belongs in the same class with 

 the spiders, the Arachnida, but in 

 external appearance it is entirely unlike 

 them, resembling the tape-worms (fig. 

 111). Hence for a long time all ento- 

 parasites, on account of their simi- 

 larity, were united into a single 

 systematic group under the name of 

 • Ilelminthes,^ comprising members of 

 the crustaceans, worms, and spiiders, 

 as well as animals of entirely different 

 groups of the animal kingdom. Only 

 by embryology was the unnaturalness 

 of this groujiing recognized. Ento- 

 parasitism therefore is one of the best 

 examples for illustrating convergent 

 development, i.e., animals of difEerent 

 systematic position acquiring, under similar conditions of life, a 

 great similarity of structure and appearance. 



Symbiosis. — Less frequent than parasitism is symbiosis, or the 

 association of animals for reciprocal advantages. Social animals 

 frequently not only hold certain animals in bondage, but even seek 

 to 2)rotect and serve them; as, for example, in the company of 

 ants are found certain blind beetles, like Ctaviger (Myrmecophily), 

 or some species of plant-lice, or even ants of other species and 

 genera. But such cases of association correspond in j^art to the 



Fig. 111. 



Fig. 113. 



Fig. 111.— Tomia nana. (After 

 Leuckart.) 



Fig. 112.—Penta^tomum tmnioides 

 female. (After Leuckart.) h, 

 hooks right and left of mouth ; 

 >n\ unpaired ovary, branching 

 into two oviducts, whicli unite 

 into the unpaired vagina (va) ; 

 the latter receives tlie outlets 

 of two receptacvla .^eniinU (rs), 

 and winds around the digestive 

 tract (d); ce, cesojjhagus. 



