Animal Parasites. 79 



wet weather returns, abandon their parasitic hiding places. L. 

 Oerley has succeeded in artificially causing a free species of worm 

 (rhabditis pellio) to live temporarily as parasites by introducing 

 them into the vagina of mice, where they continued to live and 

 multiply, although when placed in the digestive canal they died or 

 were expelled. Larvae of flies often resort to parasitism as oppor- 

 tunity is afforded; meat flies and other insects which are ordinar- 

 ilv saprophagous often oviposit upon some animal in wounds or 

 ulcers and the resultant maggots obtain their nutrition in the 

 same location as parasites, although succeeding generations con- 

 tinue as usual under other conditions of existence. It may be 

 supposed that frequently such assumptions of a parasitic existence 

 are brought about by the force of circumstances and end with 

 the death of the organism if it be incapable of adaptation, even if it 

 should succeed in getting nourishment from the body of the ani- 

 mal host. Only in case the intruding worm (or other form of 

 life) can successfully withstand the mechanical and chemical influ- 

 ences which oppose it, only in case it obtains a habitat where it is 

 possible for it to live, is it likely to be changed into a parasite. 



Some worms may be -parasitic in different animal species; 

 trichinella is parasitic in man, hog, rat, mouse, cat. fox, polecat 

 and bear, and may be artificially induced to infest the dog, rabbit, 

 guinea-pig, sheep, horse and other mammals. In the same way the 

 liver fluke is found in a number of species. Other parasites, on 

 the contrary, occur in only one fixed species. For example, tcenia 

 solitmi and tcciiia saginata are found only in the human intestine; 

 crassicolis only in the cat ; in the intestine of another species they 

 die and it is impossible or only an exceptional case that they 

 should persist. Such exceptions, however, do occur naturally as 

 well as in artificial attempts to infest. For example, bothrioce- 

 phalus latns, whose normal host is man. is occasionally found in 

 the intestine of the dog; echynorhynchus gigas, a parasite of the 

 intestine of the hog, is in rare instances met in man. Such cases 

 are to be explained by exceptional opportunities for transmission. 



The influence of parasites upon the host varies much accord- 

 ing to their habits and situation. In many instances the presence 

 of a parasite occasions no noticeable disturbances ; dogs may have 

 dozens of tapeworms in them without showing any evidence of 

 sickness, although in other cases even a small number of the same 

 tapeworms cause digestive disturbances. At one time it was sup- 

 posed that some parasites were of use to the host, as the larvae 



