8 THE ANIMAL PARASITES OF MAN 



worms into the vagina of mice, where the parasites remained alive and 

 multiplied. Lcptodera pellio dies in the intestines of mammals and 

 man ; it remains alive in frogs, but always escapes into the open 

 with the faeces. 



Recently the incidental parasites of man have also been called 

 " PSEUDO-PARASITES" or " PSEUDO-HELMINTHES." Formerly, how- 

 ever, these terms were applied not only to living organisms that do 

 not and cannot live parasitically, and that only exceptionally and 

 incidentally get into man, but also to any foreign bodies, portions 

 of animals and plants, or even pathological formations that left the 

 human system through the natural channels, and the true nature 

 of which was misunderstood. Frequently these bodies were described 

 as living or dead parasites and labelled with scientific names, as if 

 they were true parasites. A study of these errors, which formerly 

 occurred very frequently, would be as interesting as it would be 

 instructive. It is better not to use the expression pseudo-parasites for 

 incidental parasites, but to keep to the original meaning, for it is not 

 at all certain that pseudo-parasites are not described, even nowadays. 



The Influence of Parasites on the Host. In a great many cases, 

 we are not in a position to state anything regarding any marked 

 influence exercised by the parasite on the organism, and on the con- 

 ditions of life, of the host. Most animals and many persons exhibit 

 few signs of such influence, an exception being infestation with 

 helminthes and certain other parasites which produce eosinophilia 

 in the blood. As a general rule, the parasite, which is always smaller 

 and weaker than its host, does not attempt to endanger the life 

 of the latter, as simultaneously its own existence would be threatened. 

 The parasite, of course, robs its host, but usually in a scanty and 

 sparing manner, and the injuries it inflicts can hardly be taken into 

 account. There are, however, numerous cases 1 in which the situation 

 of the parasites or the nature of their food, added to their number and 

 movements, may cause more or less injury, and even threaten the life 

 of the host. It stands to reason that a Cysticercus cellnlosce situated in 

 the skin is of but slight importance, whereas one that has penetrated 

 the eye or the brain must give rise to serious disorders. A cuticular 

 or intestinal parasite is, as a rule, less harmful than a blood parasite. 

 A helminth, such as an Ascaris Ininbricoides or a tapeworm, that 

 feeds on the residues of foodstuffs within the intestine, will hardly 



1 Liihe, M., " Ueber d. Fix. d. Helm. a. d. Darmwand ihrer Wirthe u. die dadurch 

 verursachten path-anat. Veranderungen d. Wirthsdarmes," Trans, of IVth Intern. Zool. 

 Cong., Berlin, 1901 ; Mingazzini, P., " Ric. sul var. modo di fiss. delle tenie alia par. int. 

 e sul loro assorbimento," Ric. Lab. Anal. Roma e altri Lab. biol., vol. x, 1904; Shipley, 

 A. E., and E. G. Fearnsides, "The Effects of Metazoan Parasites on their Hosts," Jotirn. 

 Econ. Biol., 1906, i, 2. 



