8 THE ANIMAL PARASITES OF MAN 



partly relates to such forms as have been so deficiently described 

 that their recognition is impossible, partly to parasites of man in 

 various regions of the earth, the Helminthes and parasites of which 

 are totally unknown or only slightly known, or finally to early develop- 

 mental stages that are difficult to identify. Creatures that usually 

 live free, and exceptionally become parasitic, may likewise be called 

 incidental parasites. In this category are included a few Anguillulidce 

 that have been observed in man ; also Leptodera appendiculata, 

 which usually lives free, but may occasionally become parasitic 

 in black slugs (Arion empiricorum) : when parasitic it attains a 

 larger size, and produces far more eggs than when living a free life. 

 In order to avoid errors, the term " incidental parasites " should be 

 confined to true parasites which, besides living in their normal host, 

 may also live in other hosts. Leuckart speaks of FACULTATIVE PARA- 

 SITISM in such forms as Leptodera. 1 L. Oerley succeeded in artifi- 

 cially causing Rhabditis pellio to assume facultative parasitism by 

 introducing these worms into the vagina of mice, where the parasites 

 remained alive and multiplied. Rhabditis 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 

 no signs of such influence. As a general rule, the parasite, which is 

 always smaller and weaker than its host, does not attempt to 



1 Oerley, L., Die Rhabditiden und ihre medicinische Bedeutung, Berlin, 1886, p. 65. 



