DR. THEODOR NEUMANN. 211 



of the animal body. It would be a rather hazardous un- 

 dertaking to give the approximate number of all known 

 species of parasites; doubtless it is several thousands, and 

 every day new forms are discovered, studied and added 

 to the list. Nearly all types of the animal kingdom 

 furnish representatives of parasites, yet the partici- 

 pation is rather unequal. Vertebrates and molluscs 

 have only very few forms ; more are found among the 

 protozoa ; a much greater number among the arthro- 

 pods and the hydrobiotic Crustacea ; while the type of 

 vermes is very largely composed of parasitic forms. 

 Parasitic worms are those which live eminently inside 

 the human body, and it was they which were first called 

 napaairoi, and still nowadays we meet the name entozoa 

 (inside animals), indicating all those animals which live 

 in the interior of another. 



In the same way that most types of the animal kingdom 

 contain parasitic forms, we may safely maintain that 

 there is probably no animal which might not be the host 

 of some parasite. Neither size nor manner of life nor 

 peculiarities of the individual are sufficient protection; 

 and it might make us smile to think of the established 

 truth that there are parasites in parasites. Another 

 rather peculiar circumstance is the experience that cer- 

 tain animals are visited with preference by only one or a 

 few kinds of parasites; on the other hand, the opposite 

 case is more frequent that one animal, especially a ver- 

 tebrate, houses and feeds a good many different parasites 

 at the same time. Man stands at the top of this scale, 

 since we know that he is the host of about eight 

 kinds of protozoa, fifty -one worms and twenty arthro- 

 pods — and these are probably not all. 



A great difference exists among parasites concerning 

 the part of the body in which they settle. Some do not 

 seem to be very particular, as they are found in nearly 

 every organ, while others confine themselves strictly to 



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