INTRODUCTION. 



THE number of parasites as yet observed in man, including some 

 doubtful species and merely temporary visitors, amounts to nearly 

 sixty. We know no other creature which harbours so large a 

 number, but it is open to question whether this may not be simply 

 due to the fact that special attention has been devoted to man and 

 human parasites. On the other hand, we can hardly suppose that 

 the whole list of human parasites is yet known to us. From the 

 results of the last few years, we are led to the conclusion that there is 

 still, especially beyond the bounds of Europe, a crowd of unknown 

 human parasites. Nor is our knowledge of European species yet ex- 

 haustive ; we know of many doubtful forms, and have been enabled 

 in our own day to render the catalogue more complete. 



Of course the parasites infesting man have not by any means equal 

 importance from a medical point of view. Besides dangerous or even 

 fatal forms, we find others which usually exert hardly any influence 

 whatever on the health of their host. The study of their distribution 

 reveals also similar differences. Some though on the whole only a 

 few are exclusively restricted to man ; J others infest other animals 

 also, and many of these occur indeed more frequently in other than 

 human hosts, which they may perhaps have visited only once by 

 chance ; and further, we know that some of our parasitic visitors 

 (larvae of insects) generally prefer free life to parasitism. 



Most parasites infest the human body only in their adult and 

 sexually mature state ; but to this statement also there are exceptions. 

 Some species, such as Tcenia solium and Trichina spiralis, are found 

 in man in two stages, while others are only parasitic in their larval 

 or intermediate form, e.g., Tcenia echinococcus, as a cystic worm. 



As regards the special parts of the organism which are infested, 

 we find that in man the skin and the intestine are, as usual, most 

 exposed to the assaults of parasites. By far the greater number of 

 parasitic species concentrate themselves in these two organs, and 

 only a fourth of the whole number are found throughout the rest of 

 the body. Yet there is hardly any part of the body which is without 



1 With the increasing completeness of our knowledge of helrninthology, the number of 

 these parasites is becoming ever smaller. Thus I have lately found in the gorilla, not 

 only Ascaris lumbricoides, but also Dochmius duodcnalis, which had previously only been 

 found in man. 



