174 INTRODUCTION. 



its parasitic guest, which may be either a definite and peculiar form, 

 or one which occasionally visits other organs, and which may perhaps 

 only occasionally wander from them. Of this we shall, in the course 

 of our systematic study, find abundant illustration, and it is at this 

 stage only necessary to observe that the distribution of parasites 

 within the human body is very diverse and unequal. While some 

 species are confined to definite, and perhaps strictly limited areas, 

 others wander to the most various organs, and some (e.g., Echinococcus) 

 hardly seem to avoid any part of the body. 



As regards their systematic position, the parasites of man belong 

 to three different divisions of the animal kingdom the Protozoa, the 

 Vermes, and the Arthropoda. The Vermes furnish the largest con- 

 tingent, the Protozoa the smallest but both only internal parasites 

 while the parasitic Arthropoda are almost wholly external. 



