from man; it has been estimated that in a severe case of trichinosis 

 60,000,000 of the worms may be present. 



Concurrent infection by two or more species in the same individual 

 is very common. Krause, cited by von Beneden, found in a single 

 horse over 500 Ascaris, 170 Oxyuris, "several millions" of Strongylus, 

 214 Sclerostomum, 69 Taenia, 287 Filaria, and 6 cysticerci ! In men, 

 as many as five species of intestinal parasites at a time, have been 

 reported (Riley, '12). 



Location xrithin the host — Larger number of species of parasites 

 occur on the surface of their host or in cavities opening to the surface, 

 such as the alimentary canal, respiratory, and urinogenital passages. 

 Other species occur in the muscles, connective tissue, nervous tissue, 

 the blood, and the lymph. They may live freely within a cavity or 

 inside of cells or even of cell nuclei. 



Early views regarding the origin of parasites — Theory of abiogene- 

 sis, or spontaneous generation taught by the earlier zoologists and 

 popularly accepted, even today. Redi's work in 1668. Others sup- 

 posed parasites inherited. 



Theory of biogenesis or development from pre-existent living forms 

 extended to internal parasites by beginning of 19th century, but by 

 most authorities it was then believed that such species originated 

 from free-living protozoa or other organisms, the theory of hetero- 

 genesis, or chance development from wholly different species. 



The development of the experimental method of study furnished the 

 key to the problem. Feeding experiments by Kiichenmeister, '44, 

 followed by Leiickart, Haubner, Cobbald, and many others. 



Life history of the parasite — "If we only know concerning a certain 

 animal that it is a parasite, we know but little; thoroughly to under- 

 stand its history, we must follow out all the separate stages and 

 conditions of its existence, and especially the circumstances under 

 which it becomes a parasite." — Leuckart, '86, p. 42. 



.4s regards relation to host, parasites may be grouped as: 



a Monoxenous parasites (monos, one; xenas, host), such as live 

 only in one genus or species of host. 



b Heteroxenous parasites (heteros, different; xenas, host), such 

 as pass different stages of their lives in different hosts, usually belong- 

 ing to different genera, and often widely distinct. 



Intermediate host — Supports asexual or immature form. 



