FORMS OF PARASITISM 9 



chus hirudinaceus (specific in pig, incidental in man) ; Fasciola hepatica 

 (specific in Herbivora, incidental in man). 



Ectoparasites (Epizoa) are those which are parasitic to the surface 

 of the body, whether burrowing into the integument, living upon it, or 

 only occasional visitors. Examples, scab mites, ticks, and other Acarina, 

 lice, flies. All of the arthropodal parasites with scarcely an exception. 



Endoparasites (Entozoa) are parasites which enter the body of their 

 host, inhabiting its alimentary canal, blood; and other tissues. Exam- 

 ples, Linguatula, larvae of the botflies, and almost all of the helminths. 



Helminthes is a term under which are grouped all of the worms 

 generally parasitic, with the exception of a small number in which the 

 body is annulated. The group is not a natural zoological one and is 

 used mostly in parasitology. 



In terms used to designate parasitic diseases it is customary to apply 

 the name of the genus, or other group name to which the parasite be- 

 longs, as the root, to which is added the suffix asis or osis. As for ex- 

 ample: 



Pediculosis, the condition produced by the presence of lice upon 

 the skin; Acariasis, the condition produced by the presence upon the 

 skin of mites and other Acarina; Filariasis, the condition produced by 

 Filaria. And thus we have Ascariasis from Ascaris, Oxyuriasis from 

 Oxyuris, Strongylosis from Strongylidse, Trichinosis from Trichinella, 

 Taeniasis from Taeniidse, Fascioliasis from Fasciola, Helminthiasis from 

 Helminthes, and Trypanosomiasis from Trypanosoma. 



In view of the many factors to be considered, the formulation of 

 exact and limiting interpretations of terms bearing upon kinds of par- 

 asitism is scarcely possible. It cannot be claimed for the above series, 

 therefore, that it is entirely satisfactory as stated and defined. For our 

 conceptions we must rely upon the behavior of the typical rather than 

 the isolated or synthetic, and be content to regard any grouping based 

 upon modes of parasitism as more convenient than exact. It is difficult 

 to circumscribe parasitism ; while we speak of the parasitic mode of life 

 as a form of symbiosis, it may well be questioned whether such insects as 

 mosquitoes and biting flies bear a true S3anbiotic relationship to their 

 hosts; their fleeting visits certainly do not constitute the living together 

 as usually impKed by the term. Again, we may not be able to draw a 

 distinct line between certain predaceous and certain parasitic forms. 

 From the more general viewpoint, however, it may be repeated that all 

 predaceous animals voluntarily, by the exercise of their powers of stealth 

 and cunning, seize upon and aim to destroy their prey at once, feeding 

 upon the body. There are parasites which use a degree of stealth in 

 approaching their victims, as certain parasitic Diptera, though the 

 .invasion of the body of its victim by the parasite is more often passive 

 than voluntary. While the parasite may appropriate a share of the 



