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found amongst aquatic animals, and it can be readily understood 

 that a delicate protoplasmic organism could only pass from one 

 host to another in a fluid medium, or by the help of special mechan- 

 isms adapted to aerial transport or transmission by contact. It 

 should be mentioned, however, that some human contagious skin- 

 diseases are suspected to be due to the agency of parasites of the 

 nature of Protozoa.* 



Lake the epizoic forms, there are many entozoic Protozoa which 

 inhabit the bodies, and especially the intestines, of other animals, 

 but which are in no way to be regarded as parasites ; they feed 

 merely on various substances to be found there, such as waste 

 particles of food, excreted or faecal matter, or on other organisms, 

 such as bacteria, yeasts, and the like in short, on substances which 

 from the point of view of the host are superfluous, or even harmful. 

 Many examples of such organisms could be cited ; a good one is the 

 common Chlamydopfirys stercorea, found in the faeces and digestive 

 tract of man and many animals. The common intestinal flagellates 

 belonging to the genus Trichomonas (Fig. 5) and other genera are, 

 similarly, not to be regarded as true parasites in any sense of the 

 word. The common Lophomonas blattarum (Fig. 45) from the 

 intestine of the cockroach feeds chiefly upon bacteria and yeasts. 

 Many of these intestinal Protozoa are perhaps useful, rather than 

 harmful, to their hosts. 



On the other hand, the vast majority of organisms, Protozoa or 

 otherwise, that live in the interior of other living creatures are there 

 for no good or useful purpose ; their habitat is alone sufficient to 

 render them suspect. Two modes of parasitism may be distin- 

 guished from a general point of view. On the one hand, the para 

 site may merely intercept the food of the host and rob it of its 

 sustenance. On the other hand, the parasite may nourish itself 

 upon the living substance or vital fluids of its host. 



Organisms which rob the host of its food may do so in one of two 

 ways. They may appropriate the raw food-material, which they 

 then ingest and devour after the strictly holozoic method of feeding ; 

 examples of this mode oflife are possibly to be found in the extensive 

 infusorian fauna to be found in the stomachs of ruminants. Or 

 they may absorb the fluid products of the digestion of the host by 

 diffusion through the surface of the body of the parasite ; examples 

 of this mode of parasitism are to be seen, probably, in the case of 

 the Gregarines so common in the guts of insects. Parasites of the 



* For example, the so-called Coccidioides immitis, a name given to bodies found 

 in certain South American skin diseases ; see Blanchard (633), p. 168. Alolluscum 

 contagiosum has also beon attributed to parasites referred by some to the Protozoa. 

 In both these instances, however, the exact nature of the parasitic bodies is far 

 from clear ; the parasite uf molluscum contagiosum should probably be referred 

 to the Chlamydozoa (p. 470). 



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