§ XI.] Harmless Parasites. 115 



It follows necessarily from what has been said with respect to 

 the variety of gradations in the mutual relations of host and 

 parasite, that the progress and issue of the disease must also 

 show manifold gradations, varying with the species on both 

 sides, and in a less degree with the individuals. The very 

 general occurrence of Trichinae, tapeworms, the itch and other 

 diseases bring them so much under the notice of all, that this 

 brief notice of them here will be sufficient. 



XI. 



Harmless parasites of warm-blooded animals. Parasites 

 of the intestinal canal. Sarciua. Leptothris:, Micro- 

 cocci, SpirillTim, Comma-bacillus of the mucous 

 membrane of the mouth. 



It seemed to me expedient to give the foregoing short review 

 of the phenomena of parasitism and of its consequences, because 

 all that we know of parasitic Bacteria are only special cases of 

 the main phenomena which occur everywhere ; and this is 

 equally true of all our suppositions concerning them. The 

 understanding of these matters will therefore be materially aided 

 by resting on old and long-known phenomena. 



In passing now to the consideration of important examples 

 of parasitic Bacteria, it will be advisable to speak first and 

 chiefly of the parasites of warm-blooded animals, including 

 mankind, and afterwards to say a few words on those of other 

 animals, and of plants. 



In the former class it will best suit our purpose to distinguish 

 the species which are the exciting causes of disease from those 

 which are less injurious or altogether harmless ; and first a few 

 words respecting the latter kind. 



The digestive canal and the breathing passages, the former 

 especially, are a favourite habitat of lower organisms, Fungi and 

 Bacteria, putting the various kinds of worms out of considera- 

 tion. A large number of Fungi make use of the intestinal canal 



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