CHAPTER XV 



PARASITES 



PARASITES are organisms, usually very small, that 

 live either on the surface or within the body of a larger 

 animal, called "the host," and gain their nourishment, 

 directly or indirectly, at its expense. In some cases the 

 parasites subsist on lifeless matter, but in other in- 

 stances they secure their nourishment directly from the 

 living tissues of the host. 



Some parasites belong to the vegetable kingdom ; for 

 example? the mold -like fungi that cause "ringworm," 

 or "barn itch," and a few other organisms of the sur- 

 face of the skin, and some that live in the lungs or 

 air- passages. Germs, or bacteria, are really minute 

 plants, but diseases produced by these organisms are 

 not ordinarily classed as parasitic. Many of them are 

 classed with infectious and contagious diseases. 



Most parasites (as the term is commonly used by 

 veterinarians) belong to the animal kingdom, and they 

 can be divided into two general classes : the insect -like 

 (including, for convenience, ticks and mites, which 

 are closely related to insects) ; and a large variety of 

 other animals, most of which are properly classed as 

 worms. As a rule, parasites do not spread from one 

 species of animal to another, as from cattle to sheep, 

 but there are some important exceptions to this. 



Some species of these parasites pass only a part of 

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