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 
(ineluding, 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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