CHAPTER XI 
PARASITISM AND DEGENERATION 
93. Relation of parasite and host.—In addition to the vari- 
ous ways of living together of animals already described, 
namely, the social life of individuals of a single species and 
the commensal and symbiotic life of individuals of differ- 
ent species, there is another kind of association among ani- 
mals that is very common. In cases of symbiosis the two 
animals living together are of mutual advantage to each 
other; both profit by the association. But there are many 
instances in the animal kingdom of an association between 
two animals by which one gains advantages great or small, 
sometimes even obtaining all the necessities of life, while 
the other gains nothing, but suffers corresponding disad- 
vantage, often even the loss of life itself. This is the asso- 
ciation of parasite and host; the relation between two ani- 
mals whereby one, the parasite, lives on or in the other, the 
host, and at the expense of the host. Parasitism is a com- 
mon phenomenon in all groups of animals, although the 
parasites themselves are for the most part confined to the 
classes of invertebrates. Among the simplest animals or 
Protozoa there are parasites, as Gregarina, which lives in 
the bodies of insects and crustaceans; there are parasitic 
worms, and parasitic crustaceans and mollusks and insects, 
and a few vertebrates. When an animal can get along 
- more safely or more easily by living at the expense of some 
other animal and takes up such a life, it becomes a parasite. 
Parasitism is naturally, therefore, not confined to any one 
group or class of animals. 
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