CHAPTER LXVI 
ASSOCIATION OF ANIMALS—PARASITES 
Parasites live at the expense of larger animals, either using 
various parts of them as aliment, or robbing them of the food 
which they have digested. The less modified forms (ecto- 
parasites) attack their “hosts” from the outside, making either 
an occasional visit, as in the blood-sucking Leeches, or dwelling 
permanently upon the skin, a condition familiarly illustrated by 
many of the Fleas. Much greater modification is found among 
those parasitic animals (endoparasites) which live within their 
hosts, ¢,g. Flukes and Tape-Worms, and many of these pass 
through a complicated life-history, in the course of which two or 
more hosts may be utilized as homes. Many forms are parasites 
for a part of their lives only, being free-living when young or 
adult as the case may be, and not infrequently there is a differ- 
ence between the sexes in this respect, one of them (especially 
the female) being a parasite and the other not. 
The origin of parasitism is not far to seek. It may be regarded 
in many cases as an outcome of the carnivorous habit. Small 
animals attacked larger ones which they were unable to kill and 
devour in a straightforward fashion, so to speak, and the con- 
venience of preying upon a highly nutritious living food-supply, 
at which it was possible to “cut and come again”, naturally led 
to further evolution of the habit. And it is not difficult to imagine 
the stages by which external parasites gradually became internal 
parasites. Sometimes, too, no doubt, parasitism has resulted from 
the association of messmates (commensalism) in which the partner- 
ship was from the first one-sided, or ultimately became so. It 
would also seem that in many instances the habit possessed by 
many female animals of seeking out some secure refuge for egg- 
laying purposes has been the starting-point of parasitic relations. 
However originated, it is at least certain that the phenomenon of 
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