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, e.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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