ANIMAL PARASITISM 429 



The word parasitism in its broad sense applies to all cases in which 

 one animal depends on another to furnish it with food; for instance, 

 ornithologists call cowbirds parasites because they lay their eggs in 

 the nests of other birds and leave the foster-parents to feed and 

 care for the young cowbirds. Most zoologists, however, use the 

 word parasitism only for cases in which the parasite lives in or on 

 the body of its host; for example, lice live on the bodies of many 

 animals, and tapeworms live in them. 



Origin of Parasitism 



How did it happen that some animals became dependent on others 

 to furnish their food, that is, how did parasitism arise f There is 

 a considerable amount of evidence for the belief that all parasites 

 are descendants of free-living ancestors, and that these descendants, 

 in the course of generations, gradually became more and more de- 

 pendent on certain hosts, until in some cases they are now abso- 

 lutely unable to make their own living. For example, certain species 

 of nematodes which are free-living inhabitants of the mud at the 

 bottom of ponds and streams are able to live in the large intestine 

 of a frog if they happen to be swallowed by a frog. Other species, 

 very similar to the mud-dwelling nematodes, have found the intes- 

 tines of frogs such a good habitat that they live nowhere else ; in 

 other words, they have become parasites. Some intestinal parasites, 

 in the course of many generations, have lost their locomotor struc- 

 tures or even their digestive organs and yet continue to thrive 

 because there is little or no need for locomotion or digestion when 

 all food is brought to the parasite already digested by the host's 

 intestine. Since such degenerate parasites are unable to secure food 

 elsewhere, they are condemned by their peculiar structure to live as 

 parasites in the intestine of their host. 



Degrees of Parasitism 



Free-living animals which sometimes become parasites when they 

 get into another animal (by being swallowed, for instance) are 

 called accidental or occasional parasites, as in the case of the mud- 

 dwelling nematodes mentioned above. ''Vinegar eels," nematodes 

 in vinegar, sometimes establish themselves as harmless parasites in 

 the human urinary bladder. Facultative parasites are able to live 



