56 Roosevelt Wild Life Bulletin 



internal organs of the host — as the animal is called which harbors 

 a parasite. When they occur in large numbers in a vital organ they 

 may seriously afifect the health of the host, and either cause its death 

 or form such a brake on its locomotor activities that it falls an easy 

 prey to its natural enemies. 



As a rule parasitic worms are in a high degree selective in their 

 choice of the host and also of the particular organ in which they shall 

 pass their existence. A species of parasite which lives and breeds in 

 a fish could not pass the same stage of its life history in a frog or in 

 any of the land vertebrates, and usually a parasite that is found in a 

 given species of fish would be found only in that species, or at least 

 in some closely allied one. Also, a species of worm that normally 

 inhabits some particular organ of its host will usually not be found 

 in any other organ. If it lives in the mouth, for instance, it will not, 

 as a rule, occur in the intestine, and a worm which may be looked for 

 in the duodenum will not be found in the rectum. A worm that 

 attaches itself to the gills and sucks the blood of the host in that 

 favored locality will never be found in the interior of the body. 



It is not intended, however, to give the impression that a worm 

 consciously seeks the particular host and organ in which it will flourish 

 best. In the course of the evolutionary history of these worms, as 

 indeed in that of all animals, each species has, as the result of the 

 operation of natural selection, become adapted to a certain more or 

 less definite environment where it finds the conditions of existence 

 favorable. Internal parasites are bound to a specific environment 

 more rigidly than perhaps any other animals, because, living as they 

 do within the body of their host and confined to a narrow space, 

 they are constantly immersed in its body fluids. The chemical and 

 physiological action of these fluids, to which the entire organization of 

 the parasite is thus adapted, varies radically in different localities 

 of the host's body, and a species of worm whose relation to its 

 host would fit it for a prosperous existence in one organ might not 

 be able to exist at all in some other, or perhaps in any other. A 

 parasite, for instance, which clings to the mucous membrane of the 

 mouth or pharynx of a bird, where the reaction of the salivary juices 

 is an alkaline one, would not survive if it should be carried into 

 the stomach, where the reaction is an acid one, but would be digested 

 like any other particle of organic matter. 



It is, in fact, hardly, possible to exaggerate the extreme delicacy 

 of the adjustment of an internal parasite to its environment, which 

 binds the parasite in most cases to a certain organ of a certain host- 

 animal. It is thus important in studying internal parasites to observe 

 carefully the localities in the host's body in which the parasites are 

 found, and all the accompanying circumstances of their life and sur- 

 roundings. Quantitative studies of these points are also important, 

 since the greater the number of observations the truer will be the 

 generalizations based upon them. One of the main objects of this 

 whole study is to determine these conditions and thus to throw light 

 upon the life history of the parasites under investigation. 



The life history of most internal parasites, however, includes much 



