15.-S0ME OBSERVATIONS CONCERNING FISH-PARASITES. 



BY EDWIN LINTON, 

 Professor of Zoology in Washington and Jefferson College. 



The purpose of this paper is twofold — first, to present, in the form of general 

 considerations, some of the results of my own work; and, second, to suggest certain 

 lines of research along which future investigations on fish entozoa should be made. In 

 pursuance of this plan, bare anatomical details and wearying recapitulation of the 

 names of helminthological writers and their works will be kept in the background. 



I. 



It is probably now pretty well known by most persons of average intelligence and 

 information that cases of parasitism are not exceptional facts in nature, but that there 

 is a fauna of considerable extent whose natural habitat is within other animals. 

 Much work remains to be done before an approximately correct estimate can be 

 made of the number of species which pass their lives within other animals. Von 

 Linstow's Compendium der Helminthologie, 1878, enumerates 1,917 species of animals 

 from which entozoa to the number of 2,755 distinct species have been reported. 

 There are probably several duplicates in this list, but they are offset many times over 

 by the new species which have been added to helminthological science in the fifteen 

 years that have passed since the publication of Von Linstow's work. 



There are few general statements in natural history that do not require qualifica. 

 tion ; and the one which would say when one animal eats another the life of the latter 

 is by that act terminated affords no exception to the general rule. For, without 

 doubt, there are thousands of species of animals which live to be eaten, and if eaten, 

 live; and, conversely, if not eaten, perish. Those forms which take up their abode 

 within other animals are, for convenience, called guests, and the animal which affords 

 such entertainment to others is called the host. 



Of all animals which nature thus requires to keep open house, the class of fishes 

 takes the lead, not only for the variety of the forms and the number of the individuals 

 harbored, but also for the -frequency of individual cases of parasitism. 



The cases of parasitism had in mind in the preparation of this paper all belong to 



those kinds which pass the entire cycle of their lives within other animals, that is, to 



the entozoa, properly so called. 



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