218 American Fisheries Society 



In general, then, very few fish are free from internal 

 parasites. The lightest infection is found in the carp, an 

 introduced species, and in the minnows and other small 

 stream fish. In the carp, which was introduced in the egg 

 stage, it is natural that the number of parasites should be 

 smaller than usual in fish of similar types and far smaller 

 than characterizes the same species in its home in conti- 

 nental Europe. It has here only the parasites of American 

 origin that could secondarily adapt themselves to its habits 

 and the brief time since its introduction has not been ade- 

 quate for the acquirement of an extensive parasitic popula- 

 tion. Only an occasional individual of this species is at all 

 affected by internal parasites. Among the fish of small 

 streams the conditions are unfavorable for infection. The 

 territory is limited, unconnected with other regions, con- 

 stantly drained of its organisms, and parasitic forms are 

 not likely to gain a footing. But because of its limitations it 

 is easy to see that when parasites are once introduced the 

 infection is likely to be heavy. It would be expected, then, 

 that the records would show either that parasites were ab- 

 sent or were abundant, and furthermore that this difference 

 was associated with definite regions. The evidence, while 

 perhaps not conclusive, clearly indicates the conditions noted. 

 In all, ten fish among those examined failed to yield any 

 evidence of parasitic invasion. 



On the other hand there are not many fish in which the 

 infection appears excessive. The tables show at a glance 

 the extent of infection, which can be taken from the column 

 indicating the average infection. One possible difficulty 

 must be avoided in making such a comparison. The num- 

 bers do not indicate the relative size of the parasite, which 

 is an important factor in the effect produced. Especially 

 among the cestodes, or tapeworms, is this difference of im- 

 portance, as some species are very small and others con- 

 spicuous by their size. A dozen of the latter will be of 

 greater influence on the host than a thousand of the minute 

 varieties. 



