210 American Fisheries Society 



numerous and most serious disease-producers. The effects 

 they produce are as varied as the species which produce 

 them. In one respect only do they differ from the higher 

 parasites. They have usually the power of multiplying 

 within the host animal, a power which most higher parasites 

 do not possess, and accordingly, even though the original 

 infection be inconsiderable, the number of parasites may 

 increase beyond the ability of the host to resist their attacks. 

 The Protozoa include thus the most dangerous animal 

 parasites. 



The effects of the parasite on the host may be classified 

 roughly as mechanical, structural, or functional. Purely 

 mechanical injuries, such as the stoppage of the alimentary 

 canal or any other passage way, pressure of the parasite on 

 important tissues, the irritation of delicate structures by 

 movement or the distortion of organs, though found among 

 fishes, are nevertheless rare and of distinctly secondary im- 

 portance. Hofer mentions a number of such cases in Euro- 

 pean fish, and notes their rarity. Similar troubles doubtless 

 occur among fish in our own waters, but their infrequence 

 renders them of little practical importance and 1 have been 

 unable to find any mention of them in the literature. 



Structural changes, such as the proliferation of muscle. 

 or other cells, are frequently produced by the protozoan 

 parasites. The importance of parasitic invasion is here 

 clearly related to the numbers and size of the parasites, and 

 in most cases the action of a few is insignificant, whereas the 

 influence of a greater number or of such as may be relatively 

 large is a serious factor in the economy of the fish. The 

 location of the parasite is also important, and most serious 

 effects are produced by insignificant organisms in the nerv- 

 ous system or other delicate or important structures. All 

 of these features are as yet relatively unstudied in the case 

 of fish parasites. Even in Europe where investigations have 

 been more extended because the culture of fishes has been 

 pursued as a commercial undertaking for more than a cen- 

 tury, observations on these points are scanty and unsatisfac- 



