BULLETIN OF THE UNITED STATES FISH COMMISSION. 431 



In severe cases, the skin of the toj) of the head, of the snout, of the 

 gill covers, and of the lower jaw may be almost completely destroyed, 

 and the atfection may extend far into the interior of the month. Cases 

 of the blinding of lish by extension of the disease over the eyes are re- 

 ported. It is also said that the gills are attacked; but, although care- 

 ful attention has been paid to this point, the gills have been unaffected 

 in every fish that has come under our notice, however severe and exten- 

 sive the disease might be. In far-advanced cases the edges of the fins 

 become ragged ; and, sometimes, the skin which invests the tin rays is 

 so completely destroyed that they stand out separately. 



All observers agree that the flesh of a diseased salmon, however ex- 

 tensive the morbid affection may be, presents no difference in texture, 

 or in color, from that of a healthy fish; and those who have made the 

 experiment declare that the flavour of a diseased fish is as good as that 

 of a healthy one. No morbid appearances are discoverable either in the 

 viscera or in the blood. Moreover, when fresh-run fish are diseased, 

 they may exhibit just as large an accumulation of peritoneal fat as 

 healthy fish. Nevertheless, it is certain that the cutaneous affection 

 causes much irritation. The fish exhibit signs of great uneasiness, 

 often dashing about and rubbing themselves against stones and other 

 hard bodies in the water. Eventually thej' get weaker, become sluggish, 

 and often seek the shallows before they die. 



The disease spreads with great rapidity after it has commenced, three 

 or four days being said to be sufficient to enable it to extend over the 

 whole body of a large salmon. 



In the early stages of the malady, the peculiar ai)pearance of the parts 

 of the skin affected might readily be, and certainly often has been, 

 ascribed to mechanical injury. It has already been remarked that the 

 scales often appear to have been detached when in reality they are only 

 hidden by the pellicle which covers them; nor, so far as inspection with 

 the naked eye goes, is there anything to suggest that the disease, in its 

 most advanced form, is anything but a sloughing ulceration of the skin. 

 But, when the papyraceous substance which constitutes the apparent 

 slough is subjected to microscopic examination, it proves to be some- 

 thing totally different from mere dead tissue of the fish, such as a true 

 slough would be. In fact, the comparison with wet paper turns out to 

 be more exactly correct than might have been anticipated ; for, like wet 

 paper, it is chiefly composed of a felted mass of vegetable filaments, 

 intermixed with which are debris of the tissues of the skin of the salmon 

 and all sorts of accidental impurities ; especially shells of Diatoms and 

 multitudes of very minute sand grains, derived from the water in which 

 the salmon swim. The filaments vary in thickness from ^^o of an inch 

 to 3 oVo of an inch, the majority lying between 30V0 ^^^^ 20V0 of an inch. 

 Each filament is tubular, composed of a thin wall, which contains cellu- 

 lose, or the essential proximate principle of wood, lined bj^ a thicker or 

 thinner layer of finely granular protoplasm, within which, again, is a 

 watery fluid. The whole filament is colorless and usually transparent, 



