314 SPENCER VVALPOLE AND T. H. HUXLEY. 



No morbid appearances are discoverable either in the viscera 

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

 they may exhibit jiisfc 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 they 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 suffi- 

 cient to enable it to extend over the whole body of a large 

 salmon. 



In the early stages of the malady the peculiar appearance 

 of ihe parts of the skin afi'ected might readily be, and cer- 

 tainly 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 slough- 

 ing ulceration of the skin. But when the papyraceous 

 substance which constitutes the apparent slough is subjected 

 to microscopic examination, it proves to be something 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 g-^o of an inch to -g-oVo of ^'^ inch, the majo- 

 rity lying between -^-^x^-o ^"^ -aoW of an inch. Each filament 

 is tubular, composed of a thin wall, which contains cellulose, 

 or the essential proximate principle of wood, lined by a 

 thicker or thinner layer of finely granular protoplasm, within 

 which, again, is a watery fluid. The whole filament is 

 colourless and usually transparent, but sometimes the 

 granules are sufficiently numerous to render it opaque, and 

 then it looks white by reflected light. Sometimes the fila- 

 ments are simple as far as they can be traced; sometimes, 

 on the other hand, they are much branched ; but they never 

 exhibit any transverse partitions, the cavity of each filament 

 being continuous throughout. Wherever the free en''; of a 



