SALMON DISEASE. 123 



important as showing that even large and unpolluted rivers 

 afforded but little immunity. Up to the present time the disease 

 had shown itself to be remarkably local ; for the Tweed, Nith, 

 Annan, Doon, Esk, and Eden were marked from all other rivers 

 in this country. In the south, the Severn and Wye, both famous 

 salmon rivers, had been up to now quite free from any trace of 

 the disease. 



It seemed to be agreed that the salmon disease was due to 

 a fungus which grew upon the cutaneous textures or skin of 

 the fish ; and not only upon salmon but upon many other kinds 

 of less important fish, the fungus was also developed in aquaria. It 

 resembled dirty cotton wool, and might be seen on all parts of 

 the fish, the fins most especially seeming to afford a suitable soil 

 for growth. Attacking the head, the fungus might extend so as to 

 cause blindness, and it might extend about the gills so as to cause 

 suffocation. It also extended into the mouth, and in some cases 

 so as to lay the bones quite bare, and occasionally causing 

 inflammation of the brain lying beneath. There was hardly any 

 part of the body of the fish which was not liable to be attacked, 

 but the parts first affected were the softer portions of the body 

 which had no scales. The effect of the disease was clearly to 

 cause pain and irritation. The fish knocked itself about, and 

 skimmed along the surface of the water, possibly to rub off the 

 fungus, or relieve the irritation. Death was brought about by 

 suffocation and the destruction of the natural function of the skin. 

 He next described the fungus as seen under the microscope. On 

 taking a portion of the fungus and allowing it to spread itself 

 out in a drop of water, it would be seen that there were 

 numerous threads spreading in all directions, interlacing and 

 joining each other, and to appearance they were colourless. The 

 base of the disease consisted of a network of similar threads, 

 which extended like the roots of a plant. The growth was 

 exceedingly rapid, and when the filaments were mature they bore 

 fruit which consisted of zoospores, rounded bodies consisting of 

 protoplasm endowed with movement, whose chief object seemed 

 to be to escape from the tube which contained them. They 

 might be seen working up and down a tube until they escaped 

 one by one from an opening at the summit, and, when free, 

 dashed away and formed tubes for themselves. In escaping from 

 the mouth of the tube, these zoospores shaped themselves to the 

 size of the opening and then regained their former rotundity. 

 Dr. Cooke spoke of them as having a pair of threads, which are 

 used as oars for propulsion. He had not seen these legs, but he 

 had little doubt that they were analogous to the cilia which they 

 were familiar with in the oyster and other lower forms of life found 



