SAPROLEGNIA IN RELATION TO THE SALMON DISEASE. 321 



case of the potato disease, Peronosjiora infestans is different 

 from all the species of Peronospora which abound upon 

 European wild plants, and will not live on them, any more 

 than these other species will live on the potato. 



However this may be, it is easily proved that the Sapro- 

 legnia is not dependent on living salmon. In fact, if a patch 

 of diseased skin is cut off and placed in a vessel of water, 

 it will be found, in twenty-four hours, to be covered with 

 a new growth of young hyphae, close set and of nearly equal 

 lengths, so that the surface resembles a miniature cornfield, 

 A piece of the diseased membranous valve of the mouth of 

 a salmon was placed in water on the 4th of March, 1882 ; 

 on the 6th it was covered vvith young hyphae one fifth of an 

 inch long; and on the 7th these had elongated and de- 

 veloped multitudes of zoosporangia. 



Moreover, there is not the least difficulty in proving that 

 the salmon Saprolegnia is not dependant on salmon at all, 

 but that it is capable of living on dead insects and pieces 

 of wet bladder. If a recently killed fly is gently rubbed 

 two or three times either over a fresh patch of diseased 

 salmon skin, or over one which has developed the fresh 

 growth just mentioned, and then placed in a vessel of water 

 by itself, it will be found, in the course of eight-and-forty 

 hours, to be more or less extensively beset with short, delicate, 

 cottony-looking filaments, which rapidly increase in length 

 and in number until, at last, the fly's body is enclosed 

 within a spheroidal coat half an inch in diameter. These 

 filaments are hyphae having exactly the same size, form, and 

 structure as those of the salmon Saprolegnia; their ends 

 give rise to zoosporangia of the same character ; and these 

 produce zoospores of the same size, which germinate in the 

 same way. 



Between December, 1881, and April of the present year, 

 repeated experiments of this kind have been made with 

 diseased salmon from the Conway, the Tweed, and the 

 North Esk, upon dead flies and small pieces of wet bladder, 

 always with the same result. There appears, therefore, to 

 to be no doubt that the Saprolegnia of the salmon, like 

 other Saprolegniee, is capable of living and flourishing on a 

 variety of dead animal matters. 



When the Saprolegnia is established on one such sub- 

 stance it is easy to transmit it to another. The Saprolegnia 

 obtained from diseased salmon was thus cultivated for many 

 weeks (from the end of December, 1881, to the first week in 

 April, 1882) in the hope of obtaining the oosporangia and 

 thus identifying it with one or other of the described forms 



