ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 539 



transplantation of the Saprolegnia of the living salmon to dead 

 animal bodies. 



The body of a recently killed common house-fly was gently rubbed 

 two or three times over the surface of a patch of the diseased skin of 

 a salmon, and was then placed in a vessel of water, on the surface of 

 which it floated, in consequence of the large quantity of air which a 

 fly's body contains. In the course of forty-eight hours, or thereabouts, 

 innumerable white cottony filaments made their appearance, set close 

 side by side, and radiated from the body of the fly in all directions. 

 As these filaments had approximately the same length, the fly's body 

 thus became enclosed in a thick white spheroidal shroud, having a 

 diameter of as much as half an inch. As the filaments are specifically 

 heavier than water, they gradually overcome the buoyancy of the air 

 contained in the tracheas of the fly, and the whole mass sinks to the 

 bottom of the vessel. The filaments are very short when they are 

 first discernible, and usually make their appearance where the in- 

 tegument of the fly is softest, as between the head and thorax, upon 

 the proboscis, and between the rings of the abdomen. These fila- 

 ments, in their size, structure, and the manner in which they give rise 

 to zoo-sporangia and zoospores, are precisely similar to the hyphte of 

 the salmon fungus ; and the characters of the one, as of the other, 

 prove that the fungus is a Saprolegnia and not an Achlya. Moreover, 

 it is easy to obtain evidence that the body of the fly has become 

 infected by spores swept off by its surface when it was rubbed over 

 the diseased skin. These spores have in fact germinated, and their 

 hyphse have perforated the cuticle of the fly, notwithstanding its 

 comparative density, and have then ramified outwards and inwards, 

 growing at the expense of the nourishment supplied by the tissues of 

 the fly. 



This experiment, which has been repeated with all needful checks, 

 proves that the pathogenic Saprolegnia of the living salmon may 

 become an ordinary saprogenic Saprolegnia ; and, per contra, that the 

 latter may give rise to the former, and they lead to the important 

 practical conclusion that the cause of salmon disease may exist in all 

 waters in which dead insects, infested with Saprolegnia, are met with ; 

 that is to say, probably in all the fresh waters of these islands, at one 

 time or another. 



On the other hand, Saprolegnia has never been observed on 

 decaying bodies in salt water, and there is every reason to believe 

 that as a saprophyte, it is confined to fresh waters. 



Thus it becomes, to say the least, a highly probable conclusion 

 that we must look for the origin of the disease to the Saprolegnia^ 

 which infest dead organic bodies in our fresh waters. Neither 

 pollution, drought, nor overstocking will produce the disease if the 

 Saprolegnia is absent. The most these conditions can do is to favour 

 the development or the diffusion of the materies morbi where the 

 Saprolegnia already exists. 



The results of the last season's observations on the salmon disease 

 appear to justify the following conclusions : — 



1. That the Saprolegnia attacks the healthy living salmon exactly 



