384 Prof. T. H. Huxley. Pathology of the [Mar. 2, 



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, innumer- 

 able 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 inclosed 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 tracheae 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 integument of the fly 

 is softest, as between the head and thorax, upon the proboscis, and 

 between the rings of the abdomen. These filaments, in their size, their 

 structure, and the manner in which they give rise to zoosporangia and 

 zoospores are precisely similar to the hyphee 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 salmon skin. 

 These spores have in fact germinated, and their hyphae have perfo- 

 rated 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 Jeast, a highly probable conclusion that 

 we must look for the origin of the disease to the Saprolegnioe 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 develop- 

 ment or the diffusion of the materies morbi where the Saprolegnia 

 already exists. 



Having infected dead flies with the salmon Saprolegnia, once from 



* So far as I know, there is only one case on record of the appeai'ance of a fungus 

 on a fish in salt water, and in this case it is not certain that the fungus was a Sa- 

 prolegnia. 



