444 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



Peronospora just before the approach of winter, tliefnnj;iis in the derma 

 may simply lie dormant, and be ready to spring into activity as soon as 

 the fish returns to fresh water. Cases of the appearance of the disease 

 in quite fresh-run fish are occasionally reported, which would be readily 

 explicable should this supposition turn out to be well fouuded. 



Another possibility was suggested by the same fact. We know that 

 the spores of the Enqnisa, a fungus which attacks living flies, germinate 

 and bore tlirough the cuticle in much the same fashion as the SaproJegnia 

 enters dead flies. But the hypha of the Empusa, which has thus entered 

 the fly, immediately breaks up into short Joints, which diffuse them- 

 selves through the body of the fly and everywhere multiply by division, 

 until they have appropriated all the nutritious matters which are avail- 

 able to them. It was therefore justiliable, on analogical grounds, to 

 suppose that the hypluie of Sojrrolefjnia, which had entered the derma 

 of a salmon, might break up in a similar way; and that the segments 

 might be conveyed through the lymphatic and blood vessels into all 

 parts of the body, and either produce blood poisoning by a septic fer- 

 mentative action, or develope centres of obstruction by lodgment in the 

 narrower channels of the vascular system. However, there is no evi- 

 dence to justify this suspicion. The liyphne in the derma show no signs 

 of division, nor have any toruloid bodies, or other structures that can be 

 regarded as derivatives of Saprolegfiia, been observed, either in the 

 blood or in any of the viscera. 



The salmon disease, in fact, appears to be a purely cutaneous affec- 

 tion ; and the fish seem to die partly from irritation and consequent 

 exhaustion, and partly, i)erhaps, from the drain on their resoui-ces, 

 caused by the production of so large a mass of vegetable matter ift their 

 expense. 



The opportunities for the investigations, the chief results of which 

 have now been detailed, have arisen only during the last three or four 

 months ; and a great deal more time and attention must be devoted to 

 the subject before it can be expected that many of the obscurities and 

 ditficulties which still hang about it can be cleared up. 



It is neeedful to discover the conditions under which the fungus ex- 

 ists in those rivers which are infested by the disease when the full- 

 grown salmon have deserted them ; whether it lingers in isolated cases 

 among the parr, trout, or the non-salmonoidfish ; or whether it contents 

 itself with the bodies of dead insects, and other dead animal, and per- 

 haps vegetable substances ; or whether, in the late summer, oosporangia 

 may not be formed and give rise to oospores, which, as De Bary's ex- 

 periments show, may have a dormant period of three or four months ; 

 that is to say, sufficient to i)reserve them till the next return of the 

 salmon. 



On all these points, persons conversant with the use of the microscope, 

 who are resident in the neighborhood of salmon streams, might obtain 

 information of great value, hardly to be procured in any other way. 



