1882.] 



Epidemic known as the " Salmon Disease." 



385 



Conway and once from Tweed fish,* I was enabled to propagate it from 

 these flies to other flies, and, in this manner, to set up a sort of garden 

 of Saprolegnice. And having- got thus far, I fancied it would be an 

 easy task to determine the exact species of the Saprolegnia with which 

 I was dealing, from the abundant data furnished by the works of 

 Pringsheim, De Bary, and others, who have so fully studied these 

 plants when cultivated on the same materials. For this purpose, it 

 was necessary to obtain the oosporangia ; and, in ordinary course, these 

 should have made their appearance on my Saprolegnice in five or six 

 days. Unfortunately, in the course of cultivations continued over two 

 months, nothing of the kind has taken place. Zoosporangia have 

 abounded in the ordinary form and also in that known as " dictyo- 

 sporangium," but, in no instance, have any oosporangia appeared. After 

 a few days of vigorous growth, the zoosporangia become scanty, and 

 the fungus takes on a torulose form, segments of the hypha3 becoming 

 swollen and then detached as independent " gemmee," wdiich may 

 germinate. Sometimes the gemmas are spheroidal and terminal, and 

 closely simulate oosporangia. 



Although, therefore, I have very little doubt that the Saprolegnia of 

 the salmon is one of the forms of the " S. ferax group " of Pringsheim 

 and De Bary, I have, at present, no proof of the fact. 



Another very carious and unexpected peculiarity of the salmon 

 Saprolegnia, both on the fish and when transmitted to flies, so far as 

 my observations have hitherto gone, is that locomotive ciliated 

 zoospores do not occur. I once saw one which exhibited a very slight 

 motion for a few minutes after it left the zoosporangium ; but 

 although thousands must have passed under my notice, with the ex- 

 ception to which I have referred, they have always been perfectly 

 quiescent and not unfrequently in different stages of germination. 

 Whether the season of the year, or the conditions under which my 

 saprolegnised flies were placed, have anything to do with the non- 

 appearance of oosporangia and of locomotive zoospores in them I 

 cannot say. But it is certain that the Saprolegnia ferax which com- 

 monly appears upon dea,d flies and other insects normally develops 

 both oosporangia and locomotive zoospores in abundance. 



From such notices by other observers as I can gather, oosporangia 

 appear to be of very rare occurrence in the Saprolegnia of the salmon 

 itself. Mr. Stirling mentions that he has met with them only four 

 times. With respect to locomotive zoospores, I can find no positive 

 evidence that they have been regularly, or even frequently, observed 

 in the salmon Saprolegnia. But these points require careful investi- 

 gation on freshly taken diseased fish. 



Whether the zoospores are actively locomotive or not, they are quite 



* And since this paper was read once more from the North Esk fish. (March 8, 

 1882.) 



