440 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



The fish do not appear to be iucoiiveuienced, and the fungus is very 

 easily washed off. In a case of this kind which we recently examined, 

 the fungus was a Saprolegnia, the mycelium and the zoosporangia of 

 which were altogether indistinguishable from those of the salmon fun- 

 gus. Moreover, the hyphse burrowed in the epidermis and distorted 

 the cells with which they came in contact in just the fashion described 

 below (p. 442). As it was not desirable to kill the fish, it was impossible 

 to determine whether the derma was penetrated or not ; but the ab- 

 sence of sores, and the ease with which large flakes of epidermis, in 

 which the Sa2)rolcfl)iia was rooted, could be detached, lead to the con- 

 clusion that the Saprolegnia had not i>enetrated beyond the epidermis. 

 The zoosporangia of the Saprolegnia taken from the fish emitted actively 

 locomotive zoospores, but no oosporangia could be detected. 



Dead flies, infected with this Saprolegnia on the ISth of March, 1882, 

 yielded an abundant growth, quite similar to that obtained in the same 

 way from the salmon Saprolegnia ; and on the 24th, that is in six days, 

 the characteristic oosporangia and antheridia of Saprolegnia ferax [mo- 

 noica) made their appearance. 



It appears, therefore, that Saprolegnia ferax is callable of attacking a 

 great variety of fishes during life, but that the concomitant pathologi- 

 cal phenomena differ in different fishes. 



Mr. Stirling's experiments on the transmissibility of the salmon fun- 

 gus to other fish yielded onl^^ negative results. Diseased salmon skin 

 was pat into a vessel containing minnows, which nibbled the skin, and 

 were none the worse. Experiments of this rough and ready sort, how- 

 ever, really i^rove nothing ; and a great deal of light will assuredly be 

 thrown upon the whole question of the salmon disease by carefully con- 

 ducted experimental investigations. 



At the present moment, we possess evidence that at least three dis- 

 tinct affections of the skin of fresh-\\ ater fishes have been confounded 

 together under the name of "Aquarium fungus." Oue of these is asso- 

 ciated with a Saprolegnia identical with that which attacks salmon ; 

 another is attended by the very closely allied fungus, Achlga; while the 

 third is not accompanied by the growth of any fungus, but is a very 

 curious morbid affection of the skin itself, apparently allied to epithe- 

 lioma. We have hitherto observed it only in carp, the head, body, and 

 fins of which sometimes appear covered with white patches, which pre- 

 sent a most deceptive resemblance to those caused by Saprolegnia^ the 

 more especially as the edges of the fins may be eroded, and ragged 

 fragments hang from the white patches. These patches, however, con- 

 tain no fungus, but result from the abnormal growth of the eindermis, 

 sometimes to eight or ten times its ordinary thickness, not unfrequently 

 accompanied by a corresponding elongation of the papillae of the tlerma. 



Having thus dealt with thequestion of the nature and affinities of the 

 fungus which is the constant concomitant of the " salmon disease," the 



