SAPROLEGNIA IN RELATION TO THE SALMON DISEASE. 325 



heen confounded together under tlie name of '^ Aquarium 

 fungus." One of these is associated with a Saprolcgnia 

 identical with that which attacks salmon ; another is attended 

 by the very closely allied fungus, Aclilya ; 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 epithelioma. We have hitherto observed it only in 

 carp, the head, body, and fins of which sometimes appear 

 covered with white patches, which present a most deceptive 

 resemblance to those caused by Saprolegnia, the more espe- 

 cially as the edges of the fins may be eroded, and ragged 

 fragments hang from the white patches. These patches, 

 however, contain no fungus, but result from the abnormal 

 growth of the epidermis, sometimes to eight or ten times its 

 ordinary thickness, not unfrequently accompanied by a corre- 

 sponding elongation of the papillae of the derma. 



Having thus dealt with the question of the nature and 

 affinities of the fungus which is the constant concomitant of 

 the '^ salmon disease,^' the next point for consideration is 

 the relation of the fungus to the affection of the skin. Is the 

 growth of the fungus the cause of that affection, or does the 

 fungus merely find a favorable nidus in the production of 

 the affection ? 



The SaprolegnicB, as we have seen, habitually grow on 

 dead animal and vegetable sul)stances ; and it is therefore a 

 fair supposition that some morbid affection may cause the 

 local death of the skin of the fish ; and that the fungus 

 simply implants itself in the dead tissue, as if it were the 

 dead body of a fly. 



On the other hand, our knowledge of the destructive epi- 

 demics caused by Empusa in flies, Botrytis in silkworms, and 

 Entomophthora in other caterpillars, and of the multifarious 

 fungi which produce bunt, smut, and mildew in plants, 

 affords at least equal ground for the supposition that the 

 ulceration and destruction of the skin are caused by the 

 invasion of healthy fish by the Saprolegnia. The decision 

 of this question is obviously of the greatest importance. 



Direct experimentation by infection of healthy salmon in 

 the manner in which dead flies were infected from the 

 diseased salmon being out of the question at present on 

 account of its practical difficulties, the only profitable way of 

 investigation lay in the study of the minute structure of the 

 healthy and of the diseased skin, so as to ascertain the exact 

 relation of the fungus to the morbid appearances. 



The skin of the salmon, like that of vertebrated animals in 

 general, consist of a superficial, cellular, non-vascular scarf- 



