328 SPENCER WALPOLE AND T. H. HUXLEY. 



with the nutrition of the tissues which they traverse; in fact, 

 their ramifications are often so close set that the proper 

 tissues of the superficial layer of the derma almost disappear. 

 Sooner or later, therefore, necrosis sets in, and then ulcera- 

 tive sloughing takes place, resulting in an open sore. No 

 doubt the morbid process thus described is accelerated and 

 intensified by the irritation caused by the innumerable small 

 grains of sand and other foreign bodies entangled by the 

 mycelium. But that the primary cause of all the mischief 

 is the parasitic fungus does not appear to be open to doubt. 

 If it were otherwise, the structural alteration of the skin 

 should precede the fungus, and not follow it, as it actually 

 does. 



In fact, the Saprolegnia is the cause of the salmon disease 

 exactly as the closely allied fungus, Peronospora, is the 

 cause of the potato disease. In symptoms, progress, and 

 results there is the closest analogy between the two maladies. 

 Peronospora, like Saprolegnia, gives rise to spores which 

 may be ciliated and actively locomotive, or may germinate 

 without passing through an active stage. When these spores 

 germinate on the surface of a healthy potato plant, their 

 hyphse perforate the walls of the cells with which they are 

 in contact, and then ramify, as a mycelium, in the inner 

 substance of the plant, carrying destruction wherever they 

 go. The mycelium gives off hyphse which pass through the 

 stomates to the surface, and there they throw off abundant 

 spores, which repeat the process until the whole plant is 

 destroyed. Even the tubers are invaded ; but in them the 

 mycelium becomes quiescent on the approach of the winter 

 season, to break out again, in full vigour, if the tubers are 

 planted in the following spring. Moreover, there is as much 

 uncertainty about the occurrence of antheridia and oospo- 

 rangia, and of any sexual method of reproduction in the 

 Peronospora of the potato, as in the Saprolegnia while it 

 infests the salmon. 



There is a great deal of reason to believe that the Sapro- 

 legjita growing on salmon is killed by salt water; and that 

 tlip injured skin may heal and become covered with a new 

 epidermis when a diseased salmon enters the sea. But the 

 discovery that the root-hyphse of the Saprolegnia ramify in 

 the derma where the sea water cannot reach them, raises a 

 curious and important question. It becomes possible that a 

 diseased salmon returning to the sea may regain a healthy 

 epidermis and appear perfectly sound; but that, like a 

 potato-tuber invaded by Peronospora just before the approach 

 of winter, the fungus in the derma may simply lie dormant, 



