438 
sluggish, seeking the shallows near the banks of the river, 
where they finally die. 
The flesh of a salmon affected by this disease presents 
no difference in texture or colour from that of a healthy 
fish ; and those who have made the experiment declare 
that the flavour is just as good in the former case as in 
the latter. So far as my observations have gone the 
viscera may be perfectly healthy in the most extensively 
diseased fish ; and there is no abnormal appearance in 
the blood. 
It is known that a disease similar to that described is 
occasionally prevalent among salmon in North America 
and in Siberia ; and I do not see any ground for the sup- 
position that it is a novelty in British rivers. But public 
attention was first directed to it in consequence of its 
ravages in the Solway district a few years ago; and, in 
1878, a Commission was appointed to inquire into the 
subject. 
The evidence taken by the Commissioners leaves no 
room for doubt that the malady is to be assigned to the 
large and constantly increasing class of diseases which 
are caused by parasitic organisms. It is a contagious 
and infectious disease of the same order as ringworm in 
the human subject, mascardine among silkworms, or the 
potato disease among plants; and, like them, is the work 
of a minute fungus. In fact, the Safrolegnia which is 
the cause of the salmon disease is an organism in all 
respects very closely allied to the Peronospora, which is 
the cause of the potato disease. 
It is a very curious circumstance, however, that while 
the Peronospore are always parasites—that is to say, 
depend altogether upon living plants for their support— 
the Saprolegnie are essentially saprophytes; that is to 
say, they ordinarily derive their nourishment from dead 
animal and vegetable matters, and are only occasionally 
Parasites upon living organisms. In this respect they 
resemble the Zacéeria, if the results of recent researches, 
which tend to show that pathogenic bacteria are mere 
modifications of saprogenic forms, are to be accepted. 
As I have said, I do not think that the evidence laid 
before the Commission of 1878 can leave any doubt as to 
the causation of the salmon disease on the minds of those 
who are acquainted with the history of the analogous 
diseases in other animals and in plants. Nevertheless, 
this evidence, valuable as it is, suggests more questions 
than it answers, and in November, 1881, hearing that the 
disease had broken out in the Conway, I addressed myself 
to the attempt to answer some of these. 
It was already }nown that when the papyraceous 
slough-like substance which coats the skin of a diseased 
salmon is subjected to microscopic examination, it is 
found to be a mycelium, or fungus-turf, composed of a 
felt-work of fine tubular filaments or Ayfh@, many of | t 
| reason to believe that, as a saprophyte, it is confined to 
which are terminated by elongated oval enlargements, or 
soosporangia. Within these the protoplasm breaks up 
into numerous spheroidal particles, each less than I-2000th | 
of an inch in diameter. These, the zoospores, are set free 
through an opening formed at the apex of the zoosporan- 
gium, and become actively or passively dispersed through 
the surrounding water. Herein lies the source of the 
contagiousness or infectiousness of the disease. For any 
one of these zoospores, reaching a part of the healthy 
skin of the same or of another salmon, germinates and 
soon gives rise to a mycelium similar to that from which 
it started. 
But I could find no satisfactory information as to the 
manner in which the fungus enters the skin, how far it 
penetrates, the exact nature of the mischief which it does, 
or what ultimately becomes of it; nor was the identity 
of the pathogenic Safro/egnia of the salmon with that 
of any known form of saprogenic Sapro/egnia demon- 
strated. It appeared to me, however, to be useless to 
attempt to deal with the disease until some of these im- 
portant elements of the question were determined. 
W.ATORE 
———__—__—_—_—_; 
To this end, in the first place, 1 made a careful ex- 
amination of the minute structure of both the healthy and 
diseased skin, properly hardened and cut into thin sec- 
tions ; and, in the second place, I tried some experiments 
on tke transplantation of the Sapfro/egnia of the living 
salmon to dead animal bodies. Perhaps it will conduce 
to intelligibility if I narrate the results of the latter 
observations first. 
The body of a recently killed common house-fly was 
gently rubbed two or three times over the surface of a 
patch of the diseased skin of a salmon, and was then 
placed in a vessel of water, on the surface of which it 
floated, in consequence of the large quantity of air which 
a fly's body contains. In the course of forty-eight hours, 
‘or thereabouts, innumerable white cottony filaments made 
their appearance, set close side by side, and radiated from 
the body of the fly in all directions. As these filaments 
had approximately the same length, the fly’s body thus 
became inclosed in a thick white spheroidal shroud, 
having a diameter of as much as half an inch. As the 
filaments are specifically heavier than water, they gradu- 
ally overcome the buoyancy of the air contained in the 
tracheze of the fly, and the whole mass sinks to the 
bottom of the vessel. The filaments are very short when 
they are first discernible, and usually make their appear- 
ance where the integument of the fly is softest, as between 
the head and thorax, upon the proboscis, and between the 
rings of the abdomen. These filaments, in their size, 
their structure, and the manner in which they give rise to 
zoosporangia and zoospores are precisely similar to the 
hyphz of the salmon fungus; and the characters of the 
one, as of the other, prove that the fungus is a Saprvo- 
fegnia and not an Achlya. Moreover, it is easy to obtain 
evidence that the body of the fly has become infected by 
spores swept off by its surface when it was rubbed over 
the diseased salmon skin. These spores have in fact 
germinated, and their hyphz have perforated the cuticle 
of the fly, notwithstanding its comparative density, and 
have then ramitied outwards and inwards, growing at the 
expense of the nourishment supplied by the tissues of the 
fly. 
This experiment, which has been repeated with all 
needful checks, proves that the pathogenic Sufrolegnia 
of the living salmon may become an ordinary saprogenic 
Saprolegnia,; and, per contra, that the latter may give 
rise to the former; and they lead to the important 
practical conclusion that the cause of salmon disease may 
exist in all waters in which dead insects, infested with 
Saprolegnia, are met with; that is to say, probably in 
all the fresh waters of these islands, at one time or 
another. 
On the other hand, Safvolegnia has never been ob- 
served on decaying bodies in salt water, and there is every 
fresh waters.* 
Thus it becomes, to say the least, a highly probable - 
conclusion that we must look for the origin of the disease 
to the Safrolegnie@ which infest dead organic bodies in 
our fresh waters. Neither pollution, drought, nor over- 
stocking will produce the disease if the Safvolegnia is 
absent. The most these conditions can do is to favour 
the development or the ‘diffusion of the matertes morbi 
where the Saprolegnia already exists. 
Having infected dead flies with the salmon Safroleguia, 
once from 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 Safrolegnia. 
And having got thus far, I fancied it would be an easy 
task to determine the exact species of the Safrolegnia 
with which I was dealing, from the abundant data fur- 
| nished by the works of Pringsheim, De Barry, and others, 
* So far as I know there is only one case on record of the appearance of a 
fungus on a fish in salt water, and in this case it is not certain that the 
fungus was a Saf» clegnia. 
(March % 1882 
