105 



them. The two kinds of fungoid parasites may be termed entozoic and DR. M. C. 

 epizoic ; since these terms will indicate their distinguishing features, which COOKE. 



corresponds to the Endophytes and Epiphytes of plants. Those which we term 



entozoic, are fungi which enter the animals in the embryonic stage, develop 

 themselves in the interior, and exhibit a tendency outwards. Such a parasite 

 is the muscardine. The epizoic parasites are those which establish themselves 

 on the external surface of the animal, and by means of their root fibres force 

 themselves inwards. Examples of this kind are more rare. The entozoic 

 parasites are most undoubtedly the causes of the results which supervene upon 

 their development. The spores germinate in the interior, produce an abundant 

 mycelium, or mass of root fibres, ultimately absorbing and replacing the whole 

 of the internal organism of the animal, gradually converting it into a mass of 

 fungoid threads, and in the unequal struggle the animal at length quietly 

 succumbs. In the case of the silk worm moth the disease called muscardine 

 follows this course. In other instances insects become the prey of different 

 species of fungus called Torrubia which destroy the insect, and then burst 

 through and reach their ultimate and perfect state in large fleshy protuberances 

 on the outside. The fungus of the salmon disease has no points of similarity 

 with these entozoic parasites. The whole interior of the salmon is not converted 

 into a mass of fungus spawn before the fungus appears on the outside. There 

 is no evidence of the fungus originating from the interior at all. No indications 

 of its belonging to the kind which I have termed entozoic fungi. 



On the other hand epizoic fungi (or epiphytic when occurring on plants) 

 attach themselves in the embryonic stage on the outside, where their life is 

 commenced, and they become parasitic by forcing their root filaments through 

 fissures in the cuticle or skin, and thus become established. This is the case 

 with the fungus which developes itself on frog-spawn, on the ova of fish, on 

 car]) and other fish in aquaria, and also on salmon in this fatal disease. 

 Undoubtedly this Saprolegnia answers to all the characteristics of an epizoic 

 fungus. In this point it accords with some of the most destructive of plant 

 diseases, with the potato-disease, with the vine disease, with the hop mildew, 

 all of which are epiphytal, and I might enumerate many others. In a 

 recent lecture on the salmon disease the lecturer stated (Land and Water, 

 1880, pp.401), when speaking of the potato and vine diseases, that '"the 

 " fungus on those plants will not grow on those which are perfectly healthy, 

 " but that a disordered condition is necessary as a predisposing cause ; " and 

 again " it is contrary to anything that I have read or known for fungus to 

 " grow on either healthy animal or vegetable." These observations should 

 have been more qualified, because, under the ordinary acceptation of the term 

 " healthy," both vine and potato must be included, for the most healthy of 

 both are attacked. The fact is that continued cultivation, " over-culture," 

 has injured the constitution of both to such an extent that they do not possess 

 sufficient force to overcome the attacks of the parasite. Strong plants when 

 attacked would repel the invader ; the weak must succumb. The essential 

 element of successful attack of epiphytic fungi on plants is a debilitated 

 constitution caused by over-cultivation. How far similar causes are in operation 

 with salmon will have to be alluded to hereafter. For the present it may be 

 remarked that the same fungus is not so successful, or so destructive, to plants 

 of the same species in a low state of cultivation, or uncultivated, as when 

 under high cultivation. 



Causes. — One of the alleged causes of the salmon disease has been referred 

 to pollution of the rivers. This may easily be dismissed on faith of the 

 evidence that it is not confined to polluted streams. But the term needs defi- 

 nition. What is meant by polluted? If it only means water unfit for 

 drinking purposes ; that is not pollution as far as inferior animals is concerned. 

 Horses and domestic cattle always drink the stagnant water of a horse pond 

 in preference to pure water, and yet the horse pond swarms with low forms of 

 animal and vegetable life. That they do prefer it is an undoubted fact, and 

 that their instinct is a better guide than our reason, as to what is best for them, 

 may be taken for granted. The entomostraca, or minute crustaceans, which 

 abound in streams and ponds, constitute in large measure the food of fish ; 

 and minute aquatic plants also contribute. Doubtless the presence of living 

 animal and vegetable organisms in water is not pollution in the estimation of 

 fish. Whether decaying animal or vegetable matter is injurious may be an open 



