Diseases of Fishes 



353 



back into the water, and if no opportunity be lost of killing 

 those sharks which feed on the food-fishes, two sources of the 

 prevalence of parasites would be affected and the sum total of 

 parasitism diminished. These remarks are made not so much 

 because such precautions are needed as to suggest possible 

 applications of knowledge which is already available." 



Parasitic Fungi. Fishes are often subject to wounds. If 

 not too serious these will heal in time, with or without scars. 

 Some lost portions may be restored, but not those including 

 bone fin-rays or scales. In the fresh waters, wounds are usu- 

 ally attacked by species of fungus, notably Saprolegnia ferox, 

 Saprolegnia mixta, and others, which makes a whitish fringe 

 over a sore and usually causes death. This fungus is especially 

 destructive in aquaria. This fungus is not primarily parasitic, 

 but it fixes itself in the slime of a fish or in an injured place, and 

 once established the animal is at its mercy. Spent salmon are 

 very often attacked by this fungus. In America the spent sal- 

 mon always dies, but in Scotland, where such is not the case, 

 much study has been given to this plant and the means by 

 which it may be exterminated. Dr. G. P. Clinton gives a 

 useful account of the development of Saprolegnia, from which 

 we take the following: 



" The minute structure and life-history of such fungous 

 forms have been so thoroughly made out by eminent specialists 

 that no investigation along this line was made, save to observe 

 those phenomena which might be easily seen with ordinary 

 microscopic manipulations. The fungus consists of branched, 

 hyaline filaments, without septa, except as these are found 

 cutting off the reproductive parts of the threads. It is made 

 up of a root-like or rhizoid part that penetrates the fish and a 

 vegetative and reproductive part that radiates from the host. 

 The former consists of branched tapering threads which pierce 

 the tissues for a short distance, but are easily pulled out. The 

 function of this part is to obtain nourishment for the growth 

 of the external parts. Prostrate threads are found running 

 through the natural slime covering the fish, and from these are 

 produced the erect radiating hyphae so plainly seen when in 

 the water. The development of these threads appears to be 

 very rapid when viewed under the microscope, although the 



