168 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 
DISEASES AND PARASITES OF FISHES. 
Fishes are subject to a variety of diseases, some of which are local 
and unimportant, having little or no effect upon the general condition 
of the fish, while others assume a very serious character, becoming even 
epidemic in their course and causing the mortality of immense numbers 
of individuals. But little attention has been paid to the nature or 
pathology of such diseases or to their treatment, and an important 
field is, therefore, open for investigations in this direction. It is well 
known that a large percentage of the disorders which have been ob- 
served among fishes is due to parasitism of one sort or another, and as 
biologists have long been interested in the natural history of the para- 
sites so concerned, much information has been secured regarding the 
structure and the development of these organisms. This branch of 
research must, in fact, precede the more special consideration of the 
relations of the parasites to their hosts and their effects upon the 
latter; but the time has come when not only the amount of material 
collected seems ample to begin upon a full investigation of this more 
practical phase of the subject, but also when the demands for accurate 
information regarding it have become sufficiently urgent to necessitate 
its being taken up without delay. Dr. Revere R. Gurley has recently 
been assigned to the study of these problems. 
The parasites which infest fishes belong to both the animal and 
vegetable kingdoms, and some fishes even are parasitic on others. The 
groups of crustaceans and worms furnish the greatest variety of known 
parasitic forms, the former occurring generally on the exterior of the 
body or in the mouth cavity, the latter in or among the viscera and in 
the tissues. A majority of these, however, seem to be entirely harm- 
less, but many produce a diseased condition of greater or less extent, 
and some at least must eventually prove fatal. The protozoan para- 
sites, called psorosperms, give rise to large excrescences on the exterior 
of the fish, making it very unpresentable in appearance, and undoubt- 
edly soon causing death. Among fresh-water fishes most harm is 
probably effected by low forms of plants, which often result in a very 
widespread mortality, as in some of the large northern lakes. Their 
attacks are not confined to the adults, but extend also to the younger 
stages and the embryos as well as to the eggs, and in the artificial 
hatching apparatus they offen cause much destruction. A great mor- 
tality also occurs among fishes, which has not been traced to parasit- 
ism, and of the true nature of which we are stillignorant. One instance 
of this character is noticed elsewhere in this report, in the account 
of Prof. 8S. A. Forbes’s observations at Lake Mendota, Wisconsin. 
The long-continued investigations by the Fish Commission on the 
Atlantic coast of the United States, especially in the vicinity of Woods 
Holl, Mass., have afforded the means of collecting and studying the 
crustacean and worm parasites of marine fishes under exceptionally 
