174 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



would seek the surface of the water, become weakened and sluggish in their movements, 

 and finally die. 



The exact pathogenic action of the parasite on the fish I am unable at the present 

 moment to explain, for although I examined a number of fish after death — many of 

 which were considerably decayed before I could study them — I found no one gross 

 lesion or set of lesions common to them all, other than the general injury to the epider- 

 mis and the enormous amount of slime present on the body and over the gills. I am 

 convinced that the parasite is not a harmless object and that the mortality of the fish 

 was not due solely to the high temperature of the water, for had this been the case 

 fish in uninfested tanks would have been affected as well as those in the infested 

 aquaria. Furthermore, even if one were inclined to claim that the temperature was 

 the chief factor in the disease, he would be confronted by a fact which would be 

 difficult to explain on that theory, *. e., that the young catfish seemed to succumb more 

 easily than the trout, although able to live in much warmer water. On the other 

 hand, as the catfish were more seriously infested than the trout, the higher mortality 

 of the former is more easily understood by assuming a pathogenic property of the 

 protozoa in question. 



Accordingly, while the specific pathogenic action still remains undetermined, I 

 think there can be no reasonable doubt that this parasite contributed largely to the 

 cause of the losses sustained by the aquarium. 



The special problem before me was to find an inexpensive solution which would 

 kill the parasites, but in which the fish could live. This in itself was not a difficult 

 matter, for several solutions were soon made which were absolutely fatal to the pro- 

 tozoa when liberated from the fish, and in which the fish were able to live for several 

 hours or even for several days. Upon applying these solutions to practical use, how- 

 ever, it was found that they had absolutely no effect upon the parasites as long as the 

 latter remained in the epidermis or in the slime. Two methods of experimentation 

 were then open: first, to find a solution which would cut or remove the layer external 

 to the parasite and thus act as a carrier of the germicide; or, secondly, to study the 

 life-history of the parasite in the hope of discovering some stage of its development 

 during which it lived free in the water, then to kill this stage and thus prevent a 

 reinfection of the fish. 



While the details of these experiments* will be given below (cf. pp. 180-189), it may 

 be stated here that the last-mentioned method was the only one which was found 

 feasible. The three solutions which gave the best results were salt water, methylen 

 blue, and eosine. To fully understand the action of these liquids, it will be necessary 

 to give a description of the parasite and the details of its life-history. 



* These investigations are to be looked upon as field experiments, rather than as minute researches 

 into the exact effect which the various solutions mentioned below have upon the fish, as they were 

 carried on in an extemporized laboratory, and with a view to meeting the epizootic at hand rather 

 than placing on record details and minutiae as to the exact number of seconds that various species of 

 fish or the parasites could live in different percentages of strength of various disinfectants. Owing 

 to the fact that I was obliged to make the experiments upon the grounds of the Exposition, it was 

 necessary for me to borrow my instruments from the different Government exhibits at the fair, and it 

 is with pleasure that I acknowledge here my indebtedness to Dr. Wiley and Mr. Fairchild for placing 

 various instruments at my disposal; Dr. Kinyoun, of the U. S. Marine Hospital Service; Dr. Lagard, 

 of the U. S. Army Hospital; John Wyeth »fc Bro., chemists, Philadelphia, and Merck & Co., New 

 York, kindly furnished me with various chemicals, thus saving much trouble and expense. 



