Diseases of Fishes 343 



always cream-white. In size and form they vary greatly. 

 The cyst in which they lie is filled with creamy substance 

 made up of spores and granual matter. 



Dr. Gurley enumerates as hosts of these parasites about 

 sixty species of fishes, marine and fresh-water, besides frogs, 

 crustaceans, sea-worms, and even the crocodile. In the sharks 

 and rays the parasites occur mainly in the gall-ducts, in the 

 minnows within the gill cavity and epidermis, and in the higher 

 fishes mainly but not exclusively in the same regions. Forty- 

 seven species are regarded by Gurley as well defined. The 

 diseases produced by them are very obscurely known. These 

 parasites on American fishes have been extensively studied by 

 Charles Wardall Stiles, Edwin Linton, Henry B. Ward, and 

 others. 



According to Dr. Linton the parasitism which results from 

 infection with protozoan parasites will, of all kinds, be found to 



w. 



FIG. 228. White Shiner, Notropis htidsonius (Clinton), with cysts of parasitic 

 psorosperms. (After Gurley.) 



be the most important. Epidemics among European fish have 

 been repeatedly traced to this source. The fatality which 

 attends infection with psorosperms appears to be due to a sec- 

 ondary cause, however, namely, to bacilli which develop within 

 the psorosperms (Myxobolus) tumors and give rise to ulceration. 

 The discharge of these ulcers then disseminates the disease. 



" Brief mention of the remedies there proposed may appro- 

 priately be repeated here. Megnin sees no other method than 

 to collect all the dead and sick fishes and to destroy them by 

 fire. Ludwig thinks that the waters should be kept pure, and 

 that the pollutions of the rivers by communities or industrial 

 establishments should be interdicted. Further he says: 



