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 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 secondary 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. [Illustration: FIG. 228.--White Shiner, _Notropis hudsonius_ (Clinton), with cysts of parasitic psorosperms. (After Gurley.)] "Brief mention of the remedies there proposed may appropriately 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: "That most dangerous contamination of the water by the _Myxosporidia_ from the ulcers cannot of course be stopped entirely, but it is evident that it will be less if all fishermen are impressed with the importance of destroying all diseased and dead fish instead of throwing them back into the water. Such destruction must be so effected as to prevent the re-entry of the germs into the water. "Railliet says that it is expedient to collect the diseased fish and to bury them at a certain depth and at a great distance from the water-course. He further states that this was done on the Meuse with success, so that at the end of some years the disease appeared to have left no trace." [Illustration: FIG. 229.--White Catfish, _Ameiurus catus_ (Linnæus), from Potomac River, infested by parasitic protozoa, _Ichthyophthirus multifilis_ Fouquet. (After C. W. Stiles.)] =Parasitic Worms: Trematodes.=--Parasitic worms in great variety exist in the intestinal canal or in the liver or muscular substance of fishes.