344 



Diseases of Fishes 



"That most dangerous contamination of the water by the 

 Myxosporidia from the ulcers cannot of course be stopped en- 

 tirely, 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 watercourse. He further states that this was done 



FIG. 229. White Catfish, Ameiurus catus (Linnaeus), from Potomac River, infested 

 by parasitic protozoa, Ichthyophthirus'muUifilis Fouquet. (After C. W. Stiles.) 



on the Meuse with success, so that at the end of some years the 

 disease appeared to have left no trace." 



Parasitic Worms : Trematodes. Parasitic worms in great 

 variety exist in the intestinal canal or in the liver or muscular 

 substance of fishes. 



Trematode worms are most common in fresh-water fishes. 

 These usually are sources of little injury, especially when found 

 in the intestines, but they may do considerable mischief when 

 encysted within the body cavity or in the heart or liver. Dr. 

 Linton describes 31 species of these worms from 25 different 

 species of American fishes. In 20 species of fishes from the 

 Great Lakes, 102 specimens, Dr. H. B. Ward found 95 speci- 

 mens infected with parasites, securing 4000 trematodes, 2000 

 acanchocephala, 200 cestodes, and 200 nematodes. In the 

 bowfin (Amia calva), trematodes existed in enormous numbers. 



Cestodes. Cestode worms exist largely in marine fishes, 

 the adults, according to Dr. Linton, being especially common 

 in the spiral valve of the shark. It is said that one species of 



