THE DISTRIBUTION AND FREQUENCE OF 

 ANIMAL PARASITES AND PARASITIC DIS- 

 EASES IN NORTH AMERICAN FRESH- 

 WATER FISH.* 



By Henry B. Ward 



In his valuable work on fish diseases, Hofer (1906) dis- 

 tinguishes two types of general infectious diseases : those 

 caused by bacteria and those produced by Sporozoa. Special 

 diseases of individual organs he classifies as due to mechani- 

 cal causes, to chemical influences, to disturbances of nutri- 

 tion, and as parasitic diseases either of plant or animal origin. 

 Under the last heading — parasitic diseases of particular or- 

 gans due to animal parasites — he differentiates further 

 those due to Protozoa, to worms, and to Crustacea of various 

 sorts, noting the occurrence of several hundred different 

 animal parasites in European fish and recording more than 

 one hundred specific diseases which are due to their action. 

 This work is of great value both to the practical fish cul- 

 turist and to the scientific student. It stands, however, alone 

 as a general work in this field and the only information 

 available in English consists of widely scattered notes in 

 works on other topics or of brief special papers published in 

 journals often generally inaccessible. In either case -it- 

 is difficult to trace these items and still more difficult 

 to secure them, so that usually one is unable to make any use 

 of the material they contain. 



Two years ago Dr. T. H. Bean read before this Society 

 a paper entitled, "A Plea for the Systematic Study of 

 Fish Diseases." He emphasized our scanty knowledge of 

 the cause and treatment of fish diseases and the inadequate 

 allotment for scientific inquiry in such lines compared to the 

 economic value of the researches. He added a brief and 



*Contributions from the Zoological Laboratory of the University 

 of Illinois, No. 13. 



207 



