Ward. — Fish Parasites and Parasitic Diseases 225 



The external crustaceous parasites of fish can all be 

 easily and readily determined by reference to the splendid 

 scries of recent studies on this group by C. B. Wilson. As 

 the older accounts are important on account of their data 

 concerning effects on fish they also are given in the list of 

 papers although they contain imperfect and sometimes 

 erroneous statements concerning the parasite and in all 

 cases reference should also be made to the later papers by 

 Wilson. Especial mention should be made of two papers 

 (Wilson, 1902 and 1911b), which contain very complete 

 synopses of the fish lice and the most frequent gill parasites 

 and hence are of special value to the fish culturist seeking 

 knowledge concerning forms of these types. Reference to 

 the original papers is necessary if it is desired to determine 

 accurately the species present. Even in such thoroughly 

 studied groups it is too much to expect that the record is 

 complete or that no other species than those listed are to be 

 found on our fresh-water fish. The young of these species 

 are free-swimming and seek out new hosts by active migra- 

 tion through the water. Tt is clear then, that small active 

 plankton-feeding fish will tend to keep down the number of 

 these parasites by destroying the young during this active 

 stage. 



