304 ILLINOIS BIOLOGICAL MONOGRAPHS [304 



may infect suitable intermediate hosts. The latter, vertebrates or in- 

 vertebrates, may migrate or be carried by currents to farther limits. 

 New hosts may be infected and these on their migrations may carry the 

 infection beyond the original limits of distribution of the parasite. The 

 introduction by man of fish into new regions is a method of widening 

 the range of a parasite. It is suggested as a further possibility that 

 fish-eating birds at the time of migration may carry the eggs of the 

 parasites ingested with the fish for some distance and these eggs being 

 discharged with the feces into the water may infect suitable intermedi- 

 ate hosts. This method of dispersal could be possible only if the eggs 

 of the parasite thus eaten are capable of withstanding the vigorous 

 digestive action of the bird. Experiments would be required to deter- 

 mine the value of this method. 



Families op Fish from Which Proteocephalids are Known 

 The data on this subject are collected in the accompanying table. 

 It is there shown that Proteocephalids are known from fifteen families 

 of fish of the group Teleostomi and representing the orders Crossop- 

 terygii, Chondrostei, Holostei, and Teleostei. The writer follows the 

 classification of Bridge (1904) and Boulenger (1904), Vol. VII, Cam- 

 bridge Natural History. Of the fifteen families ten belong to the order 

 Teleostei. The latter contains by far the larger number of fresh water 

 fishes. Proteocephalid species are as yet unknown from the Dipnoi. A 

 comparison of the collection records and of the accompanying table 

 shows that certain species of parasite seem to be characteristic of cer- 

 tain families of fish. Those ,cases in which the parasite has been recorded 

 but once or twice probably should not be considered as characteristic 

 of their host families. Among such cases are to be mentioned the occur- 

 rence of P. pentastoma in the Polypteridae, P. skorikowi in the Acipen- 

 seridae, P. singularis in Lepisosteidae, etc. There are certain species 

 which have been recorded several times and the majority of these 

 times from a certain family of fish. Such species are to be considered 

 as being characteristic for the family. The species which fall in this 

 group are: P. longicollis and P. exiguus, characteristic of Salmonidae; 

 P. torulosus of the Cyprinidae ; P. macrocephalus of Anguillidae ; P. 

 pinguis of the Esocidae; P. fUicollis of Gasterosteidae ; P. ambloplitis 

 of Centrarchidae ; P. percae and P. cernuae of the Percidae. The re- 

 maining species are too little knoA\Ti to justify an attempt to determine 

 this relationship. 



