We pass now to the examination of monogenetic trematodes which are 

 indicated as parasitizing different families of fishes which, however, are related 

 to the same order. 



Five species of monogenetic trematodes are indicated as parasitizing four or 

 more species of fishes related to different families of the same order: Capsala 

 laevis (Verrlll), Diplozoon paradoxum Nordmann, Discocotyle sagittata (Leuckart), 

 Hexostoma grossum (Goto), and Microcotyle pomacanthi MacCallum. 



Capsala laevis is encountered on several species of Histiophorus and Tetrap- 

 turus (Histiophoridae) and then on Xiphias gladius (L. ) (Xiphiidae) and finally on p. 23i 



Coryphaena hippurus (L. ) (Coryphaenidae). All three families are undoubtedly i 



related to each other (Gregory places them in Scombrodei). 



Diplozoon paradoxum is known from more than 30 species of various Cyprinidae 

 and from several species of Cobitidae. Both these families are closely related. It 

 is true that in the literature there are still a number of data concerning the finding 

 of D^ paradoxum on different fishes of other families or even orders which were not 

 taken into consideration by us during the composition of Table 4. However, during 

 the verification (these were found to be, nobis) probably erroneous. Thus, in the 

 work of Wagener (Wagener, 1857a), the indication of the pike as a host of D^ para- 

 doxum is based on the finding of the egg and not the worm itself on the gills of Esox i 



lucius L. The finding on the Ling (given in the Russian as Burbot, Eelpout-- Lota , ! 



nobis) is indicated by Luhe (Luhe, 1909) was not substantiated by any other literary 

 indications. Concerning Cottus gobio L. --there are data in the work of Hausmann 

 (Hausmann, 1897) based apparently on accidental findings. Finally, the data about ' 



sticklebacks as hosts of D^ paradoxum cited by Sproston (Sproston, 1946) are clearly 

 incorrect. Numerous researchers in our laboratory and particularly in the Labora- 

 tory of Fish Diseases of VNIORH speak with certainty that D^ paradoxum does not 

 parasitize any fishes except representatives of the two above-mentioned families. 



1 

 Discocotyle sagittata is encountered undoubtedly in a number of species of the , 



genera of Salmo, Salvelinus and Coregonus (Salmonidae) and on Thymallus t hymallus (L. ) { 



(Thymallidae). Both families of hosts are undoubtedly closely related, and Thymallidae ; 



have only recently been separated from Salmonidae as an independent family (Suvorov, I 



1948). 



Hexostoma grossum was discovered on several representatives of Thunnidae and 

 was indicated for the carangid Seriola aurevittata Tem. and Sch. (=S^ quinqueradiata auct. ). 

 The latter arouses certain doubt in us, but if we consider that the data of Ishii and | 



Sawada (Ishii and Sawada, 1938a) are correct, they reflect the views on genetic links 

 or ties of Thunniformes with other Perciformes. Thus, the majority of ichthyologists 

 consider Thunnidae closely linked to Scombroidei (Scombridae, Cybiidae, 



254 



