The second group characteristic for Salmonoidei consists p. 3u8 



of representatives of the genera Gyrodactyloides and Discocotyle , i. e. , 



T 



The indication to the presence of Encotyllabe on Salmonidae is not 

 taken into consideration (see page 262 ). 



very heterogeneous, for both genera pertain to different subclasses. 

 All species of Gyrodactyloides without exception are encountered only 

 on Salmonoidei. As is further indicated, this genus is a derivative of 

 Gyrodactylus-like ancestors (see page 399), consequently to speak 

 about the antiquity of its parasitizing the Salmonddei is very difficult. 

 By origin it is undoubtedly a marine genus, which is confirmed by a 

 number of considerations (Bychowsky and Poljanski, 1953). With its 

 hosts it enters also into fresh waters but it has never been encountered 

 in purely fresh waters on the purely fresh water family of Salmonoidei-- 

 Thymallidae. Hence, we can make a more or less definite conclusion 

 about the relatively recent time of separation of Gyrodactyloides as an 

 independent genus; for, without any doubt, if this genus had separated 

 before the separation of Thymallidae it would have been peculiar to 

 this family. 



The genus Discocotyle is widely distributed on Salmonoidae 

 and Thymallidae and is encountered only in fresh water. Just as the 

 remaining genera of Discocotylidae parasitizing Cypriniformes it is a 

 very much altered representative of Mazocraeidae which undoubtedly 

 became adapted secondarily to parasitizing the present group of hosts. 

 More detailed information about the correlations of this genus with 

 others is given somewhat later (see page 311). 



The third group peculiar to both the Salmonoidei and 

 Esocoidei consists of representatives of Tetraonchidae and Gyrodactylidae, 

 i. e. , it is again different in origin. The only genus of Tetraonchidae, 

 Tetraonchus, is characteristic for it; it is encountered exclusively on 

 a number of genera of Salmonidae, on Thymallidae and Esocidae. As 

 regards Gyrodactylidae, in addition to the genus Gyrodactyloides in- 

 dicated earlier, a number of species of Gyrodactylus are also en- 

 countered on the families of fishes enumerated above (both on Salmonoidei 

 as on Esocoidei). Undoubtedly these are parasites which became 

 secondarily adapted to these fishes. Conversely, one can suppose that 

 Tetraonchus is phylogenetically primary for Salmonoidei which then 

 transfered onto Esocoidei. A number of primary (primitive?, nobis ) 

 traits in the organization of these worms serves as a substantiation of 

 this (see page 388). It is also noteworthy that L, C. Berg groups the hosts 



357 



