them one must distinguish three groups with a different nature of f 



relations with their hosts. The first group, to which Axine , Dactylogyrus , I 



and Benedenia belongs, can be characterized as containing genera whose 

 basic mass of species are encountered on related families of fishes, and 

 in addition to that in small numbers on the (hosts, nobis) unrelated to the 

 first ones (and to each other -- Dactylogyrus) . ^ 



_ 



As a matter of fact, the genera Ancyrocephalu s, Cleidodiscus, and 

 Urocleidus, which were excluded from discussion, can be included in this 

 in this group. 



The second group of genera ( Gyrodactylus, Heterobothrium , 

 Metahaliotrema and Acolpenteron ) are characterized by the occurrence of 

 more or less similar small numbers of species (1-2) on different unrelated 

 families of fishes, that is, it does not indicate any normal links between 

 the occurrence of the genera of Monogenoidea and the consanguinous re- 

 lations of their hosts. 



Finally, the third group, to which Entobdella belongs, is dis- 

 tinguished by the fact that here takes place the occurrence of representatives 

 of the genus only on two unrelated (between them) groups of related families 

 and on each one of them in more or less considerable number of species. 

 This group points to a special case of divergence of a species of the genus 

 apparently connected with ecological differentiation. As a matter of fact, p. 269 

 it is too early to speak about it now because this problem will be evaluated 

 considerably later (see page 301 ). 



Thus, it can be said that it would be more correct to attribute 

 only four genera (second of the examined groups) to the genera of mono- 

 genetic trematodes which actually parasitize unrelated families of fishes. 



As a result of the examination of the complete material on the 

 occurrence of the genera of monogenetic trematodes on the families and 

 orders of fishes we can consider the following as fully established: 



1. The vast majority of genera of monogenetic trematodes is 

 encountered on one genus of fishes or on several related to one family. 



2. When representatives of the genera of monogenetic trema- 

 todes are found on several families or even orders of fishes it can be shown 

 in a majority of cases that the latter are related to each other. 



3. For an insignificant nunnber of genera of monogenetic trema- 

 todes which contain predominantly nnany species, the following characteristics 

 can be observed: a) either the finding of a basic mass of species on related 



306 



