The transfer of these species took place probably from some 

 forms of extinct groups of hosts at the very beginning of the formation 

 of Pleuronectiformes and Gadiformes. The Clupeiformes, which have 

 ancient forms of parasites of both subclasses, are distinguished by a 

 much more original fauna. However, as a wTiole the fauna of Clupei- 

 formes is nevertheless relatively poor. Practically only two, it is 

 true the most numerous orders --Perciformes and Cypriniformes, are 

 characterized by a large distinct fauna. Attention is attracted by the 

 fact that one of their orders is basically marine whereas the second 

 one of these orders is almost completely fresh water. The existing 

 exchange by the separate groups of Monogenoidea between these two 

 orders is clearly secondary and only underscores the possibility of 

 transfer in certain conditions (limited or confined conditions, nobis) 

 of fresh bodies of water. The distinctiveness of the fauna of the three 

 largest orders of Teleostei shows undoubtedly not only the plurality 

 of the possible "ecological niches" but also the relative antiquity of 

 these faunas. It is impossible not to note that all these three orders 

 of fishes originate approximately at the same time because their 

 fossil remnants begin to be encountered at the same period- -in the 

 Cretaceous period. ^ It is understandable that the Clupeiformes are 



_ 



Clupeiformes are known from the upper middle Triassic, whereas 

 Clupeoidei--only from the lower Cretaceous, and Salmonoidei--even 

 from the lower Eocene. Cypriniformes are known from the upper 

 Crateceous just as are the Perciformes, 



somewhat more ancient than the two remaining orders especially since 



the origin of Perciformes is even linked with Clupeiformes (Suvorov, p. 320 



1948). Hence we can conclude that the beginning of the development of 



the faunas of parasites of these three orders can also be supposed as 



linked either with the Cretaceous period or with a somewhat earlier 



period, That is much later than the origin of the fauna of the examined 



groups of hosts. 



A general preliminary conclusion from the examination 

 of the faunas of monogenetic trematodes of different groups of their 

 hosts can be that in the analysis of phylogenetic relations of separate 

 groups of Monogenoidea in addition to the evaluation of their own inter- 

 relations the inspection of the nature of the origin of separate groups 

 is also indispensable. The application of such inspection will give 

 clear notions about the general progress of evolution. 



373 



