secondary parasites of Selachii only a few forms separated to the degree of 

 independent genera, but the majority at best reached the degree of well- 

 delineated species. This secondary fauna does not give us basis to speak 

 about phylogenetic links between Elasmobranchii and Teleostei, but the 

 entire fauna of Monogenoidea of Selachii shows that its appearance took 



TrygofiLdae 

 MylLobatlctae 



\ 



Fig. 262. Diagram of occurrence of monogenetic trematodes on the 

 families of Selachii. The diagrann of phylogenetic relations of Selachii 

 is draw^n according to Suvarov, 1948. Only the families on which mono- 

 genetic trematodes are encountered are retained in it. 1 --Hexabothriidae; 

 2--Microbothriidae; 3--Loimoidae; 4--Amphibdellatidae; 5--Acantho- 

 cotylidae; 6--Monocotylidae. 



place not before the separation of this class of fishes, but after its formation. 

 However, we shall return to the last question somewhat later (see page p. 305 



449). An important circumstance which we can see during the analysis of 

 the fauna of Monogenoidea of Selachii is that it practically consists of a 

 number of families which developed independently and which are not 

 connected with each other by close phylogenetic links and each of which 

 became adapted to parasitizing the present groups of hosts independently 

 from the others. Such beyond any doubt are Amphibdellatidae, Acantho- 

 cotylidae, Hexabothriidae, and Microbothriidae. As regards Monocotylidae 

 and Loimoidae, even though one can allow the commonness of origin^there 

 are nevertheless many more bases to think that it is lacking. The phylo- 

 genetic origin of their fauna on Selachii undoubtedly is of great significance 

 to the analysis of the evolution of Monogenoidea because we see in this 

 fashion ways of evolution different from the notorious parallelism. It is 

 interesting from this point of view to analyze the Monogenoidea of Selachii 

 within the limits of the primary fauna in connection with separate groups 



353 



