Before speaking about correlations within the limits of the 

 order Dactylogyridea we must direct attention to the innportant break 

 which occurs in our data concerning the time of separation of the trunk 

 of Polyonchoinea from Oligonchoinea on one hand, and on the other- - 

 about the supposedly more recent time of the appearance of some of the 

 orders of the first subclass. We have said above that the separation of 

 both subclasses of both Monogenoidea took place most probably in the 

 Devonian and have just established that all three orders of Polyonchoinea 

 appeared not earlier than the end of the Jurassic period at best. Thus, 

 a huge span of time passes between the appearance of the differentiated 

 ancestors of the separate orders of Polyonchoinea and the time of 

 divergence of the trunk of Oligonchoinea, by which we judge the divergence 

 of both trunks. What happened during this period? First of all one can 

 think that there is a mistake in our present considerations, namely that 

 the separation of the trunk of Oligonchoinea from the common "tree" of 

 Monogenoidea does not indicate that Polyonchoinea also became separated 

 in the same period. It is quite possible that the separation of the first 

 took place by means of a quick change in the structures (new formations !) 

 of promonogenetic trematodes; whereas the process of oligomerization, 

 characteristic for the formation of Polyonchoinea, proceeded considerably 

 slower. One can even suppose that after the separation of Oligonchoinea, 

 the trunk which lead to Polyonchoinea was already characterized by forms 

 which had a larger number of edge hooks than the Gyrodactylidea and 

 Tetraonchidea. It is understandable that all is a region of pure speculation 

 however, one can consider it as an indubitable fact that the primitive 

 ancestors of Polyonchoinea, whatever structure they may have had, existed 

 for a relatively long time on some sort of fishes w^hich have not persisted 

 to the present time, and it is also possible, on Selachii which already 

 inhabited the fresh and marine bodies of water at that period. 1 A simiilar 



1 



This enables us to suppose that the study of the fauna of Monogenoidea 

 of Selachii and particularly on the more primitive sharks which have not 

 yet been studied (Heterodontidae and Chlamydoselachidae) can add many 

 supplementary miaterials for a clearer understanding of the phylogeny of 

 the monogenetic trematodes. 



break also takes place between the time of the separation of the 16 -hooked 

 forms and the appearance of the first Tetraonchidea. Here again one can 

 suppose an extended period of existence of the primary undifferentiated 

 Tetraonchidea on fishes different than the ones that are their hosts at the 

 present time and most probably which have died out subsequently. The 

 latter supposition is based on the fact that it is improbable that the species 

 living on fishes which exist until the present time became extinct on them. 

 Such a case is theoretically possible but practically could hardly have a 

 mass character (primary significance, nobis). Theoretically its probability 



545 



