etc. We have already written about this in 1937. On the other hand, taking 

 into consideration that the order Chimaericolidea, the most specialized, 

 which at the same time has a number of very primitive traits, is encountered 

 only on chimaerids and that we do not observe any transitions between this 

 order of Monogenoidea and the one closest to it (Diclybothridea), one can 

 say with very great certainty that this order is connected in its evolution 

 with the hosts approximately from the moment of the separation of the 

 latter. As confirmation for this can serve also the extremely wide geo- 

 graphical distribution of Chimaericolidea with their very great poverty of p. 450 

 genera and species. Inasmuch as there are no doubts whatsoever that the 

 divergence of both subclasses of Monogenoidea preceded the formation of 

 the present order, for notwithstanding their specialization and prinnitive 

 traits they are indisputably representatives of the subclass Oligonchoinea, 

 in any case this divergency took place before the Jurassic period, because 

 contemporary chimaerids are known from that period (Berg, 1940). How- 

 ever, we can make the time of divergence of both subclasses of mono- 

 genetic trematodes more precise if we take into consideration that the 

 separation of Chimaericolidea must have preceded the separation of the 

 second order of the trunk of Oligonchoinea- -Diclybothriidea. The fact that 

 this process took place precisely so and not otherwise is shown convincingly 

 by the comparison of the morphology of the representatives of both orders. 

 Actually it does not seem possible to derive diclybothriids from the structure 

 of Chimaericolidea because the former are a secondarily simplified and 

 regressing group in many aspects. This is understandable from what has 

 been expressed earlier (see pages 40vS and 410). Diclybothriidea, which are 

 principally connected with the Elasmobranchii, undoubtedly descend from 

 ancestors common to them and Chimaericolidea which parasitized some 

 sort of fossil fishes, chimaerids or Elasmobranchii, with an equal degree 

 of probability. Inasmuch as both are known from the Devonian (Berg, 1940), 

 one can suppose that the divergence of both trunks of Monogenoidea should 

 be linked precisely with this period. 



At the same time, we must remember that the retention of the primary 

 characteristics peculiar to monogenetic trematodes is characteristic for the 

 first stages of development of the first trunk and for the second- -the 

 appearance of completelv new peculiarities; of a changed attaching apparatus, 

 representing among them a combination of the primary chitinous hooked 

 armature and the delimiting of the muscular formation "of the sucker -clamp" 

 around part of the hooks. ^ Actually, only the appearance of this peculiarity creates 



_ — — 



This name is unsuitable but points to the fact that in the beginning there 

 was an organ which provided the ability of attaching by both means --suction 

 and pinching (just as takes place among Dlclidophoropsis see page 437). 



540 



