Oligonchoinea in the real scope of the group because the mere oligomeri- 

 zation of the part of the hooked apparatus does not indicate the principal 

 difference from the initial forms although it has a very great significance. 

 In other words, there is a sharp difference in the nature of the develop- 

 ment of the initiaL and then partially also of the subsequent, stages of formation 

 of both subclasses --in the Polyonchoinea we deal with the normal process 

 of oligomerization of the primary attaching apparatus; whereas in 

 01iogonchoinea--with new formation which take place at the locations of 

 the apparatus with a stabilized quantity of edge hooks. Taking into con- 

 sideration that these are different means of adaptation to the same thing, 



i. e. , --attachment to the body of the host and primarily on the same 

 places, one cannot fail to notice that the solution of the same physiological 

 problem proceeds along different directions. Hence the correctness of the 

 considerations of V. A. Dogiel about the nature of the interrelations between 

 oligomerization and the other means of progressive evolution is clear. 



We pass now to the examination of the correlations within the 

 limits of separate trunks (branches, nobis) of development, i.e., of the 

 subclass of Monogenoidea. The first trunk, i.e., Polyonchoinea, consists 

 of 3 orders; Gyrodactylidea, Tetraonchidea, and Dactylogyridea. The first 

 two are characterized by the presence of the attaching armature, consisting p. 451 

 of 16 edge hooks and a third of 14 and even partially of 12. This forces us 

 to suppose that the first are closer to the initial ancestral forms, judging 

 by the present characteristic. However, this does not give us the right to 

 speak with certainty about their greater antiquity. In order to make 

 corresponding conclusions, let us examine all the 3 orders in sequence. 

 Thus, Gyrodactylidea consists of 3 families of which one--Sphyranuridae-- 

 clearly separated from Polystomatidae recently, as was indicated by us 

 earlier (see page 401). Thus, Gyrodactylidae and Polystomatidae belong 

 to the initial discussion as groups which have arisen prior to the third 

 family. Beyond any doubt the divergence between the two families is 

 ancient, it arose before Gyrodactylidae acquired the ability of viviparous - 

 ness and the Polystomatidae acquired contemporary peculiarities of the 

 attaching apparatus. Thus, the branch of Gyrodactyloidea takes its origin 

 in more remote times than the families which compose it. Taking into 

 consideration that Polystomatidae could appear, as we wrote earlier, 

 alnnost simultaneously with the separation of contemporary Amphibia, one 

 can think that this process took place approximately in the beginning of 

 the Cretaceous period or the end of the Jurassic. This is substantiated 

 indirectly also by the data about Gyrodactylidae, which are historically 

 younger than Polystomatidae and became separated most probably some- 

 what earlier than the separation of the contemporary Salrnonoidei (see 

 page 124 ). Inasmuch as the latter can be considered as having arisen not 

 later than the Paleocene period (Osmeridaeon which part of Gyrodactyloidea 

 is encountered, undoubtedly separated as early as the Cretaceous period, 

 see Berg, 1940), the data reproduced on Polystomatidae and Gyrodactylidae 

 are sufficiently trustworthy. Let us suppose, however, that our con- 



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