such a thing is quite possible, it can hardly explain those peculiarities of 

 distribution on the hosts and the degree of morphological differentiation 

 which were indicated in the preceding text. If one accepts the last point 

 of view it will be completely obscure why Chimaericolidae, for instance, 

 did not transfer peculiarities of the structure of their sex system to 

 Mazocraeidae or Diclybothriidae, although undoubtedly both last groups 

 descend from the trunk of Chimaericolidae ? The fact that the given 

 peculiarity appeared after the branching off of both younger branches. I.e. , 



that which served as the basis of our considerations, can serve as the 

 only explanation. 



Let us return, however, to the further analysis of the details 

 of the phylogenetic scheme. The commonly accepted view is that biological 

 progress on the basis of morphological regressive development is charac- 

 teristic for parasitic animals. Monogenetic trematodes cannot serve in any 

 measure as substantiation for these views, quite to the contrary, their 

 evolution as a whole is built on the progressive development of morphological 

 structures. This is understandable because the basic changes in structure 

 concern two systems of organs --attaching and sex. Nevertheless, in 

 separate cases regressive processes take place in one just as in the other. 

 As regards the attaching apparatus, these cases are relatively few and 

 embrace entire systematic categories for the most part. Let us note along 

 the way to avoid misunderstanding that we do not consider the normal process p. 464 

 of oligomerization of edge hooks as the indication of morphological regression, 

 although at first glance the decrease in the number of hooks should be under- 

 stood precisely as such. However, taking into consideration that this is a 

 special qualitative difference and fully distinctly expressed process, we 

 exclude it from consideration in the present case. We see regressive 

 changes of the attaching apparatus present among very few Dactylogyridae 

 (reduction of middle hooks in some genera, or more rarely, species of 

 Capsalidae, Dactylogyridae, Calceostomatidae), in Sphyranuridae from 

 Gyrodactylidea, in Hexostomatidae from Mazocraeidea, and finally in the 

 entire order Diclybothriidea. As regards regressive changes in the sex 

 system they bear a different nature and concern separate private peculiari- 

 ties --chitinous armature of the copulatory organ etc. Thus, the 

 reductions of the sex system do not have any principal significance and do 

 not characterize more or less separated groups. Let us note also that in 

 speaking about reductions we must differentiate between them and the under- 

 developed particular structures, although this presents considerable 

 difficulties in separate cases. 



The general progressive nature of the development of morpho- 

 logical structures, connected principally with fully determined adaptive 

 tendencies (see page 324) causes numerous convergent similarities within 

 the linnits of this class. We have already written several tinaes about 

 separate cases of their appearance (Bychowsky, 1933b, 1949, 1959; Bychowsky 

 and Gussew, 1955; Bychowsky and Nagibina, 1954); however, chiefly only 



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