CHAPTER rV 



PHYLOGENETIC INTERRELATIONS OF THE FAMILIES 

 OF MONOGENETIC TREMATODES 



We now pass to the structure of the phylogenetic "tree" of p. 448 



monogenetic trematodes. At the present time, such structures do not 

 attract wide recognition of systematists and morphologists . It is con- 

 sidered that it (the construction of such trees, nobis ) is simply a specula- 

 tive .occupation which neither justifies the labor nor the time spent on it. 

 However, this point of view appears deeply erroneous. We are in complete 

 agreement with A. N. Severtsov that the elucidation of factual correlations 

 in the .phylogenesis of separate groups of animals in relation to the pre- 

 ceding history of the animal world is a paramount problem of zoologists. 

 Its solution presents not only historical interest but it also clarifies to us 

 the ways of subsequent development and in a number of cases allows us 

 to understand not only "normal" lines of the future, but also the possibilities 

 of their active acceleration or change. 



We have already spoken about phylogenesis of Monogenoidea 

 and noted its basic way and directions in the section which precede the 

 presentation of the system. The problems of the present chapter lead to 

 the specifying or clarifying of concrete lines of development and corre- 

 lations anaong contemporary Monogenoidea. Practically, this chapter 

 represents the phylogenetic deciphering of the system proposed by us. In 

 addition to that, we shall then attennpt, on the basis of this deciphering, to 

 make certain general conclusions which it seems to us will be of general 

 biological interest. 



First of all, examining contemporary Monogenoidea and comparing 

 them with the hypothetical promonogenetic trematodes which were discussed 

 earlier, we see that closest to them are representatives of the lowest mono- 

 genetic trematodes --Dactylogyridae- -and groups close to them on the one hand, 

 and Tetraonchidae and Amphibdellatidae- -on the other. All these families 

 have a number of more primitive and more specialized characteristics; 

 however, as a whole they stand approximately at the same level of develop- 

 ment. At the same time, in spite of the smaller number of edge hooks and the 

 bifurcation of the intestine, the Dactylogyridae have perhaps a somewhat 

 larger number of primitive traits. It would seem that this provides 

 sufficient indication that this group is more ancient as a whole and formed 

 the foundation for all the subsequent groups of Monogenoidea, especially 

 within the limits of the first trunk (branch, nobis ) of the development of 

 the latter. Practically, our system departs from Dactylogyridae and then 

 passes to groups morphologically more complex. However, the analysis 

 of the correlations of the groups and the tendencies of development show that 

 in building the phylogenetic "tree" in such a fashion we would have made a 

 very grave error. Dactylogyridae is one of the youngest branches of the 



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