The data on the biology of Monogenoidea Avere utilized by us for 

 the correction of the comparative anatomical materials and during the 

 analysis of the occurrences of the worms on their hosts. Thus, this group 

 of facts bore a purely auxiliary nature. 



The business with materials on postembryonic development is 

 quite different. As is known, the studies from the time of Mueller and 

 especially Haekel represent the classical part of every phylogenetic re- 

 search. In the analysis of these materials from the same position as in 

 the relation of the comparative anatomical data it seems to us that we 

 succeeded in obtaining very important general conclusions for the under- 

 standing of the phylogenesis of the groups. Inasmuch as the analysis of 

 the postembryonic period proceeded in accordance with the comparative 

 anatomical aspect it allowed us to establish with a great degree of proba- 

 bility the traits of the promonogenetic trematodes and the basic ways of 

 divergence of the group. The latter was extremely important because it 

 enabled us to conduct an analysis of the true homologies on the one hand and 

 to establish the convergent sinnilarities and parallelisms on che other. 



It seems to us very important to apply the study of the life 

 cycles of Monogenoidea during the establishment of the phylogenesis of 

 the group. First of all we succeeded in showing that a complex life cycle 

 is characteristic for Monogenoidea, not in the sense of the presence of a 

 number of phases of development as in digenetic trematodes, tapeworms 

 and other parasitic worms, but in the sense of its unusual adaptability to 

 the life cycle of the host, chronologically as well as in relation to attach- 

 ment to determined phases and stages of the life of the latter. These 

 peculiarities of the life cycle indicate the duration of existence of the 

 biological pair, parasite-host, which in its turn has a great phylogenetic 

 significance. Thus, the analysis of life cycles yields important confir- 

 mations to those principles which are discovered during the analysis of 

 the occurrence of the parasites on their hosts. 



We have attributed great significance to the occurrence of 

 Monogenoidea, and it seems to us that this was substantiated. The 

 analysis of the nature of the distribution of monogenetic trematodes on 

 their hosts showed their exclusive selective ability in relation to determined 

 species and larger groups of hosts in natural conditions. Basing ourselves 

 on these data we have analyzed the nature of the faunae of Monogenoidea on 

 specific groups of hosts, and taking into consideration the data on the 

 paleontological antiquity of the latter we established the time of appearance 

 of the different groups of monogenetic trematodes on the corresponding 

 groups of hosts. At the same time, what is especially important for the 

 establishment of the phylogenesis of Monogenoidea, this analysis enabled 

 us to answer the question about which groups of worms were primarily 

 connected with a given group of hosts and which- -secondarily. 



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