first trunk and undoubtedly became separated rather late. This disparity p. 449 



between the contemporary morphological structure of the separate groups 



of parasites and the time of the origin, or to be more precise of their 



separation, makes the problem relative to the structure of the phylogenetic 



"tree" very complicated. Consequently' our analysis cannot be based on 



the morphology and embryology of the corresponding animals only but 



must arise from a combination of these data and the data on occurrence 



and history of the interrelations of parasites and their hosts, determined 



both by occurrence and by the nature of the origin and development of the 



faunistic complexes of parasites on specific groups of hosts. At the same 



time the materials about the life cycles and biology both of the parasites 



just as of their contemporary hosts must be also considered. It would be 



tempting to utilize the data on phylogenesis of the hosts but these data used 



uncritically can lead us to greater errors than purely morphological analyses, 



especially since the materials on the phylogenesis of the hosts arouse very 



great doubts of their accuracy in a number of cases. Nevertheless in spite 



of all these difficulties the attempt at establishment of the phylogenesis 



of monogenetic trematodes seems to us quite likely and in the essential 



traits appears to us as possible to be solved clearly enough. 



During the appraisal of the basic lines of development of mono- 

 genetic trematodes, we see that their attaching armature is regular and 

 changes in direction from the larger number of edge hooks toward the 

 smaller. Thus, among the lowest Monogenoidea this number changes from 

 16 to 12 and among the highest is more or less constant, and equals 10. 

 The tendency is towards disappearance or metamorphosis or, i.e., 

 actually to oligomerization. It would be very tempting to attempt to build 

 the system in the form of a single number of species changing by the 

 characteristic of the edge hooks from the many to the few hooked ones; how- 

 ever, this is impossible because the materials on development point to the 

 presence of two easily distinguishable branches. The divergence of these 

 branches, about the size and peculiarities of which we have already spoken, 

 took place in very ancient times. The study of the nature of the faunas of 

 the Monogenoidea of different groups of hosts as well as the morphological 

 analysis fully confirnns this. Regardless of how one pictures the speed of 

 the evolutionary process, one can say with certainty that the divergence be- 

 tween both subclasses of Monogenoidea is so great that long geological 

 periods were required for its occurrence. Would it be possible to determine, 

 albeit tentatively, the time of its divergence? It seems to us that this 

 question can be solved positively. First of all it can be considered that the 

 appearance of monogenetic trematodes as a separate class probably occurred 

 only after the separation of fishes, i.e. , approximately during the Silurian 

 period. The supposition that Monogenoidea separated earlier is improbable 

 because they are undoubtedly specialized parasites of vertebrate animals, 

 the entire life cycle of which, from the very beginning, was not connected 

 with the presence of a number of intermediate hosts as is the case (the 

 presence of intermediate hosts, nobis) in digenetic trematodes, tapeworms. 



539 



