reconstruct its past and those genetic links which can be utilized for the 

 building of the system on phylogenetic foundations with a high degree of 

 reliability. It is impossible not to note that parasitic animals are more con- 

 venient in this connection because tneir examination provides supple- 

 mentary, purely parasitic criteria for the reconstruction of the past history 

 of the group. 



It seems to us that the importance of the study of phylogenesis of 

 any given group of animals must not be underestimated. Only the concrete 

 knowledge of the phylogenesis of separate groups permits us to represent 

 in an accurate way a general nature of the evolution of the animal world 

 which has great theoretical significance and which also appears as a basis 

 for the exact classification of animals. Along with this, we are firmly con- 

 vinced that only an adequate, complete knowledge of the phylogenesis of a 

 gr up can serve as the sufficient basis for the solution of natural methods of its 

 full economic utilization. As applied to the parasitic animals, the know- 

 ledge of phylogenesis of a group is indispensable for the correct planning 

 of methods of biological struggle with parasitic diseases, and also for a 

 clear understanding of the potential danger of any single group of parasites 

 or of a specific parasite fauna. 



Our research on phylogenesis of parasitic flatworms rs still far 

 from complete, but it is already possible to expediently sunn up the study 

 of phylogenetic relationships of monogenetic trennatodes and also to build 

 a system of the group on the basis of these interrelationships. Our present 

 wOrk is dedicated to these two problems. 



Data from the literature and new material collected by us, our 

 students, and colleagues forms the foundation of our system. In many 

 instances the published data, to our regret, are entirely unsatisfactory 

 because the authors did not pay proper attention to certain peculiarities of 

 morphology and embryology which, from our point of view, have the great- 

 est significance. This forced us to re-examine or repeat the experiments 

 which was extremely burdensome because of the difficulty of securing 

 original material. For these basic reasons we were forced to consider with 

 extreme care the data pertaining to the discovery of a particular species of 

 monogenetic trematodes on a particular host. One of the reasons is the in- 

 exactitude in identifying the host or parasite. The second, much more com- p. 7 

 plex, precisely the (reporting of a, nobis ) finding of a parasite not on the host 

 peculiar to it in normal conditions, but on the strength of local discovery of 

 the host during the examination or during the capture of the fish host (in the 

 instruments of capture of the fish or during the transport) when a mechanical 

 transfer or an independent transfer from one host to another might occur. 



The third and most important reason, which in numerous cases 

 demands special analysis, involves the description of a parasite from a 

 host which is not habitual to it as a result of the grouping for varying periods 



XVIII 



