INTRODUC TION 



Our first observations on the monogenetic trematodes were begun P- "^ 



during student years at the Institute of Natural History in Peterhof, when in 

 1927 we became acquainted with the morphology and growth of a series of 

 representatives of the species of Dactylogyrus Diesing and later the poly- 

 stomes of the frog. 



The striking resemblance of the larvae of these two groups, despite 

 the great dissimilarities of the adults, forced us to consider the question of 

 family relationships within the limits of the group of the monogenetic trematodes 

 even then. Further work on parasitic worms led to the firm conviction of the 

 necessity for special research on the subjects of phylogeny and interrelation- 

 ships of the parasitic flatworms. At the same time, the first studies of the 

 monogenetic trematodes convinced us of the essential significance of this 

 group to an understanding of the phylogenies of all of the groups of parasitic 

 worms. 



Already in 1932, while examining the interrelationships of the vari- 

 ous species of Gyrodactylus von Nordmann and Dactylogyrus Diesing, we 

 came to the conclusion that it is impossible to understand systematic relation- 

 ships and phylogeny without careful study of the embryological development 

 of monogenetic trematodes, because, in a number of cases the true relation- 

 ships are masked by the resemblances of the adults which are connected with 

 the analogous conditions for existence. 



About 1935-36 we were convinced of the incorrectness of the wide- 

 spread view concerning the close kinship between monogenetic and digenetic 

 trematodes, and the monolithic relationship of the group of Cestodaria, and 

 in 1937 we published a work containing an effort to build a system of parasitic 

 flatworms on the basis of the scheme of their phylogenetic interrelationships. 

 This work was primarily preliminary in character because of the insufficiency 

 of factual material. Its publication forced us to pay more attention to the 

 questions of the interrelationships within the class Monogenoidea, inasmuch 

 as it seemed to us that the analyses of these interrelationships would give a 

 more solid base to our entire system. Henceforth, we set for ourselves a 

 problem of studying all aspects of the monogenetic trematodes for the purpose 

 of re-establishing the phylogenesis of the given group and a thorough treat- 

 ment of its systematics. 



The study of the interrelationships of many contemporary inverte- p. 6 



brates, particularly the worms, presents considerable difficulties because of 

 the absence of fossil remains. As a result, the researcher who is interested 

 in phylogeny is faced with the difficult problem of reconstructing historical 

 processes on the basis of materials dealing with the morphology and life 

 history of contemporary animals. The method of mutual verification of data 

 from the studies of different phenomena of the group under study allows us to 



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