CHAPTER VI 



THE POSITION OF MONOGENETIC TREMATODES 

 IN THE SYSTEM OF FLATWORMS 



We now pass to the discussion of the question of the position of p. 470 



monogenetic trematodes in the system of flatworms. Our considerations on 

 this subject were expressed in an article in 1937 devoted to correlations of 

 ontogenesis and phylogenesis of parasitic flatworms. In this work is given 

 a short- historical survey of views on the origin of parasitic flatworms, and 

 it is indicated that monogenetic trematodes were joined by different authors 

 to different groups. In particular, according to D. F. Snitsin (Snitsin, 1911) 

 Monogenoidea descend partially from digenetic trematodes by means of 

 simplification of their life cycles. Conversely, Janicki (Janicki, 1921) con- 

 siders that monogenetic trematodes, descending from Rhabdocoela, gave rise to 

 digenetic trematodes. The last point of view is most widely distributed 

 (Meixner, 1926, and Fuhrmann, 1928-1932, and others). As regards the 

 flatworms, their direct connection with monogenetic trematodes was recog- 

 nized only by Spengel (Spengel, 1905), whereas the remaining scholars con- 

 sidered them as descending directly from straight-intestined Turbellaria. 

 During the consideration of the interrelations of monogenetic trematodes 

 with digenetic ones we came to the conclusion that the contemporary data 

 on morphology, development and life cycle of these groups do not provide 

 any basis to suppose the commonness of their origin. The existing 

 similarity in the structure of Monogenoidea and hermophidic mature digenetic 

 trematodes is a purely converging phenomenon which does not in any way 

 indicate their actual phylogenetic consanguinity. As regards the inter- 

 relations between the tapeworms and the digenetic treinatodes, we have 

 indicated the complete correctness of Fuhrmann's point of view who considers 

 the rapprochement of both groups as impossible. As a result we came to 

 the conclusion that digenetic trematodes are very far from Monogenoidea and 

 Cestoidea, in connection with which we thought it necessary to divide mono- 

 genetic trematodes and digenetic trematodes into two independent classes. 

 Passing to the consideration of the interrelations between tapeworms and 

 monogenetic trematodes we have purposely stopped on the question of the status 

 of Gyrocotylidae, which relates to Cestodaria according to the system 

 commonly accepted at that time. As is known, the question about the orien- 

 tation of the body of these peculiar worms had not yet been completely solved 

 at that time and considerable discussions were going on on the subject of 

 which of their ends was homologous to the anterior end of the rest of tape- 

 worms. In the work of Ruszkowski, which came out in 1932, it is shown with 

 sufficient basis that the posterior end of Gyrocotylidae is the one which is p. 471 

 equipped with the rosette and bears embryonic hooks during the time of 

 development, characteristic for the lycophore of these animals. In connection 

 with the clarification of the proper orientation of the body of Gyrocotylidae 

 we were forced to examine their morphological and histological structure 

 more attentively. This re -examination led us to the unexpected con- 



clusion that this group should be considered as a completely independent 



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