merization of the organs and of their role in the evolutionary processes 



of the animals. It seems to us that the ideas of V. A. Dogiel are very 



fruitful and must always be kept in sight during phylogenetic research. 



Similarly, in a number of cases it is indispensable to remember and to 



use the consideration of V. N. Beklemishev about the importance of 



the basic plan of the structure of the animals. General architectonics 



and its study undoubtedly play an important role in any phylogenetic 



research. Finally, as we will see later, one must not forget also the 



teaching of A. N. Severtsov about the way of evolution which often gave 



the key to the understanding of general directions of evolutionary process p. 324 



in a given concrete group. 



Let us now pass to the direct exposition of the basic con- 

 sideration about evolution of monogenetic trematodes and, without 

 touching upon the question of the origin of the group as a whole, to 

 which a separate chapter is dedicated, make an attempt to re-establish 

 the structure of the primary, the most primitive monogenetic trematodes 

 and the original ways of their development. 



In the chapter about individual development of monogenetic 

 trematodes, we already indicated on the basis of the structure of the 

 larvae of contemporary species that the primary form could have the 

 structure represented in Fig. 115. Let us dwell somewhat more in 

 detail on the morphology and other peculiarities of this hypothetical 

 form. As was indicated in the chapter on occurrence (see page 291 ), 

 as the first monogenetic trematodes can be considered forms which 

 differ from the straight-intestined (Rhabdocoela nobis) Turbellaria both 

 by means of alimentation and by way of morphological adaptations to 

 the constantly moving attachment on the body of the host. These 

 morphological adaptations, characteristic for the present evolving group, 

 undoubtedly were chitinous attaching formations (see page Q6 ). They 

 undoubtedly were not new formations but derived by way of the change 

 of some sort of skin-thorns of Turbellaria. 



Apparently the primary armature of nnonogenetic trematodes 

 had the shape of small thorns lying on the ventral side and vmdoubtedly 

 predominantly on the posterior end of the body. The latter was connected 

 with the fact that during the process of feeding on the coverings of the body of the 

 host the anterior end of the parasite had to have the ability for quick 

 movements (locomotion), and by this very fact its fixation had to be 

 achieved by contrivances other than the thorns which, with their con- 

 siderable number and weakly developed nervous system, could hardly have 

 easily acted in a co-ordinated fashion. This is confirmed by the study 

 of contemporary forms. With the significant number of chitinous 

 attaching formations located thereon, the locomotion of the posterior end 



378 



