differences of t'wo extreme members of one phylogenetic branch. Using a 

 similar approach for the analysis of consanguinous ties we can arrive at 

 very unsuccessful conclusions about consanguinous relations even among 

 representatives of one species of animals, not to speak about larger 

 taxonomic units. Nevertheless, a number of important peculiarities in the 

 structures of Monogenoidea and Cestoidea (see a little further) are very 

 close to each other and are sharply juxtaposed to the corresponding structure 

 of Trematoda. 



Finally, to suppose that the unusually striking similarities in 

 the structure of the attaching apparatus of monogenetic trematodes and the 

 larvae of tapeworms is a converging phenomenon as D. N. Fedatov does is 

 hardly justifiable. The common tendencies of oligomerization, the 

 peculiarities of development and also of morphological similarity of the 

 chitinous elements of the cercomere of both classes give sufficient bases to 

 consider that here occurs a commonness of origin, i.e., homology of 

 corresponding structures. During the development of any characteristic 

 a careful morphological analysis can always show which peculiarities 

 are developed convergently because under these conditions one never observes absolute 

 similarities of structures. As a matter of fact, we have already spoken 

 about convergent similarities and, recognizing their important significance p. 474 

 in the development of Platoda, we cannot recognize the appearance of the 

 cercomere in all three classes of cercomeromorphous flatworms as con- 

 vergent. 



In conclusion, we think that the considerations of D. N. Fedotov 

 and his interpretation of the interrelations of the flatworms, which correspond 

 to a formerly widely distributed opinion, are hardly justified even though 

 they deserve attention. Contemporary materials force us to suppose that 

 the separation of the groups of cercomeromorpha is proper and to attribute 

 phylogenetic significance to it. 



Without wishing to repeat what has already been said before, 

 we permit ourselves nevertheless to dwell on certain new facts and consider- 

 ations substantiating, from our point of view, the scheme of the interrelations 

 of Monogenoidea with other classes of flatworms as accepted by us. 



Very noteworthy is the clarification of the peculiarities of develop- 

 ment among certain Microcotylidae among which the shedding of the posterior 

 part of the attaching disc is observed (see page 212). This process very 

 closely resembles the shedding of the cercomere which is characteristic 

 for a number of tapeworms (including Cestodaria). At the same time, it is also 

 curious that the part of the disc with 3 pairs of hooks (2 pairs of middle and 

 one pair of edge) is shed, which numerically coincides with the ones among 

 tapeworms. It is understandable that these 3 pairs of hooks do not corre- 

 spond to the 3 pairs of hooks of the cercomere of Cestoidea (although in 

 connection with one pair one can suppose that here exists a real homology). 



567 



