as very considerable. If one does not count the differences in details be- 

 tween different systems, the entire combination of the basic traits of 

 monogenetic and digenetic trematodes is so different from the basis of the p. 472 

 structure of tapeworms that it is hardly possible to divide the trematodes in- 

 to two phylogenetically distinct classes and even more to unite nionogenetic 

 trematodes and tapeworms into one superclass. If ontogenesis will not give 

 further substantiations of the similarity between tapeworms and monogenetic 

 trematodes, it will be necessary to recognize the formation of the posterior 

 attaching apparatus among both groups as a convergency and as an adaptive 

 characteristic which arose in connection with the conditions of larval 

 development of both. It is simpler to refuse recognition of the homology of 

 the exterior characteristic than to accept a number of traits of internal 

 structure of monogenetic and digenetic trematodes and tapeworms as con- 

 vergent. The comparison of organization between monogenetic trematodes 

 and Gyrocotylidae in Bychowsky is better founded; it is possible that he is 

 right in bringing these groups closer together. It is hardly allowable, even in the 

 form of a diagram, to present the picture of evolution with the indication 

 of geological periods and the scope of the branches in different periods for 

 forms the fossil remains of which do not exist. Bychowsky is undoubtedly 

 right in that tapeworms should not be derived from digenetic trematodes. 

 These groups are too specialized but, properly speaking, the monogenetic 

 trematodes are also too specialized to be considered as the ancestors of 

 tapeworms. There are no paleontological proofs of the great antiquity of 

 monogenetic trematodes. Compared with tapeworms their ancestry is 

 different and the embryology of both so far does not yield any similarities 

 except for the early development of the attaching disc. 



Further substantiation of the anatomical and embryological 

 proximity of monogenetic trematodes is necessary for the acceptance of 

 the phylogenetic suppositions of Bychowsky, but for the time being it is more 

 likely to allow the origin of monogenetic and digenetic trematodes from a 

 common Rhabdocoele ancestor. The tapeworms also descended from some 

 sort of rhabdocoelid but their ancestors apparently were ectoparasites at 

 first, which led to the development of strong organs of attachment among 

 them which are comparable but not homologous to the ones among mono- 

 genetic trematodes and which now appear in the embryological develop- 

 niental details in the shape of phylogenetic "remnants". 



"The miorphological differences between trematodes and tape- 

 worms are great but they are obscured by the variety which is observed 

 among Rhabdocoele turbellarians, in connection with which it is quite 

 possible to derive a primary ancestor from this group of worms for 

 trematodes as well as for tapeworms. " 



However, the objections of D. N. Fedotov seem insufficiently 

 convincing to us. The point of departure about the proximity of the internal 

 structure of monogenetic and digenetic trematodes understood by D. N. 



565 



