individual cases were discussed and this problem was not envisaged more 

 widely --within the scope of the entire group. But the examination of greater 

 material shows that the nature of the developing convergencies is very- 

 different and deserves naore meticulous attention. 



First of all we can distinguish convergencies of homologous and 

 non-homologous organs and structures. For instance, the undoubtedly con- 

 verging similarity of the secondary disc of Acanthocotylidae (see page 383 ) 

 with the primary discs of other monogenetic trematodes can serve as an 

 example of the latter. Likewise, thesucker -shaped pulvilli of Tetraonchoididae 

 (see page 31 ) are very similar to the suckers of Polystomatidae, however, 

 they have a different origin than the latter. The number of similar cases 

 could be considerably increased; however, they are completely understandable 

 by themselves and also in their greater part they are generally sufficiently 

 coarse and superficially evaluated from the point of view of their origin and 

 evolutionary significance. Convergencies of homologous organs are much 

 more complex and interesting. Among them we observe similarities 

 peculiar only to one specific structure or system of formations, to separate 

 organs or a number of them both linked with each other as. well as not connected 

 functionally. With this we have a whole gradation of convergencies by 

 degrees of phylogenetic proximity of species possessing correspondingly 

 similar peculiarities. Speaking about separate structures, one can point 

 to the structure of the middle hooks in a number of Dactylogyridae. Their 

 origin can be interpreted differently; however, their homology to each other 

 does not arouse any doubts. The appearance of a special indented fracture 

 on the interior edge of the curvature of the point of the middle hooks is 

 characteristic for many representatives of the genus Dactylogyrus. Often 

 this break also finds its reflection in a small thickening of the exterior 

 edge of the point and then the hook acquires a special indented edge. Never- 

 theless there are no doubts that this peculiarity arises each time inde- 

 pendently in a number of cases. Thus, it is peculiar to D. varicorhini 

 Bychowsky (Fig. 311, A) which was found on middle -Asian Varicorhinus spp. 

 probably linked with each other in their origin, but it appears (also, nobis) p. 465 

 in D. markewitschi Gussew (Fig. 311, B) from the Amur fishes, Saurogobio 

 dobryi Bl. , not connected in any way with the middle -Asiatic species. 



In addition to that, in the genus Falciunguis there is a very 

 similar change in the initial part of the hook (Fig. 311,C); the representatives 

 of the genus Dogielius (Fig.. 147) are arranged analogously. The first of 

 these genera is clearly of Eastern, apparently Chinese origin, and the second 

 middle -Asiatic. The genetic links of their hosts are also very remote (the 

 first is known from Cyprini and the second from Barbini). Just as the case 

 of Acolpenteron and Pseudacolpenteron, discussed in the work of Bychowsky 

 and Gussew cited above, the examples cited here pertain to closely related 

 species in one genus or of close genera of the same family. 



557 



