the attaching apparatus. These peculiarities, namely- -the disappearance 

 in one fannily of a part of the chitinous armature (Hexabothriidae) while 

 it is preserved in the second but with a reduction at the same time of 

 the narrowed part of the disc, convincingly speak for the very ancient 

 divergence of both families. Nevertheless, it remains a fact that both 

 these fanailies are much more related to each other than to the rest of 

 Oligonchoinea. 



There are no bases whatsoever to assume that Diclybothriidae 

 chanced upon Acipenseriformes from Selachii secondarily in a relatively 

 recent time. The biology and physiology of the host and the nature of 

 the distribution of Diclybothriidae on them speaks against this. For 

 instance, one can doubt the fact that Diclybothrium armatum has been 

 encountered on Polyodontidae from the time of their separation; it is 

 possible that it transferred to Polyodontidae at a much later time, but 

 there is no basis whatsoever to doubt that this species has been dis- 

 tributed in Acipenseridae from very ancient times and it is much more 

 likely that it passed the entire period of their evolution with them, 

 from the upper Cretaceous at a minimum. Most tempting, however, 

 is the supposition that Diclybothriidae are the descendants of the initial 

 Monogenoidea parasitizing ancestors common for thennand for Selachii, 

 that they have an even more ancient history. Only this supposition 

 permits us to understand the reasons for finding Diclybothriidea which 

 are related to each other on fishes which are very distant from each 

 other and the total absence of both on Holostei. In other words, one can 

 suppose that the specialization of Diclybothriidae was already so far 

 advanced at the moment of formation of Holostei that it excluded the 

 possibility of transfer onto the latter. 



A completely different picture is offered by the second p. 307 



group of monogenetic trematodes of Acipenseriformes--Capsalidae, 

 among which only one genus Nitzschia is encountered on the fishes 

 which interest us. It is true that it is sharply differentiated from 

 other species and that it forms a special subfamily (see page 382). 

 Nevertheless, the distribution of Capsalidae, about which we have 

 already spoken (page 276. )» allows us to maintain that they are phylo- 

 genetically secondary parasites of Acipenseridae which appeared on 

 the latter much later than the Diclybothriidae. Taking into consideration 

 that for a number of reasons the White Sturgeon • ( Huso huso L,.) can be 

 considered as the basic host of Nitzschia, it is very probable that the 

 separation of the genus took place historically not earlier than the 

 Pliocene (Berg, 1940). As a result, we can say that the fauna of mono- 

 genetic trematodes of Acipenseriformes shows on one hand a sharp 

 differentiation of this group of fishes from the remaining Teleostomi 

 and on the other hand a different chronological sequence in appearance 

 on a given order of both families of parasites which comprise it. 



355 



