interests us. The data about occurrence force us to believe that Tetra- 

 onchidae appeared not earlier than the Cretaceous period and not later 

 than the Oligocene period (see page 308); Tetraonchoididae, Bothitrematidae 

 and Amphibdellatidae--not earlier than the Eocene (Berg, 1940, see also 

 pages 390 and394 ), Thus the occurrence of the families of Tetraonchoidea 

 allows us to say that apparently Tetraonchidae is the most ancient family. 

 The remaining three appear to be younger and one should consider that 

 Bothitrematidae and Tetraonchoididae became separated last on the basis 

 of indirect considerations and mainly of the data on morphology. Taking 

 into consideration that the ancestors of Tetraonchidea are undoubtedly 

 morphologically close to those of Gyrodactylidea one can suppose that both 

 orders having the attaching disc with 16 edge hooks are closer to each 

 other than to the 14-hooked forms which are united by the order of 

 Dactylogyridea. One cannot fail to note the fact that within the 



limits of Tetraonchidea we observe the tendency both to the preservation of the 

 primitive attaching apparatus with insignificant changes (Tetraonchidae and 

 Amphibdellatidae ) and to the transformation of the disc into a single sucking 

 apparatus (Bothitrematidae) and even to the peculiar division of the latter 

 into sections by septa, and at the same time the appearance of suckers on 

 the disc, true not at all the same as in Polystomatidae and not on the base 

 of the primary edge hooks (Tetraonchoididae). All these tendencies lead, 

 as we will mention in more detail later, to converging similarities of the 

 representatives of the present family with families related to other 

 systematic groups. 



As a whole the entire order of Tetraonchidea gives the impression 

 of not being clearly delimited and it is possible that it will require later 

 reconstructions. Nevertheless, the peculiarities which separate it under- 

 score its relative antiquity, whereas the differences of separate families 

 indicate their rapid divergence from each other. 



On the basis of what has been expressed in the preceding sections, 

 the question about the time of the appearance of the third order, Dactylogyridea, 

 is solved relatively easily. 



One can speak with certainty that the ancestral forms of this order 

 appeared later than the ones of Gyrodactylidea but undoubtedly somewhat 

 earlier than Tetraonchidea*, in other words, the separation of this order 

 can be considered most probably linked with the Cretaceous period. 



In conclusion, the question about the separation of separate 

 orders of Polyonchoinea is solved rather unexpectedly. Although the order 

 Gyrodactyloidea appears to be the most ancient, possessing the greater p. 454 



number of edge hooks of the attaching disc, the second order- -Tetraonchidea, 

 which possesses the same peculiarity, is younger than Dactylogyridea which 

 have the 14-hooked attaching apparatus. 



544 



