elusions are erroneous and that the branch of Gyrodactyloidea appeared at 

 a much later period. Then we will have to suppose that in the first place 

 the Gyrodactylus - like ancestors of Gyrodactyloidea transferred to parasitizing 

 Salmonoidei at a certain period when this group already had reached its full 

 development. This supposition is possible, but then the relatively narrow 

 adaptability (specificity, nobis) of this genus to a deternnined circle of hosts 

 of very wide geographical distribution which gives, one would think, the 

 possibility of contact with many other fishes of the same superfamily (see 

 page 308), becomes unclear. In the second place, the widest (practically 

 world-wide) distribution of Polystomatidae under the condition of the unusual 

 adaptability of the life cycle of separate species of parasites to their hosts 

 speaks for the indubitable antiquity of their interrelations; furthermore, the 

 life cycle of Polystoma shows that the representatives of this genus first 

 became adapted to life on the ancestors of contemporary Amphibia having 

 gills during their entire life, because the transfer to parasitizing in the 

 urinary bladder is undoubtedly a secondary phenomenon (see page 124 ). The 

 supposition that the ancestors of Polystoma first became adapted to the life 

 on the gills of the larvae of contemporary Ranidae cannot be considered 

 trustworthy in any way, which again speaks for the probable appearance of 

 Polystomatidae in relatively recent times (let us remember that even the 

 contemporary genus Rana separated not later than the Eocene, whereas it is 

 possible not to doubt the earlier appearance of Polystomatidae, for the reasons 

 indicated above .^ 



Thus the suppositions about the greater youth of the branch of 

 Gyrodactylidae contradict the facts which we have at our disposal. We 

 especially dwell on this question somewhat more in detail in order to demon- 

 strate the manner of our reasoning, which gives us a known certainty of 

 their correctness even though it is based on indirect material. We shall also 

 note along the way that interrelations within the limits of the families of 

 Gyrodactyloidea are clear fromi what has been said earlier (see page 397 ) and p. 452 

 from the drawing in Figure 310 which does not require special explanations. 



The second order--Tetraonchidea- -presents much greater diffi- 

 culties in the determinations of the correlations of the families composing 

 it, as well as for the formation of opinion about the relative antiquity of the 

 separation of both the families and of the order as a whole. The presence 

 of a sac- or pipe-shaped intestine is characteristic for 3 families, 

 Tetraonchidae, Tetraonchoididae and Bothitrematidae. There is no doubt 

 that this is a primitive character which is not encountered in any other 

 families of the same subclass. Thus, according to the given characteristic 

 the fourth family of the order - Amphibdellatidae, differs already by a newer 

 characteristic - bifurcation of the intestine. Two families, (Tetraonchoididae 

 and Bothitrematidae) have one pair of middle hooks each and the remaining 

 2, two each. Although the presence of one pair of hooks is a more primitive 

 characteristic it can have both primary and secondary origin. Because of 

 this it is not possible to attribute great significance to it in the relation which 



542 



