This order includes Z families which were united into one 

 before our common work with Gussew. This formerly undivided family 

 was ascribed to the superfamily Polystomatoidea Price, 1936, i.e., it 

 was placed closer to forms which had a completely different origin and 

 only convergent similarity with the group under discussion. This similarity 

 was of a very coarse nature. Thus, the presence of the 6-suckered "disc" is 

 common, but it was not taken into consiaeration that the 6-suckered condition p, 

 of Polystomatidae is a primary phenomenon, whereas the same number on 

 the anterior, widened part of Diclybothriidea is secondary, and that the 

 suckers of the latter are not homologous to the corresponding pairs of 

 suckers of the former. Undoubtedly the location of the edge hooks in the 

 suckers appears similar, but in the first place this phenomenon bears a 

 more common nature and is not only encountered among these groups (see 

 page 37 ), and in the second place the nature of the correlations between 

 the hooks and the soft part of the suckers- -clamps is different in both 

 groups. Furthermore, if one considers the sex system, the latter has a 

 different nature in Polystomatidae than among all the representatives of 

 Oligonchoinea, as is apparent from what has been said before. One could 

 consider the presence of the ductus genito-intestinalis as a rather important 

 characteristic which links Polystomatidae to Diclybothriidea but as we have 

 already indicated, this characteristic arises independently within the limits 

 of both subclasses of monogenetic trematodes (see page 32 ). Strange as it 

 may be, it seems to us that the most astonishing similarity is the unstable 

 characteristic of the number of the eyes of the larva. Thus, in Polystomatidae 

 (just as in the majority of Polyonchoinea) and among Diclybothriidea the 

 larvae (and adults) are equipped with 2 pairs of eyes, whereas in the 

 majority of Oligonchoinea usually there is only one double eye. As regards 

 the larvae of Hexabothriidae it is more probable that they don't have any 

 eyes at all, which, as is known, is not a rarity among larvae of naonogenetic 

 trematodes. Nevertheless the presence of 4 eyes points to the likelihood 

 that this is a very ancient characteristic peculiar to common ancestors of 

 both subclasses. As a whole, there are no bases to attribute important 

 significance to this characteristic, taking into consideration the frequent 

 reduction of eyes in a particular subclass, and the presence of 2 or even one 

 merged eye in separate cases is found also in Polyonchoinea, and 4 eyes 

 also among other Oligonchoinea (Microcotylidae, see page 214 ). Thus, all 

 the characteristics common to Polystomatidae and Diclybothriidae are 

 either primary or are actually convergent, to which one cannot attribute 

 any significance which would liken these groups phylogenetically. 



Two families --Diclybothriidae Bychowsky and Gussew and 

 Hexabothriidae Price enter into the composition of the order. 



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