2. Order Tetraonchidea Bychowsky ord . nov. 



Tetraonchinea Bychowsky, 1937. 



Polyonchoinea, having larvae with 16 edge hooks on the attaching 

 disc. Adult fornns having an attaching armature consisting of 16 edge hooks, 

 1-2 pairs of nniddle hooks, and one connecting plate, which is sometimes 

 absent (Tetraonchoididae). Sometimes there is supplementary armature in 

 the shape of plates of varying forms. The copulatory organ has a 

 chitinous pipe and a supporting apparatus. The cephalic end is ot the 

 dactylogyrid-type or with an undifferentiated glandular edge or it even 

 forms 2 glandular lobes (Bothitrematidae). The digestive system has 

 one intestinal trunk which does not have any lateral outgrowths; one family 

 has 2 intestinal trunks terminating blindly (Amphibdellatidae). 



Parasites of marine and fresh water Teleostei and marine 

 Elas mobranchii . 



Into the circle of this order belong 4 families --Tetraonchidae p. 388 

 Bychowsky, Amphibdellatidae (Carus) Bychowsky, emend . , Tetraonchoididae 

 Bychowsky, and Bothitrematidae Bychowsky, fam . nov . 



During the study of monogenetic trematodes relative to Poly- 

 onchoinea after the publication of our work in 1937, we were forced to 

 separate the families belonging to the suborder Tetraonchinea further from 

 the rest of the Dactylogyridea and in this connection isolate them into a 

 completely independent order. In spite of the considerable similarity of 

 the lowest Tetraonchidea with Dactylogyridea, undoubtedly these groups are 

 far removed from each other. As is clear from what has been said earlier, 

 the Tetraonchidea have a very primitive characteristic- -a large number of 

 edge hooks in comparison with Dactylogyridea. As we have often indicated, 

 the number of edge hooks is a very constant sign characterizing large groups 

 of Monogenoidea and, as a result of this distinction by this characteristic 

 a lone, one should consider the representatives of Tetraonchidea as being far 

 removed from the groups of species having the other number of them. With 

 this, however, it is also characteristic that the edge hooks of Tetronchidea, 

 in contrast to the large majority of the lowest Dactylogyridae, either do 

 not grow at all or almost do not grow during the postembryonic period-- 

 this is an indication of considerable specialization and of the transfer of 

 the function of attachment to other formations. Indeed, the characteristiv- 

 tendency for Tetraonchidea is at first the strengthening of the role of 

 the middle hooks and at the same time with a larger degree of co-ordination 

 and then toward the transfer of the function of attachment to the disc itself. 

 Although it is analogous to the one which we observe in Dactylogyridea this 

 process takes place completely differently. This is seen in the case of the 

 formation of the septa and of the secondary sucker-shaped pulvelli of 

 Tetraonchoididae or in the characteristic strengthening of the disc in 



464 



