Bothitrematidae. One cannot fail to note also that the pipe or sac-shaped 

 structure of the intestine is characteristic for the majority of the families. 

 There is no doubt that this is a very primitive characteristic which in this 

 case has important significance and, as will be seen later, is encountered 

 almost exclusively in Tetraonchidea. Unfortunately the material at our 

 disposal on the development (enabryology) of the representatives of the 

 present order was insufficient, but we can surmise that subsequent studies 

 in this connection will show the correctness of our views. 



1. Family Tetraonchidae Bychowsky, 1937 



(Figs. 33, I, 64, 191, 192) 



Tetraonchinae Monticelli, 1903, part . ; Avielloidea Sproston, 

 1946. 



Tetraonchidea having small or middle sizes in the adult state. 

 The attaching apparatus of chitinous armature has 16 edge hooks, 2 pairs 

 of middle hooks and 1 connecting plate consolidating all 4 middle hooks into 

 one system. The intestinal tract is in the shape of a long single trunk 

 without lateral outgrowths. The copulatory organ is chitinous consisting 

 of a pipe and a single wavy plate supporting it. There are eyes. Other 

 characters are similar to the ones among typical Dactylogyridae. 



Parasites of fresh water and transitory Clupeiformes (Salmondel and 

 Esocoldel). p. 389 



Type and only genus, Tetraonchus Die sing, 1850. 



This family was separated by us in 1937 from Dactylogyridae on the basis 

 of differences in the number of edge hooks, different relations between the connecting 

 plate and the middle hooks, and finally because of the presence of a one-trunked intestine, 

 instead of a two -branched one as among Dactylogyridae. As regards the correlations be- 

 tween the middle hooks and the one connecting plate of Tetraonchus , such a type of for- 

 mation of a simple, single four-hooked attaching system is absent among preceding families. 

 The closest to this type is the nature of articulation of the hooks and of the two connecting 

 plates among the genus Actinocleidus (Dactylogyridae, Ancyrocephallnae, see page 466). 

 However, there are no bases whatsoever to suppose that the plate of Tetraonchus is the 

 result of the merging of two and the existing similarity is, in such a fashion, only a 

 coarse convergence. All this taken together enables us to consider it as fully justifiable 

 to separate the family Tetraonchidae, in spite of the fact that in a number of characters 

 they are very close to the typical four-hooked representatives of another order- -Dactylo- 

 gyridae. As a matter of fact, this similarity is fully understandable if we take into con- 

 sideration the consanguinous relations between the two orders. 



465 



