In such a way, returning to Diclybothriidae, it seems clear that 

 their larvae belong in the category of the second type despite some differ- 



ences, just as the family itself stands within the limits of Oligonchoinea (see 

 page 402 ). As a peculiarity of the family connected with the morphology of 

 the adult forms appears the fact that, as has been pointed out, all chitinous 

 elements, with the exception of the fifth pair of edge hooks, grow intensely 

 during the first embryonic period. Apparently the family of Hexabothriidae 

 (see page405 ), which was studied by us, has the same peculiarities. 



The subsequent development of the larvae of both types proceeds 

 with a varying degree of speed according to the degree of complication of the 

 attaching apparatus, on the one hand, and of the progressive development of all 

 systems of internal organs on the other hand. The complication of the 

 attaching apparatus from the larval stage to the one possessed by the adult 

 forms is observed in the following directions: 1) intensified development of 

 the chitinous attaching apparatus of the hook type; 2) intensified development 

 of the attaching disc itself as an organ of attachment; 3) the development of 

 the suckers or attaching clamps on the attaching disc in the process of 

 development; 4) the appearance of new disc -shaped organs of attachment 

 not homologous to the initial (or primary, nobis) attaching disc. 



The development of the hooked chitinous apparatus proceeds 

 along the line of complication and differentiation of edge and middle hooks 

 and the appearance of supplementary chitinous formations supporting the 

 hook apparatus and coordinating its work. The edge hooks of the more 

 primitive Monogenoidea have the same shapes in all stages of life and grow 

 synchronously with constant speed. Among more highly -organized species 

 the process develops along two lines --on the one hand deviation in sizes 

 of different pairs of edge hooks often accompanied by a change in shape 

 takes place, and on the other hand the changes proceed along the line of 

 preservation of the initial sizes of the hooks and a gradual loss of their 

 significance with the increasing role of the middle hooks. The latter have, 

 as was indicated earlier, varying shapes and sizes and a tendency toward the p. 103 



increase of the number from one to two or even three., pairs. In connection 

 with the complication in the structure of the hooks and their more powerful 

 development, connecting plates of different types which were already mentioned 

 (see page28 ) usually appear. As a rule, these connecting plates are developed 

 more powerfully in forms having a larger disc and more powerful armature. 

 In an original form of Heteronchocleidus buschkieli Bychowsky (see page 164 

 and Fig. 118) the adult worms have three powerfully developed middle hooks, 

 the fourth remains undeveloped and the two connecting plates which appear 

 are located in such a way as to serve for the connection of all three hooks 

 into one coordinate system. Among many Dlplectaninae the three con- 

 necting plates (Fig. 56) lie in such a way that on the one hand they support 

 the disc in the completely unfolded shape, and on the other hand, they are 

 connected with the four middle hooks so that the latter, lying in pairs, play 

 the role of two attached, pincer -shaped systems and not four independent 

 formations. 



105 



