themselves only by suckers, whereas the larvae have only the chitinous 

 armature. At first glance there appears alnnost no correlation in the p. 100 



structure of the attaching apparatus of the larvae and in the adults; how- 

 ever, it is far from that. The edge hooks of the larva which have just 

 emerged from the eggs of the more simply organized monogenetic trematodes 

 have not yet reached their final sizes and shapes, whereas among highly 

 organized representatives they acquire their permanent sizes and shapes 

 in the embryonic period. Similarly, we observe that edge hooks of the first 

 (group, nobis) reach relatively larger sizes than those of the second. At 

 the same time with this, the lowly organized forms have a more complexly 

 arranged handle of the edge hooks which annong them is often differentiated 

 into a number of divisions well -delineated from each other. Finally, very 

 often separate pairs of edge hooks are distinguished from each other by 

 size which happens only in exceptional cases among highly organized mono- 

 genetic trematodes of this group. 



Along with the changes of edge hooks, we observe the appearance 

 of middle hooks, which also have a regular character. Among the more 

 lowly organized groups they are absent from the free-swimming larvae and 

 also among adult fornns (the absence of middle hooks appears as a secondary 

 phenomenon among highly organized specie^. Later, middle hooks appear, 

 at first- -after the embryonic period, and then during it in such a way that 

 the inception of the hooks takes place successively during more or less 

 early stages, and finally - entirely in the egg. The free -swimming larvae 

 grow a successive number of middle hooks from one pair to three as is the 

 case among adult forms. Just as in the edge hooks, one can note that the 

 relative tempo of the growth of the middle hooks decreases with the increase 

 in the organization of the adult animal. 



Finally anmong those specieswhich have been studied, the larva 

 of Sphyranura oligorchis already has one pair of suckers in addition to the 

 chitinous armature aa it emerges from the egg, just as the adult form. The 

 growth of the middle hooks and suckers of Sphyranura continues even later. 



The second group of larvae demonstrates basically the same normal 

 complications of the attaching apparatus. Among the adult animals related 

 to it, attachnnent by means of clamps or the presence of sucker-shaped 

 clamps is characteristic, as among Diclybothriidae and Diclidophoridae. 

 Their number varies greatly within limits of the entire group, but the 

 general tendency goes toward the increase of their number from four pairs 

 to several tens or even hundreds. The larvae emerging from the egg have 

 chitinous arnnature of the hook type and some more highly organized forms 

 also have clamps. Among Mazocraeidae and Diclidophoridae, the free- 

 swimming larvae have five pairs of edge hooka, Discocotylidae have three pairs 

 of edge hooks (among Diplozoon paradoxum they are absent), one pair of middle 

 hooks and one pair of attaching clamps. Microcotylidae usually have five pairs of 

 edge hooks and two pairs of middle hooks [among Microcotyle spinicirrus MacCallum, 

 six (?) pairs of edge hooks 



102 



