As was already indicated, the larva of Diclybothriidae (Fig. 

 221) appears somewhat different from the other larvae of this group; it has, 

 in addition to the typical five pairs of edge hooks, two pairs of middle hooks 

 and the latter are of strange shape not resembling any of the middle hooks 

 of other species which have been studied. In our connmon work with A. V. 

 Gussew (Bychowsky and Gussew, 1950) we wrote: "Homology of the chitinous 

 hooks of the larvae with the ones of the adult animals does not occasion ajiy 

 doubt. The anterior three pairs of edge hooks correspond to the hooks of 

 the suckers--clamps, the fourth pair--to the third pair of hooks of the 

 narrowed part of the disc, the fifth pair corresponds to the small hooks of 

 the posterior end and in this fashion it is the only one of them all which is 

 not subjected to any noticeable change in sizes and form. The first and 

 second pairs of middle hooks of the larvae correspond to the ones of the 

 narrowed part of the disc of the adult individual. It is curious to note that the 

 latter hooks and the second pair of middle hooks of the larvae, which 

 strongly differ in shape acquire considerable similarity during further develop- 

 ment. One must note this circumstance in light of the evaluation of the inter- 

 relationships of the chitinous formations of the adult individuals for the 

 building of phylogenetic links within the limits of the group. " From what 

 has been said, one must consider that in comparing the chitinous armature of 

 Diclybothriidae with the one of Microcotylidae , the first pair of middle hooks p. 102 

 of the latter corresponds to the second pair of the fornner and conversely 

 the second pair, that is actually the middle hooks of Microcotylidae, 

 correspond to the first pair of the hooks of Diclybothriidae. 



At first glance the presence of two pairs of pigmented eyea, which 

 are not observed among all other larvae of the second type, appears to be a basic 

 difference between the larvae of Diclybothriunn and the other larvae of this second group 

 as well as the majority of the larvae of the first type. However, we have often indicated 

 that within the limits of the most diversified 



group of Monogenoidea there exists a tendency toward the reduction of (the 

 size of, nobis) eyes and of their number. Taking into consideration also 

 that within the limits of one family eyes can be either present or absent 

 among closely related types, this circumstance cannot have serious phylo- 

 genetic significance. This is substantiated by the very convincing data on 

 the embryology of Axine which appears to be a typical Oligonchoinea. The 

 larvae of this genus which were examined have four eyes and it is essential 

 that among Axine sp. I both pairs are normally developed, among Axine sp. 

 II the anterior pair fuses and the second is normally developed and relatively 

 larger whereas, among A. belones Abildgaard, the anterior pair is fused 

 while the posterior is very weakly expressed. There is clearly a tendency 

 toward the disappearance of the second pair of eyes and the preservation of 

 one fused eye, which is characteristic for typical Oligonchoinea 

 (see page 214 ). 



104 



