directions and ways of morphological changes become understandable. The 

 same functional problems can be solved differently morphologically and at 

 the expense of different parts of the attaching armature. For illustration of 

 this condition we shall cite several examples. Among D. anchoratus 

 (Dujardin) and the species close to it which do not have the supplementary 

 plate, the interior extensions of the middle hooks are extremely elongated 

 and it is they that support the upper edge of the attaching disc, strengthening 

 and conditioning its fixed dimensions. The same problems among worms 



Fig. 146. Dactylogyrus cornu Linstow, the development of the supplementary 

 plate of the attaching disc of worms from the gills of Abramis brama (L. ) 

 from the Bay of Finland near Peterhof. Explanation in text. 



from the groups of D. sphyrna Linstow are solved constructionally differently, 

 not by way of length but by a considerable widening of the interior stem of 

 the middle hooks and, in addition to that, by the powerful development of one 

 of the two pairs of the edge hooks which are increased twice in length and many 

 times in width. Finally in the group of D. kulwieci Bychowsky the same role 

 is fulfilled not by the offshoots of the middle hooks but basically by the 

 supplementary plate which is strongly developed and almost reaches the 

 dimensions of the middle hooks in length. 



Concluding the description of the development of Dactylogyrus 

 one must say a few words about the chitinous parts of the copulatory apparatus. ' 



Their inception takes place at rather early stages of the development and 

 further growth and differentiation takes place rather quickly (just as that of 

 the entire sex system). In connection with this, the worms become mature I 



and first deposit eggs sometimes even before the final formation of the i 



attaching armature. The last circumstance has a meaning in the work on ij 



systematics of a given genus and sometimes leads to undesirable results i; 



when the stages of development of earlier known species are described as p. 148 i 



individual species as happened in the works of Nybelin (Nybelin, 1936) and | 



Alarotu (Alarotu, 1944). The duration of the development of Dactylogyrus 

 from the free-swimming larva until adult mature individuals is very 

 different. The most precise data were obtained by N. A. Izumova from 

 D. vastator Nybelin (which are presented in the third chapter, page 104 ). 



157 



