Ancyrocephalus (s. lat. ) cruciatus (Wedl. )--We also know only 

 the structure of the free-swimming larva of Anc. cruciatus (Wedl.). It 

 was obtained by us seven days after the deposition of the eggs by the worm, 

 from the gills of the Viun [ Misgurnis fossilis (L. )] , in June 1938 (at the 

 Peterhof Institute of Natural Science hear Leningrad). The larva is 0. IZ 

 mm in length and has a width of 0. 03 nrun. The head end has weakly ex- j^^j 



pressed lobes. Two pairs of eyes are strongly developed. The ciliary 

 epithelium is located in the same fashion as it is in A. paradoxus Creplin, 

 The attaching disc is weakly expressed. On it are located 14 edge hooks 

 (Fig. 153) of the usual dactylogyrid form, about 0.017 - 0.018 mm in length 

 and with the length of the terminal hook 

 about 0.006 mm. These dimensions re- 

 main without change also among adult 

 worms. The first pair of middle hooks 

 in the shape of plates of about 0. 020 mm 

 in length which are slightly curved and 

 sharpened on the lower end, already Fig. 153. Ancyroce- 



lies in the center of the attaching disc phalus (s_. lat. ) cruciatus 



along with the edge hooks. Besides that, Wedl, attaching arma- 



a slightly curved very tender connecting ture of a free -swimming 



plate of the first pair of middle hooks larva, 



lies freely between them. It is about 



0. 008 mm in length. There are no traces of the second pair of middle 

 hooks or of their connecting plate. In such a fashion we see a different 

 development in time of the middle hooks in Anc . cruciatus (Wedl. ), and 

 the inception of the first pair takes place even during the embryonic 

 development considerably earlier than the appearance of the traces of the 

 second pair. As is known, the adult Anc. cruciatus (Wedl. ) (Fig. 154) 

 has a structure of the middle hooks and of the connecting plates similar 

 to that of Dactylogyrus and also a similar inner organization, so that one 

 can easily visualize, in generalcharacter the pro.cjress of further develop- 

 ment with the exception of the correspondence of stages of development 

 of separate parts in time. In connection with the latter, one can indicate 

 that the growth of the first pair of middle hooks continues after the in- 

 ception of the second pair, for we observed young immature worms with 

 underdeveloped hooks on both pairs. 



Ancyrocephalus (s. lat. ) vanbenedeni (Parona and Perugia)-- 

 During the work at Karadaga Biological Station in August of 1957 we 

 hatched the larva of A. vanbenedeni several times. The period of develop- 

 ment in all instances was about 4 days. The larvae which emerged from 

 the eggs had a length of about 0. 075 mm with a width of about 0. 025 mm. 

 The larva (Fig. 155) is torpedo-shaped with a powerfully developed "little 

 tail" at the posterior end. The ciliary epithelium is distributed in three 

 zones just as among the preceding species. The attaching disc has 14 

 edge hooks; no traces were discovered of the middle hooks in the process ^^^ 



of inception. The edge hooks are of identical shape with a weakly 



164 



