studied in the Delta of the Volga (June, 1954). The development of the 

 eggs took place approximately with the same speed as on the Island of 

 Hanka and the ennergence of the larvae was observed on the third day. 

 The material obtained was also partially impregnated with silver. Further 

 description is based upon the study of the preparations from both regions. 



. . viUli,„H//((/,;(,/i 



,v,.V J.'. -A: 



0.01mm 



Fig. 191. Tetraonchus monenteron (Wagener), location of ciliary cells 

 of the body of a free-swimming larva. The larva on the left is in dorsal 

 view, and on the right in ventral view. Impregnated with silver. 



The larvae which emerged from the eggs have the customary 

 elongated form. Their length is about 0. 12 mm with a width of 0. 04 mm; 

 in fixed state they are somewhat shorter--0. 08 - 0. 10 mm long with a 

 width of 0. 06 to 0. 07 nnm. The ciliary covering is located in three zones. 

 In the silvered specimens (Fig. 191) it is evident that each zone consists of a 

 different number of large, rounded cells (with a diameter about 0, 005 mm). 

 The anterior zone lies basically on the dorsal side from the very edge of 

 the body and reaches posteriorly to the level of the first pair of eyespots 

 The edges of the anterior zone extend to the ventral side where they termi- 

 nate, reaching not further than the corresponding eyespot on each side, 

 and in such a fashion the middle of the ventral surface of the anterior edge 

 of the body is deprived of ciliary covering. The total number of cells of 

 the anterior zone equals approximately 32, of which 18 lie on the dorsal 

 side and 6 on each side of the ventral. In opposition to the preceding 

 anterior one the middle zone of ciliary cells lies mainly on the ventral 

 side, only partially extending to the sides of the body. It consists of two 

 sections (about 14-15 cells), which start from the level of the pharynx anc 

 extend posteriorly somewhat further than the edge of the first half of the 

 body of the larva. Both sections of the ciliary cells of the middle zone 



p. 17: 



186 



