The sizes of the larvae: A. vistulensis - -according to our 

 preparations the length is 0. 06 - 0. 08 mm and the width is 0. 02 - 0. 04 rnm 

 and correspondingly 0. 11 and 0. 04 mm, according to the data of Siwak; the 

 length of the edge hooks is 0. 015 - 0. 016 mm and the inceptions of the 

 middle hooks 0, 016 - 0. 019 mm; in A^ siluri the length is 0. 05 - 0. 08 

 mm and the width is 0. 02 - 0. 04 mm and the length of the edge hooks is 

 the same as in the preceding species and the length of the middle hooks 

 is 0. 022 - 0. 026 mm; A, strelkowi - -length up to 0. 15 mm, the width being 

 0. 03 mm, the length of the edge hooks 0. 013 - 0. 015 mm and the inceptions 

 of the middle hooks 0. 012 mm; A. varicus --the length and the width of the 

 larva has not been established by A. V. Gussew, the length of the edge 

 hooks is 0. 013 - 0. 015 mm and of the inceptions of the middle ones 0. 015 mm. 



p. 161 



Fig. 172, Ancylodiscoides sp. sp. attaching armature of the free-swimming 

 larvae. A--A. siluri (Zandt); B--A. strelkowi Achmerow. 



The edge hooks among representatives of this genus apparently 

 either do not grow completely or almost completely and retain their 

 embryonic traits. Siwak indicates that during the further development of 

 the larva of A. vistulensis the first pair of middle hooks reaches con- 

 siderable length and is already equipped with a connecting plate when the 

 second pair is incepted. At that time the larva reaches 0,15 mm in length, 

 the first pair reaches final sizes when the larva is 0. 23 mm long whereas 

 the second pair develops in full toward the time of the cessation of the 

 growth of the worm (length 0. 74 - 1. 14 mm). According to our obser- 

 vations on A. vistulensis, which in general are similar to the data of 

 Siwak, the first pair of middle hooks and their connecting plate reach 

 the final stage of their development before the inception of the second 

 pair (Fig. 173, A). The latter is incepted in the shape of two slightly 

 curved little parentheses above the attaching disc (Fig. 173, B) and 

 reaches rather large sizes before the second connecting plate begins to 

 form. The supplementary plates are incepted last. The chitinous arma- 

 ture of the sex system has already begun to form about that time. 



p. 162 



The development of A. siluri takes place in a similar fashion 

 to that of A. vistulensis. Apparently during the early stages the 

 difference is expressed only in the sizes of the body and the chitinoxis 

 parts of the attaching disc. However, during later stages among A. siluri 



173 



