and rounded at the upper end. The hooks of this pair are very similar to 

 the corresponding ones in Nitzschia. Their sizes are 0. 026 - 0. 028 mm. 

 In their description of the development of B. melleni, Jahn and Kuhn do 

 not indicate that the growth of the middle hooks is accompanied by changes 

 in their shapes. In B. derzhavini they are considerable which is apparent 

 from the comparison of the nniddle hooks of the larvae with those of the 

 adult animals (Fig. 201 and 202). The growth of the separate pairs of the 

 middle hooks is also unequal in B. derzhavini; thus the first pair increases 

 in length about 6 times^ whereas the second about five times and the third 

 9 times. Thus the tempo of growth and its correlation among middle hooks 

 of both types of Benedenia which were studied is completely different. The 

 development of the larvae until the adult stage is analogous to the one which 

 we saw in Nitzschia sturionis Abildgaard and Benedenia melleni (MacCallum) 



p. 182 



16. The development of Polystoma Zeder 



We have at our disposal materials on the development of three 

 species: P. integerrimum Froelich, P. nearcticum (Paul), P. ozaki 

 Price. A number of authors studied P. integerrimum (Zeller, 1872a, 

 1876; Halkin, 1901; Goldschmidt, 19"02a, 1902b; GaTlien 1932a, 1933, 

 1934b, 1935). It was studied with a considerable degree of completeness 



starting from cleavage. From the 

 fall of 1927 until the fall of 1929 we 

 also conducted research on the 

 development and life cycle of P. 

 integerrimum, which unfortunately 

 remains unpublished. We shall note 

 that P. integerrimum is characterized 

 in the adult stage by the presence of 

 6 suckers on the attaching disc which 

 also serve for the attachment of the 

 animal. The chitinous armature of 

 the disc, which exists among adult 

 worms, either does not play any role 

 at all in attachment (edge hooks) or 

 an extremely insignificant role 

 (middle hooks). As is known, there 

 are two mature forms in the life 

 cycle of P. integerrimum : one 

 which grows slowly and parasitizes 

 Fig. 203. Polystoma integerrimum the urinary bladders of frogs, while 

 Froelich, free -swimming larva. the other, with a quick tempo of 



Natural size about 0. 3 mm. (Accord- development lives in the gill cavity 

 ing to Halkin, 1902). of the tadpoles. Taking into con- 



sideration that both of these forms, 

 have, in addition, a different morphology (Figs. 22, 127) one could expect 



197 



