by Grobin (Grobin 1940) are completely inaccurate. 



The middle hooks (Fig. 141 and 142) are incepted already after 

 the attachment of the larva and its growth to a certain size, and in the be- 

 ginning they are located not in the attaching disc but somewhat in front of 



it in the parenchyma of the posterior 

 part of the body, and only later do 

 they descend into the disc. At the 

 earliest stage of development of the 

 middle hooks they have the appearance 

 of two almost parallel threads of 

 more or less equal thickness, with 

 somewhat sharpened lower ends 

 (corresponding to the point of the 

 fully developed middle hook) and with 

 the upper end slightly curved toward 

 the middle of the body of the larva. 

 Just as among edge hooks its further 

 development takes place by means 

 of certain thickening of the existing 

 parts and mainly by way of the 

 accretion on the upper end, at first 

 on the base of the hook, and then at 

 both extensions. During this, in a 



^^:i/ 



Fig. 140. Dactylogyrus similis 

 Wegener, attaching apparatus of 

 a disc of an adult individual from 

 the gills of Leuciscus leuciscus 

 (Li. ) from the Gulf of Finland near 

 Peterhof. Sixth and seventh pairs 

 of edge hooks are (sizes and shapes.') 

 represented in addition to the 

 middle ones. 



number of cases, no changes in form 

 take place in the existing parts, in other cases during the early stages 

 (until the beginning of its functioning), the hook is somewhat straightened 

 in comparison with its normal form and bends only during the period of 



p. 144 



Fig. 141. Dactylogyrus alatus Linstow the developmient of middle hooks 

 of the disc of the worms from the gills of Albumus alburnus from the Delta 

 of the Volga. 



growth of the second half of its base part. The inception of the connecting 

 plate already takes place (Fig. 143) at the time when the middle hooks 

 occupy the final position on the attaching disc but are still in the early 



152 



