flfl/«« 



Generally this hook closely resembles the edge hooks in structure. The 

 sizes of middle hooks of the third pair are about 0. 034 - 0. 036 mna. 

 After a relatively small interval of time after emergence of the larva 

 from the egg, the edge and the middle hooks of the attaching disc "cut 

 through," i.e., their edges protrude outside, while the disc itself 

 unfolds and the larva acquires the ability for attachment. A gradual 

 change of the larva into the adult stage takes place after the attachment 

 to the body of the host and the shedding of the ciliary epithelium. The 

 head thickenings change into the so-called attaching grooves, and the sex t 

 system is developed and the nervous and excretory systems grow and 

 acquire their final form. The pharynx grows quickly and becomes barrel- 

 shaped, whereas the intestine develops and forms a number of lateral 



branches. Finally, the attaching 

 disc strongly increases in di- 

 mensions and acquires the shape of 

 a powerful sucker. As has already 

 been indicated, the edge hooks do 

 not grow, whereas the middle ones 

 not only grow very intensively but 

 also change their form. The growth 

 of the middle hooks takes place 

 differently (Fig. 195). The first 

 pair grows approximately 4 to 5 

 times and the growth takes place, 

 as in all middle hooks of the usual 

 type, by way of accretion at the free 

 end of the basal part. The larval 

 hook remains in such a fashion as 

 an unchanged lower part of the hook. 

 The second pair also grows approxi- 

 mately 4 to 5 times, but this growth 

 is different. Here takes place not 

 only accretion at the free end but 

 also a general thickening so that the 

 exterior, lower edge of the hook of 

 the adult individual corresponds to 

 the hook of the larva. Finally, the 

 third pair grows approximately 4 

 times and growth develops in all directions although in a larger measure 

 in the basal part than near the edge so that although the hook greatly 

 changes in shape it nevertheless remains bifurcated in the lower part 

 just as in the larva. 



A few words about the biology of the larvae. According to 

 our observations) the attachment of the larvae by their anterior ends to 

 various underwater objects takes place only when the attaching disc functions 

 fully; until that time the larva only feels the encountered obstacles. 



Fig. 195. Nitzschia sturionis 

 (Abildgaard), attaching armature 

 of an adult worm from the buccal 

 cavity of Huso huso (L. ) near the 

 Island of Sara (Caspian Sea). 



176 



190 



