In a number of groups the role of the chitinous hook armature 

 decreases in proportion to the growth of the animals and the powerfully 

 developed disc acquires the main significance. It transforms into a sucker 

 similar to that of leeches or digenetic trematodes. During this process 

 the edge hooks do not grow as a rule but retain their initial sizes while 

 the middle hooks remain without change, losing their signficance (Calceo- 

 stomatidae), or continue to grow, partially preserving the attaching role 

 and become mainly a supporting apparatus (Nitzschiinae and others). 



Characteristic for Polystomatidae is the fact that the chitinous 

 armature is preserved during the appearance in the postembryonic period 

 of suckers on the attaching disc (the latter also increases strongly, ^ and the 



Cases of its disappearance, although it may be partial, are known. 



middle hooks grow and for a cer- 

 tain time still function, whereas 

 the edge hooks remain partially 

 on the posterior and anterior edges 

 of the disc and partially in the 

 centers of the developing suckers 

 (one in each sucker) without 

 changing sizes, but cease to function 

 completely. 



Among species in which 

 attaching clamps develop in the 

 disc, the fate of the chitinous larval 

 arnniature differs, but generally it 

 doesn't grow (exception, Diclybo- 

 thriidae, see above) and is not 

 fully preserved. For the most part 

 the edge hooks enter into the 

 composition of the clamps which 

 are in the process of formation p 



and thus they disappear and one 

 pair is either preserved for the 

 entire life of the worms without 

 change or is cast off completely. One pair of middle hooks that apparently 

 represents modified edge hooks with changed shapes, as a rule does not 

 grow (just as for edge hooks the exception is Diclybothriidae), whereas in 

 a number of cases the second pair grows (Mazocraeidae) and in others 

 remains without changes (Microcotylidae). In a number of Microcotylidae 

 both pairs of middle hooks and one pair of the edge hooks, which was 

 already indicated, usually lie in common in a small narrowed section of 



Fig. 118. Heteronchocleidus buschkieli 

 Bychowsky, adult worm from the gills 

 of little aquarium fish Macropodus 

 opercularis (L. ) Leningrad. 



104 



106 



