20 centimeters in length, on Stellated Sturgeons up to 45 centimeters in 

 length, on Sturgeons up to 43 centimeters in length; according to the data 

 of Dubinina (1952) and of 184 Sturgeons and 16 1 Sterlets measuring up to 

 40 centimeters from the lower part of the Volga, only 1. 7 per cent were 

 infected. Apparently the largest individuals were the most infected. In 

 the autoreference of N. L. Nichiava's dissertation (1953) there is an indi- 

 cation that 52. 94 per cent of the young Sterlets examined in the region of 

 Seratov were infected with D. armatum . Unfortunately the author does not 

 indicate the sizes of the fishes', it is thought that she had relatively large 

 fishes -- which become infected first in natural conditions. Only yoxing 

 individuals of herring and mackerel which are completely devoid of Mono- 

 genoidea come to the Barents Sea near the region of the Murmansk Biological 

 Station, whereas the adult individuals living near the shores of Norway and 

 England are intensely infested by Mazocraes and Octostoma . It would have 

 been possible to show a significant number of similar examples but we shall 

 only indicate further that the dependence on the incidence of occurrence of 

 the age of the host was well examined on Polystoma integerrimum Forelich 

 by Zeller (Zeller, 1872a). His researches, which were done at the same 

 time in a single place ,showed that frogs aged from 6 to 7 months are 90 per 

 cent infected; that those aged one and one-half years, 33. 3 per cent ; those 

 two and a half years old, 42 per cent ; and those three and a half years, 



27 per cent; and those four and one -half years, 10 per cent. 



The salinity of the water exercises great influence on the inci- 

 dence of occurrence. Thus ,in the freshened part of the Aral Sea we found 

 Dactylogyrus simplicimalleata Bychowsky in 80 per cent of the cases and 

 in strictly marine water--40 per cent, whereas in fresh water--93. 3per 

 cent. Interesting studies were conducted by us on the lakes of the Bara- 

 binskaya Steppe (Bychowsky, 1936a). The influence of salinity on the inci- 

 dence of occurrence of Dactylogyridae on the fishes was clearly apparent. 

 It was indicative that in the most saline parts of Bolshoi Chan and Mali Chan, 

 the monogenetic trematodes were almost absent. Moreover, the only finding 

 of Dactylogyrus nanus Bychowsky on Bream in Bolshoi Chan showed, during 

 the checking of the material, that the infected individual of the fish had a 

 different tempo of growth than all the others and undoubtedly came to Bolshoi 

 Chan very recently and had not succeeded in freeing itself of the parasite. 

 However, side by side with the species vvhich change incidence of occurrence 

 depending upon the salinity, there are also species which behave quite in- 

 differently to it. These are a number of species of Gyrodactylus parasitizing 

 the Three-spined Stickleback which is infected by them both in fresh and 

 ocean water. In addition to the existing direct observations on a number of 

 regions, S. S. ShuUmann conducted very informative experiments at our 

 request. He transplanted sticklebacks infected with Gyrodactylus sp. sp. 

 from fresh straight into sea water and observed that after a short peoriod p. 80 



of "salt-shock" the worms continued normal movement and apparently did 

 not experience any harmful consequences. Reverse experiments in retrans- 

 plantation of fishes and worms from the sea into fresh water gave the same 



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