which is not inferior to the species which reproduce hy means of egg depo- 

 sition, which at first glance may seem completely improbable because it 

 is commonly accepted that viviparousness is linked with a decrease in the 

 numbers of the progeny with the better insurance of survival. The second 

 deduction from what has been said is that epidemic outbreaks which are 

 observed in carp industries can be easily explained even with the presence 

 of a weak infection in the producers which are the initial source of the in- p. 136 

 fection of the young ones. Finally the third conclusion from these theo- 

 retical calculations is that the most desirable conditions in the carp indus- 

 tries is the most rapid transfer of young fishes from the spawning ponds 

 into the growing ponds which completely coincides with direct observations 

 on the dynamics of the numbers of Dactylogyrus and Gyrodactylus in 

 natural conditions. 



One must say that the calculations cited and the discussion 

 about the ideal quantity of Dactylogyrus and Gyrodactylus , even though 

 they explain certain phenomena observed in carp industries under con- 

 siderable crowding of hosts, are nevertheless far from what exist in 

 natural conditions. Actually the quantity of individuals depends not only 

 on the productivity of the worms but also on a great number of external 

 factors which, in a great majority of cases, determine the quantity of 

 parasites. This also applies in considerable measure to the carp indus- 

 tries, but it is more strongly expressed in nature. 



At the head of the factors which greatly influence the quantity of mono- 

 genetic trematodes, one must place the means of infection, the 

 temiperature regime of the surrounding medium, the correlations between 

 the place of deposition among the egg -depositing forms with the place of 

 the most frequent presence (occurrence, nobis) of the schools of the hosts 

 and the frequency of their populations. and so forth. 



It is interesting to note that the question about the methods of 

 infection of the hosts of Gyrodactylu s cannot be considered as completely 

 solved even at the present time. As was already pointed out the repre- 

 sentatives of this genus apparently have different life cycles, and at the 

 samie time one must consider tentatively the considerable differences be- 

 tween the theoretical accretion of numbers of the worms and the one which 

 is observed in nature, as a rule, where a large number of Gyrodactylus on 

 the body of the host, even in conditions of spawning ponds of the carp indus- 

 try, is an exception. Consequently, the infection by these worms has 

 certain peculiarities which prevent the survival of the majority of born 

 individuals. Whether this is a result of the complexity of the infection 

 of the fishes by means of adult worms or whether some other factors play 

 a role here so far is unknown. 



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