not with the same intensity (Fig. 119) depending upon external conditions, 

 mainly on the tennperature and oxygen regime. Data of N. A. Izumova 

 (see page 86 ) show that under average conditions in the Leningrad 

 region during the summer, _D. vastator deposits from 4 to 10 eggs in the 



space of 24 hours. She also noted 

 that under unfavorable conditions 

 during the rising temperature and 

 a worsening of oxygen regime the 

 number of eggs deposited increases 

 considerably. The observations of 

 Izumova were conducted on worms 

 which were located in natural con- 

 ditions on the gills of their host. 



■^ Contrary to her data, all infor- 

 mation about the deposition of eggs 

 of D. vastator cited in the works of 

 Groben, Lyman and others has no 

 significance to the understanding of 

 the life cycles because it concerns 

 the deposition of the eggs by worms 

 which are located in artificial and 

 obviously completely unfavorable 

 conditions. These data basically 

 can be utilized only for the clarifi- 

 cation of the rate of the production 

 of the eggs. 



An important factor to the p. 109 

 understanding of the nature of the 

 life cycle of D. vastator is that in 

 natural conditions the levels of in- 

 fection in the host fluctuates, not 

 only with the season of the year, but 

 also with the age of the Carp. Thus, 

 according to the data of Lyman (1951b) 

 infection of the fishes takes place first at the age of not less than 10 days 

 and mainly toward the end of the first month of their existence. These data 

 are substantiated in the experimental studies of Izumova. 



During the following two months of the life cycle of the Carp, 

 the percentage of infection and the number of the individuals of the parasite 

 increase very greatly and later toward the fall-winter period, it (D. vastator , 

 nobis) is encountered also among the older ages of the fishes on a considerably 

 smaller number of individuals and in a decreased percentage of infection. 

 The reasons for these seasonal age changes in the nature of infection have 



not been clarified and can be interpreted differently. Thus, one can suppose 

 that a relative age immunity enters into play here, which, however, seems 



8 9 10 II 12 13 14 15 16 17 18 19 

 Growth of worm in a 27 hour period 



Fig. 119. Dactylogyrus vastator 

 Nybelin, dependence of the tempo 

 of the deposition of eggs on the age 

 of the worms. Observation on egg 

 deposition of worms located in 

 natural conditions on the gills of very 

 young carp. Fish Industry "Ropscha, " 

 Leningrad region (According to 

 Izumova, 1953). 1--Very young 

 fish. 2--Young fish no. 2. 

 3--Young fish no. 3. 4--Young 

 fish no. 4. 5--Young fish no. 5. 



112 



