improbable to us. In our opinion, the reasons for this phenomenon lie in 

 the range of conditions for existence among various adult categories of 

 the host, and in the lesser possibilities of the contact of the latter with free- 

 swimming larvae, D. vastator (see also further page 110 ). It is possible, 

 however, that different factors take place and also that 

 the tissues of the gills of the carp of different ages possess unequal 

 tenderness, and by this very fact the possibilities of attachment of the larva 

 to the gills in different ages of the carp are not the same. 



Thus, the life cycle of D. vastator consists of three stages 

 closely connected with the cycle of the host and with the yearly changes of 

 the conditions of the body of water. Reproduction of D. vastator starts in 

 the early spring and continues for the entire summer during which a signifi- 

 cant number of generations are passed, and during the period of the warmest 

 months the nunaber of parasites increases extremely. As a result of the 

 fact that toward the period of mass emergence of the free-swinnming larvae 

 the developing fish of the year are susceptible to infections and are located 

 in places where a significant deposition of the eggs of D. vastator occurred, 

 the bulk of the latter at first develops on young -of -the -year fishes and not on 

 the older ones which go away into places which are not suitable for infection. 

 Toward the fall the tempo of reproduction and the speed of embryologic 

 development decrease and finally the period of winter depression 



begins during which the small number of worms which were preserved on the fishes 

 decreases to such an extent that toward the spring only isolated individuals 

 remain. In the spring the period of reproduction of the surviving individuals 

 (including those •which spend the winter without reaching maturity in the fall) 

 begins, and the infection of fishes takes place first by the larvae which 

 emerge from the hibernated eggs, and then from the eggs deposited by the 

 surviving adults. In the begiiuiing, the infection develops among the older 

 ages of fishes and only later in the newly appearing generation. Thus, in 

 natural conditions the crowding of fish is less and the conditions for contact 

 between the larvae of D. vastator with different ages of the host are not the 

 same and basically insufficient so that the infection of the fishes is usually 

 small and the outbreaks of fatal epidemics are practically absent. The result 

 is different in culture farms where artificial conditions inevitably lead to the 

 increase of infection of earlier ages which causes epizootics of Dactylogy r us . 

 Hence, the necessity for constant active reference of parasitologists to the 

 conscious (detectable, nobis ) change in the character of the quantity and in- 

 fection of D. vastator in order to prevent the outbreak of epidemics. 



The cycles of the majority of freshwater Dactylogyrus have a p. 110 



similar nature and differ mainly in details. Thus, D. solidus Akmerow 

 is apparently a more cold-loving type (Bauer and Nikolskaya, 1954), and in 

 this connection in the conditions of our carp farms the nature of the 

 change of the infection of fishes by it is somewhat different and the maximum 

 quantities of the parasite fall at a time of the sximmer period. 



113 



