of the host we mean, in the majority of cases, individuals up to two years 

 old among polyannual hosts or even younger annong quickly-maturing and 

 growing fishes; however, one must bear in mind that for each type of host 

 of egg -depositing Monogenoidea there is a proper period before which it 

 (the host, nobis) is not infected. The reasons for this are not clear and it 

 is possible that they are hidden in the nnorphological peculiarities of the gill 

 apparatus which the large mass of monogenetic trematodes parasitize. 

 N. A. Izumova, who was interested in this question, established that among 

 Dactylogyrus vastator the infection of young carp is possible only when they 

 reach the sizes from 10 to 12 millimeters and she came to the conclusion 

 that this stands in direct relation to the degree of morphological differentia- 

 tion of the gill apparatus of the fishes. 



The representatives of the viviparous family Gyrodactylidae 

 have a special type of life cycle. Its basic peculiarity appears to be the 

 absence of a special morphological differentiating stage which serves for 

 the infection of the new individuals of the host. The infection, as we 

 succeeded in establishing in special studies, takes place by fully mature 

 parasites which transfer from one host individual to another when they 

 come close to each other. One must say, however, that as yet much re- 

 mains xinclear because the reasons causing the wornns to leave the host are | 

 not fully understood, for we never observe direct contact between fishes in | 

 normal conditions, even among the majority of gregarious forms. Con- I 

 sequently, the worms and especially those which are located as a rule on ' 

 the gills must make special effort to come out of them and actively try to 

 transfer to another host. We hope that further experimental research will 

 clarify this interesting question. The life cycle of Gyrodactylus has ap- 

 parently a relatively simple character. Multiplication of worms takes 

 place during the entire summer period and apparently more or less evenly 

 all the time. The daughter individual is born when it is already fully formed 

 and does not differ from the miother individual either in structure or in 

 size (Fig. 131). Based on our observations of Gyrodactylus sp. sp. from 

 the Stickleback, in the mother individual, after a rather short period of 

 time following the birth, a new egg enters the uterus and begins to cleave 

 and the process of its development until birth of the new daughter individual 

 lasts normally about 4 to 5 days. The number of births in one mother indi- 

 vidual has not been exactly ascertained, but according to the indirect evidence 

 it is not less than 3 to 4. We happened to observe that after one of the births 

 of the daughter individuals the mother individual almost immediately perishes, 

 whereas in other instances the mother individual after a certain period of i 

 depression becomes normal, beginning to feed and to move actively. Thus, | 

 according to all observations the life span of a particular individual Gyro- 

 dactylus lasts not less than 12 to 15 days and possibly even considerably longer. 

 The fate of the daughter individual is somewhat different than the mother 

 individual, for after the birth it has already in the uterus a strongly developed p. 132 

 embryo which is born approximately a day later and which in turn contains 

 an embryo in the uterus. Since up to four ennbryos usually result from one 



138 



