in length (that is, approximately, 2- or 3-year olds ) takes place at a fast 

 tempo. The development of eggs lasts from 8 to 11 days and the further 

 development of the larvae on the flounder continues very quickly, apparently 

 with the same speed as among the Dactylogyrus. Taking into consideration 

 that we found several hundred larvae that were in similar stages of develop- 

 ment on the gills of the flounder at the same time and also taking into consider- p. 112; 

 ation the small number of adult worms, one can consider that the deposition 

 of eggs takes place in large quantities during a relatively short period. The 

 infection of large individuals also takes place but both the percentage of in- 

 fection and the quantity of parasites among them are very low. Thus, in the 

 Bay of Anama we found Pr. strelkowi i n flounders of 28 to 32 centimeters 

 in length (4 years and older) only 3 times among the large numbers of fishes 

 examined and, in all cases, bearing a single individual each. From what 

 has been said we can establish that the life cycle of this worm is adapted 

 to the conditions of the littoral zone where Pr. strelkowi i s constantly pre- 

 sent on its host. Infection of mature individuals is almost lacking pre- 

 cisely because of the fact that adult flounders either connpletely or almost 

 completely avoid these depths. We could not explain the continuity of the 

 life of the worms with precision but it is evidently not less than 8 months, I 

 because the three-year-old flounders depart into depths when they are 

 highly infected and toward spring they return either completely uninfected 

 or infected only rarely. Thus the infection takes place only among the ' 

 young groups of fishes which do not migrate very far and which return to the 

 shore earlier. Consequently, in the example of Pr. strelkowi w e see a 

 special type of the adaptation of the life cycle to a determined age compo- 

 sition of the host and to a determined place of habitation within the limits 

 of the range of the latter. In the case of _D. vastator Nybelin, we could 

 not precisely indicate the reasons for the weak infections of the older ages 

 (of the host, nobis) and express only the hypothesis that here the absence ! 

 of contact between the free -swimming larvae and the host and not an age 

 immunity is of prinaary significance. But, in the case of Pr. strelkowi , 

 we can ascertain that the absence of infections in older ages (of the host, 

 nobis) is a result not of immunity, but of special correlations of the life 

 cycles of the parasite and the host. 



I 



The life cycle of Diplozoon paradoxum Nordmann, a widely 

 distributed and well-known parasite of freshwater carp fishes appears to 

 be rather complicated. The basic stages in the life cycle of Diplozoon 

 were described by early researchers, mainly by Zeller (Zeller, 1872c). 

 Certain observations which coincide in principle with Zeller' s were also 

 conducted by us. In the winter period the adult D^ paradoxum are located 

 on the fish in an inactive state. According to Zeller, their sex system not 

 only does not function but also their sex products are as yet developed 

 very weakly. Thus, Zeller observed that a quick development of egg cells 

 and vitelline cells takes place in the spring with the natural rise of the 

 temperature of the water, or during the artificial transfer of the parasite 

 with its host into a warm aquarium even in the winter, and that egg forma- 



116 



