when they are adult worms, whereas the egg -laying forms infect, in the 

 free -swimming larval phase, through the medium which surrounds the host. 

 In their turn, as was described before, egg -laying forms fall into two groups -- 

 egg -laying proper, that is depositing eggs into the body of water in which 

 the host and parasite are located, and into the egg-attaching group in which 

 the eggs delay themselves for development on the body of the host. Evalu- 

 ating what has been said above we can say that the deposition of eggs on the 

 bottom appears to be more primitive, whereas the viviparous type with in- 

 fection of the fish by means of contact appears to be more complex and un- 

 doubtedly is the latest historical development. 



The sinnplest life cycle is of the form with a prolonged period of 

 egg deposition in which eggs are deposited on the bottom of the water reser- 

 voir and with a free -swimming larva which has the ability of infecting the p. 107 

 host in all, or more precisely, in almost all phases of its (the host's nobis) 

 life cycle. Such a type of life-cycle we observe among fresh-water repre- 

 sentatives of the genus Dactylogyrus. The most studied is the cycle of an 

 important parasite, D. vastator Nybelin. Because of the fact that a disser- 

 tation and a number of special works of N. A. Izumova (1953, 1956a, 1956b) 

 are dedicated to the biology of this species, we shall dwell on the oviparous 

 cycle only briefly. 



The worms deposit eggs which develop in different periods de- 

 pending upon the temperatures of the water. A number of authors (Nybelin, 

 1925; Nordquist, 1925; Wunder, 1929, and others) maintain that D. vastator 

 has two types of eggs --"summer", relatively small and developing quickly 

 and "winter", large and developing slowly. According to Nybelin, during 

 the warm months reproduction takes place only by means of "summer" eggs 

 and during this period a number of generations of the worms take place. 

 Then, with the arrival of low temperatures, the worms begin to deposit large 

 "winter" eggs which hibernate at the bottom of the body of water and termi- 

 nate their development toward the following summer. The worms them- 

 selves all perish in the fall and are absent during the winter. Beth questions, 



i.e., concerning "winter" and "summer" eggs and about the existence of 

 adults during the winter tinne, appear to be fairly complicated and were 

 solved only in recent times, and mainly by the work of the Soviet researchers. 

 During the study of the periods of the development of the eggs of ^ vastator , 

 we established (Bychowsky, 1933d)that at a temperature of 21.5 to 24. 5° the 

 larvae will emerge from the egg on the fourth day, at a temperature of 18. 2° 

 on the fifth day, temperature 17. 7°--in six days, temperature l6.4°--the 

 seventh day, and somewhat later we observed that at a temperature of 15. 5° 

 the development continues for more than ten days. According to E. M. 

 Lyman (1951a) who especially studied this question, at mean temperatures 

 of 28° the development continues three days and at temperatures of 24 -- 

 four days, of 20° --five days, 16°- -six to seven days, 12°- -ten to eleven 

 days, 8°--27 to 28 days, 4°--is totally absent. In such a fashion, a 

 severe lowering of temperature not only retards development but also 



110 



