The reproduction of monogenetic trematodes can take place 

 during fixed (or limited, nobis ) periods, or it can be nnore or less extended 

 in time. As a rule reproduction does not take place during the winter 

 among fresh-water forms, whereas among marine forms this occurs in a 

 number of cases. The majority of Dactylogyridae and Tetraonchidae 

 apparently reproduce more or less steadily during the course of the entire 

 warm period, but the conditions of temperature and oxygen diet influence 

 the tempo of reproduction to a great extent. Thus, under existing conditions 

 of the middle of the European U. S. S. R. , common species of Dactylogyrus , 

 Ancyrocephalus and Tetraonchus produce eggs beginning from April up to 

 September and, depending upon the nature of the year, much later. We 



do not have any data about the time of reproduction of Monocotylidae and 

 Microbothriidae and a number of other Polyonchoinea. According to our 

 data, Capsalidae reproduce mainly during the first half of the summer 

 months. The presence of embryos among Gyrodactylidae is observed all 

 year round, although their birth is apparently adapted to the warm months. 

 Among the highest species, a part reproduces during all the summer months, 

 and some also in the winter months, whereas others have a fixed period p. 82 



connected with the peculiarities of the life cycle of the parasite and of the 

 host. For instance, among Polystoma integerrimum Froelich the repro- 

 duction takes place only during a very short period (directly after the 

 emergence from the places of hibernation), and lasts about a month --at 

 the latitude of Leningrad from the end of April to the beginning of May ( sic) . 

 The gill forms of Polystoma integerrimum reproduce during the entire 

 period of its maturity until the end of life^ which coincides with the end of 

 the metamorphosis of the tadpoles, that is, one and one half to two months 

 Let us note that the species which reproduce periodically and, in this connection 

 which lay a relatively much larger number of eggs in a short period of time 

 than the species with extended laying, in a majority of cases have a longer 

 or more voluminous uterus, sometimes containing a great number of eggs 

 (among P. integerrimum more than 100 eggs, among Microcotyle gotoi 

 Yamaguti up to 150). For more detailed information concerning repro- 

 duction see pages 105-137. 



Among species with extended periods of fertilization, the activity 

 of the male eex system takes place more or less steadily during the entire 

 warm period of the year. Among such species during that time we always 

 find all or almost all stages of spermatogenesis in the testes, and in the 



seminal ducts and in the vesicula seminalis a greater or lesser number of 

 ripe spermatozoids ready for fertilization. Amongspecies which h^ve a 

 more or less short period of fertilization the male sex system acts periodi- 

 cally. Thus, among P. integerrimum increased spermatogenesis begins in 

 the period which follows the laying and ends at the end of the summer, and 

 at that time almost the entire testis and partially also the vas deferens is 

 filled with ripe spermatozoids. The latter are preserved in this shape until 

 spring and during the period of fertilization are almost completely used up. 

 During the time of fertilization the vas deferens among Polystoma is strongly 



80 



