The life cycles of monogenetic trematodes have been almost 

 unstudied. There are data concerning the life cycles and the sex cycles 

 of several representatives of the genus Dactylogyrus in the literature 

 (Nybelin, 1925; Wilde, 1937; Groben, 1940; Lyman, 1951b; Bauer, p. 106 



1954; and others), Polystoma integerrimum (Zeller, 1872a, 1876; Gallien, 

 1934c, 1935), Diplozoon paradoxum (Zeller, 1872b), Benedenia melleni 

 (Jahn and Kahn, 1932), Microcotyle spinicirrus (Remley, 1942) and certain 

 others (see chapter on development and the appendix thereto, pages 138 

 to 216). Along with these materials, which are far from complete, data 

 about separate phases of the life cycles of various fresh water and marine 

 Monogenoidea are scattered in numerous systematic works. Thus, this 

 important question has not been subjected to serious specialized research 

 until the present time. Our material is not exhaustive either, but neverthe- 

 less it furnishes a great deal (of information, nobis) for the understanding 

 of the life cycles of monogenetic trematodes and contradicts the usual 

 notions concerning their extreme simplicity. 



When it becomes necessary to examine fishes on nnonogenetic 

 trematodes (sic) of ciny body of water (when it becomes necessary to 

 examine the monogenetic trematodes of fishes from any body of water 

 Hobis) , especially marine, many instances provoke perplexity. First 

 of all, in an overwhelming majority of the cases we find that only adult 

 parasites, predominantly of the same age group, are encountered on these 

 fishes, whereas different stages in development and the young worms 

 either are encountered not at all or extremely rarely. Furthermore, what 

 seems incomprehensible is the fact, as was mentioned earlier (see page 78 ), 

 that very frequently thewornns are encountered in 100 per cent of the cases 

 in one age category of the host, but are absent in other (age categories of 

 the same host, nobis). Still greater perplexity results from questions which 

 arise from the study of certain pelagic fishes, as for instance the mackerel 

 of the Black Sea. The eggs of monogenetic trematodes which parasitize 

 this fish are deposited on the bottom. If one considers the conditions (of the 

 bottom, nobis ) of the Black Sea, one wonders how fishes can be infected 

 100 per cent when the hydrogen sulfide zone prevents eggs which fall to the 

 bottom from developing. All these phenomena and many others which appear 

 during the study of the distribution of monogenetic trematodes in nature are 

 clearly a reflection of the peculiarities of the life cycles and not a chance or 

 accident connected with the shortcomings of the research. 



To have a correct approach to the understanding of the life cycle 

 of monogenetic trematodes one must, first of all, clearly understand some of 

 the biological groups. As has already been pointed out, Monogenoidea can 

 be divided into two groups according to the method of reproduction, that is 

 into a relatively small group of viviparous (only representatives of Gyro- 

 dactylidae) and another group of egg -laying types to which the overwhelm- 

 ing majority of species are related. This division points, also, at the same 

 time, to a considerable difference in the method of infection of the host be- 

 cause the viviparous forms infect by means of direct contact with a host 



109 



