results. We should also note that many species occurring on various hosts \ 



have varying incidences of occurrence for each one which is fully within 



the law (normal, natural- -in conformity with the established natural law or principles, nobis ).! 



Thus, Ancyrocephalus paradoxus Creplin is discovered in a considerable 



percentage on Lucioperca and in a very small percentage on Perca, The 



incidence of occurrence of Nitzschia sturionis (Abildgaard) on the White 



Sturgeon or Beluga (Huso huso, nobis) is always significantly higher than 



in the Stellated Sturgeon, Sturgeon and Small Sturgeon (Acipenser stellatus , Aci- 



pen-ser , and Acipenser nudiventris, nobis) . 



The intensity of infection of Monogenoidea fluctuates extremely 

 among the various types. Certain species are encountered mostly in a few 

 individuals per host, whereas others occur by the thousands on a single 

 host individual. For instance, within the limits of one genus Gyrodactylus ; 

 G. marinus Bychowsky and Poljansky are encountered on Cod in the Pacific 

 Ocean by the thousands per host, whereas^ groenlandicus Levinson is 

 encountered on Bullhead (Cottus) only in a few individuals per host. 



The clarification of the factors which influence the incidence of 



occurrence should be considered very desirable because it will be possible , 



in this connection to establish rational nneasures controlling illnesses caused ^ 



by them under conditions of pond culture. I 



There is almost no information concerning the life span of mono- 

 genetic trematodes. What is known without a doubt is that along with the j 



forms which live less than a year (a majority of Dactylogyridae , Gyrodactylidae) , | 

 there are numerous types which live several years. Thus, Dactylogyrus I 



iwanowi Bychowsky lives not less than two years, which is evident from its 

 life cycle (see page 110 ). Diplozoon paradoxus Nordmann begins to lay eggs , 



only in its second year and apparently lives not less than a year after the 

 first laying. Mazocraes alosae Hermann .which parasitizes Caspian herrings, 

 infects the fish at the age of not less than a year plus, and perishes together 

 with the host at the age of two to three years during the death of the fish 

 after spawning. Polystoma integerrimum Froelich which infects frogs only 

 in the tadpole stage, is often encountered among frogs which are six years 

 old and thus lives not less than five to six years under favorable conditions. 

 On the other hand, a form of this same P. integerrimum which lives in the 

 gills of tadpoles in the semi-adult condition (see page 121 ) has a life span of 

 not more than one and one half to two months. On the basis of the data of 

 life cycles on Monogenoidea, one can expect that generally the majority of 

 the highest forms have a polyannual existence, although this is not significantly 

 substantiated by specific observations. The continuity of life of the com- I 



mercially important Dactylogyrus vastator Nybelin has not been fully eluci- | 



dated up to this time in spite of numerous studies of its biology. According I 



to the experiments of N. A. Izumova, which were recently carried out, the 

 normal span of life for the majority of D. vastator fluctuates between 25 and 

 40 days, however, it must be taken into consideration that certain individuals 



78 



