CHAPTER IV 

 LIFE CYCLES OF MONOGENETIC TREMATODES 



Studies of the life cycles of parasitic animals are of great inter- p. 105 

 est from the practical as well as the theoretical point of view. A precise 

 knowledge of the life cycles gives into the hands of the specialists who are 

 conducting a struggle with parasitic diseases the possibility of active inter- 

 ference with the course of the cycle during the periods which are suscepti- I 

 ble to human countermeasures so as to disrupt the further normal course of 

 life of the parasite and by this very action to eliminate any parasitic infection. 

 The works on preservation of man from infection by Dracunculus medinensis 

 in Central Asia serve as an outstanding example of this. | 



The theoretical significance of the study of the life cycles is not 

 less important. Their knowledge opens the way to the origin and the processes 

 of the establishment of parasitism and gives us understanding of the reasons 

 for the nature of various parasites and also shows the role of historical factors 

 in the function of interrelations between the parasite and the host. On the 

 basis of the analysis of the life cycle of the parasite, we can ascertain the 

 degree of the inherited fixation in its relations with the host, the role of 

 factors of the external medium in relation to the parasite and the host, and 

 the role of the host as an element of the medium of the parasite. All this 

 taken together provides considerable material for the understanding of the 

 evolutionary processes and thus is of general biological significance. ' 



i 

 Before speaking about life cycles of monogenetic trematodes^ we ' 



must also indicate what meaning we attach to this definition. A life cycle is i 



a very complex pnenomenon. It should not be considered apart from the relations j 



between the animals and the medium in a simplified manner, as is customarily done ' 



when it denotes a period (extending, nobis ) from the deposition of the egg by the mater- 

 nal individual to the formation of the egg by the filial individual or that offspring 

 which is equivalent to the maternal (stage, nobis) which is formed after a certain 

 number of intermediate morphologically or ecologically distinctive phases. Into the i 



understanding of the life cycles, in our opinion, must enter all phenomena which 

 take place in the complex parasite-host-surrounding mediunn, from the for- 

 mation of the egg of the maternal individual until the death of the progeny from 

 this egg, including all stages of development of the daughter individual as well 

 as the generations issuing from her but not equivalent to her morphologically. j 



Within the life cycle, understood in this fashion, we differentiate the sex cycle 

 connected with the reproduction and limited by the time from one period of 

 reproduction to the other, the yearly cycle in which enter all the processes 

 which take place during the year, and the cycle of development which connprises 

 the above-mentioned period extending from the deposition of eggs until the 

 formation of the individual which is equivalent to the maternal one. I 



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