determined host, or to penetrate into its body. Finally; the third is the 

 ability to reach maturity within the given host and to await progeny which 

 can survive or to reach a determined phase of the nornnal cycle. If the 

 last two conditions refer in equal measure also to forms with passive nneans 

 of infection, then the first sharply differentiates these two groups. Sonne - 

 what exaggerating, it is possible to say that in the first case the parasite 

 "selects the host" and in the second, conversely, the host "picks up" the 

 parasite. 



During the active attack of the host, two cases are theoretically 

 possible. Either there is no "selection" and the larva falls on any living 

 being which it encounters and its infection can take place in all cases or 

 partially because of some certain peculiarities of the structure and physi- 

 ology of the host as well as the parasite itself, or the larva selects a certain 

 object for some characteristics which are not yet known to us (most probably 

 some odors or some other attracting peculiarities of biochennical nature) 

 and infects it mostly in the "obligatory" order so to speak (it is under- 

 standable that "errors" are possible and that larvae which attack a particular 

 species of host will not be able to develop further on it because of some 

 combination of their own peculiarities and those of the "unsuccessful host"). 

 If,during the first variation, one can expect the infection of a nnore or less 

 wide circle of hosts "suitable" to the "demands" of the parasite, then in the 

 second the narrow adaptability to the parasitizing of a limited number of 

 species is much more probable. It is understandable that in all our dis- 

 cussions one must not forget about the role of the external medium (in 

 relation to the larva and the future host) which has a determining significance, 

 for the contact between the larva and the host is possible only under favorable 

 conditions. Both of the "theoretical" cases considered above undoubtedly 

 occur in nature. Thus, we can consider that very many cercariae of 

 digenetic trematodes are quite indifferent to the selection of the second 

 intermediary host where they are transformed into the encysted phase, 

 the adolescaria. There are known cases when the same species of 

 adolescaria is encountered on the most varying invertebrate and verte- 

 brate animals (for instance, of adolescaria of certain Strigeidae and others). 

 However, where the larvae actively penetrate into the final host the second 

 variation, that is, a clearly expressed selective ability apparently takes place 

 as a rule. Consequently, we observe in nature a usually relatively small circle 

 of hosts among species with the active method of infection of the final host. The 

 means of infecting the host apparently reflects different ways of 



historical formation of the bioceonotic pair, parasite-host, In those cases 

 where parasitism arose from predation, commensalism or space -parasitism, 

 one should consider the active means of infection as primary and where the 

 origin of parasitism was through fortuitous penetration into the digestive 

 system of the host, the passive means is characteristic. Complex cases 

 when the same parasite has different means of infection in different phases 

 of the life cycle are obviously secondary phenomena, as is the phenomenon 

 of alternation of generations (Dogiel, 1947). Although we indicated that a 

 relatively narrow circle of hosts is customary only with active means of 



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