itself. As was already indicated (see page 93), we clearly differentiate 

 two periods in the life of the free-swinnming larva: non-invasional, when 

 the larva is still not in condition to infect the host; and invasional (there 

 is still another non-invasional period when the larva already is not in a 

 condition to infect the host and thus is condemned to perish). The first of 

 these enumerated periods is of special and extremely important signifi- 

 cance because it enables the larvae emerging from the egg which is located 

 on the host individual to infect another individual. In addition to the presence 

 of morphological peculiarities, this is accomplished by the action of a 

 positive phototaxis which has a direct significance in the dissemination of 

 the larvae, whereas the relative speed of their mobility creates the possi- 

 bility of a sufficient eloignment (separation, nobis) of the host on which the 

 parent individual was located. The periods of development of the eggwhich 

 undoubtedly also have adaptive significance are also closely connected 

 not only with the internal peculiarities but also the external ones, and in 

 the first order primarily with the temperature factor. This is clearly 

 apparent in the case of "winter" eggs of Dactylogyrus vastator Nybelin and 

 in the case of the coincidence of the periods of development of the eggs of 

 P. integer rimum with the time of the development of roe and the larvae of 

 the frogs which also depends on temperature. 



The historical process of the adaptation of the life cycle of the 

 parasitic nnonogenetic trematodes to the peculiarities of life of their hosts 

 proceeds along the line of limiting the possibilities of infecting unusual 

 hosts or unsuitable stages of the life cycle of the host and leads in a 

 number of cases to the development of a very narrow specificity in the true 

 sense of the word. However, the life cycle of monogenetic trematodes 

 reveals to us also a number of other methods of limiting in tinne and space 

 the possibility of infecting hosts during less suitable stages of their 

 existence, while preserving the potential capabilities toward infecting any 

 stages and even other types of hosts. This extremely important pecixliarity 

 is of tremendous biological significance. This question will be analyzed in 

 detail in the chapter concerning specificity and incidence of occurrence of 

 Monogenoidea (see pages 283-299 ). 



In the process of adapting to the infection of the host during 

 the specific stages of its life we see two basic tendencies, the first of them 

 leads to the development of adaptations toward infection and conditioning p. 131 



the entire life cycle of the parasite to the younger ages cfthe host as takes 

 place for instance among Protancyrocephalus strelkowi Bychowsky and the 

 second leads to the ability of normal infection of the host, basically in its 

 younger stages but after the parasite has reached maturity on adult indi- 

 viduals. Apparently both tendencies are connected not only with the peculi- 

 arities of the embryology of the host but also with the degree of advance- 

 ment of organization of the parasites and the duration of their lives. As a 

 rule the first tendency is observed among the lowest and the second among 

 the highest of monogenetic trematodes. When we speak about yoxing stages 



137 



