On the basis of this short analysis of the composition of 

 the fauna of monogenetic trematodes of separate orders of fishes and 

 also of the materials on Amphibia and Reptilia,one can make a number 

 of important conclusions. First of all this pertains to the primary and 

 secondary nature of the links of the parasites with a particular group 

 of their hosts. As a result of the discussion we derived a diagrana 

 which gives the idea about the distribution of families of Monogenoidea 

 on large groups of hosts (Figure 264). From this diagram it is apparent 

 that practically speaking the basic large majority of Monogenoidea 

 parasitizes Teleostei with which is linked the flowering not only of the 

 lowest but also the highest Monogenoidea. As regards the remaining 

 groups of hosts, along with the ancient forms which arose during the 

 early stages of phylogenesis of monogenetic trematodes, on them are 

 also encountered species which occur upon them secondarily and which 

 do not change at all on them, i.e. , the same as the ones on their 

 basic hosts, or evolving not more than to the degree of an independent 

 species. The exception is formed by parasites of Acipenseriformes-- 

 the genus Nitzschia w hich undoubtedly transferred to the Acipenseri- 

 formes secondarily and more likely from Percifornaes and,nevertheles3, 

 separated not only to the degree of genus but also formed a special 

 subfamily. 



What has been said applies to the parasites of fishes. As 

 regards the parasites of Amphibia and Reptilia,heie takes place a pro- 

 gressive widening of a circle of hosts beyond the limits of the original 

 hosts- -fishes, in connection with which the change of the group bears 

 a different progressive character with the formation of new species, 

 genera, and even with the appearance of new families (Polystomatidae, 

 Sphyranuridae). 



Further, in the analysis of contemporary Monogenoidea of 

 separate large groups of hosts we can conclude that between the 

 parasites of these groups there exists a minimal number of transfers 

 which have no phylogenetic significance. Practically, with the faunas 

 of monogenetic trematodes of Holocephali, Elasmobranchii, Acipenseri- 

 formes, Teleostei s. str.^ and Amphibia and Reptilia, each is fully dis- 

 distinctive and descends not from each other, but from ancestral fauna 

 which are not preserved. The supposition that these fauna were 

 peculiar to the same groups of hosts and then became extinct on them is 

 unlikely. It is more likely that they were peculiar to the extinct 

 ancestors of the contemporary groups of fishes and that their divergence, 

 apparently very complex in separate cases, took place in the period 

 between the appearance of the primary Monogenoidea and the formation 

 of their contemporary faunas and not groups( !), i. e. , during very 

 remote geological times, at any rate not later than the period of for- 

 mation of the classes of fishes, i.e., not later than the Silurian period. 



370 



