and establishment of the period (geological tinne, nobis) of the separation of 

 the group. Actually if one should evaluate the orders of monogenetic 

 trematodes from these points of view he will see full substantiation of what 

 has been said. Without entering into details, which are quite clear from 

 what has been said before, it is possible to say accurately that within the 

 limits of Polyonchoinea, Dactylogyridea are the niost simply organized 

 groups followed by Tetraonchidea and finally Gyrodactylidea, and among 

 Oligonchoinea one should place Mazocraeidae (Mazocraeidea ? nobis) first, 

 then Diclidobothriidea and Chimaericolidea. Thus, the morphological data 

 and the analysis of occurrence lead us to conclusions which appear at first 

 glance as paradoxical-- i.e. , the older the order of Monogenoidea basically 

 the less it resembles and the further it is morphologically from the ancestral 

 forms. The reasons for this phenomenon, which clearly contradict the 

 commonly accepted view about the mutual parallel evolution of the parasite 

 and the hosts about which we have already spoken (see page 296 )> apparently 

 lead us to the fact that the hosts which have a lower organization generally 

 retain it because of insignificant changes of conditions of their existence, 

 whereas their parasites at the beginning little adapted to them, during the 

 long period of their presence on these hosts become adapted more and more 

 precisely to the characteristic peculiarities of the host. 



With this, the continuation of existence does not give the parasite the time 

 to stop in the process of adaptation at a fixed level and forces it to evolve 

 in a corresponding direction. In this apparently we can see a substantial 

 peculiarity of the general pace of evolution of a number of parasitic 

 organisms. Without attempting now to analyze this phenomenon as a 

 whole, one cannot fail not to note that we also observe an analogous situation 

 among tapeworms (Fuhrmann, 1928 - 1932). 



The fact that in the analysis of the occurrence we often see 

 transfers of contemporary species from one group of hosts to another serves 

 as an interesting substantiation of what has been said; however, these 

 transfers are made according to definite norms (patterns, nobis), namely -- 

 basically they do not take place among more ancient phylogenetic groups. 

 Thus, one cannot fail to note that Gyrodactylidea and Tetraonchidea from 

 Polyonchoinea and Chimaericolidea and Diclybothriidea from Oligonchoinea 

 practically do not naake any transfers to any group of hosts which are not 

 primarily peculiar to them, and if they make them, they are made only in 

 exceptional cases and then only regressively, if one can so express himself, 

 i.e., on historically more ancient groups (see pages 300 and 411 ). Thus, 

 the ancient groups of parasites have a closer link with their hosts, which 

 again underscores the peculiarity of the evolutionary process of Monogenoidea 

 indicated above. 



Together with this, there is another important trait in the nature 

 of interrelations of Monogenoidea. It is the clearly expressed small quantity 

 of species and genera in the more ancient orders, which is distinctly apparent 

 from the phylogenetic diagram. From our point of view, the reasons for this 



553 



