of parasites with their hosts. In order not to overload the text by excessive 

 enumerations, the data on this question are presented in Fig. 262. From 

 it we can see that each of the families of Amphibdellatidae and Acantho- 

 cotylidae is encountered only on one family of Selachii, L,oimoidae--on 

 two very close, Microbothriidae --on two very close and one somewhat 

 more removed; whereas Monocotylidae and Hexabothriidae are widely 

 distributed within the limits of the entire group Selachii and are encountered 

 on a vast majority of the families (probably in the future it will be found 

 also on a number of other insufficiently studied families of sharks and 

 skates). At the same time one must draw attention to the distinct difference 

 between the biological peculiarities of both of these last families. Thus, 

 Microcotylidae are wornns which move along the body of the host freelyj 

 whereas Hexabothriidae at best are capable of small changes of location 

 on a very limited section. If at the same time one takes into consideration 

 that Hexabothriidae are discovered also on Hexanchidae while Micro- 



cotylidae are not, one can suppose with a large degree of certainty that 

 the origin of the first is more ancient and that they appeared on Selachii p. 306 



earlier than Monocotylidae. Again we shall return to this question later 

 (see page 448 ). 



It is rather easy to characterize the fauna of monogenetic 

 trematodes of the Elasmobranchii and Holocephali as a whole, the one of 

 Teleostomi.on the other hand, can be evaluated only with great difficulty, 

 because the basic mass of all Monogenoidea is on these fishes. Exceptions 

 are formed by groups, already naentioned before, parasitizing Selachii and 

 Holocephali and also the suborder Polypisthocotylinea (order Gyrodactylidea) 

 peculiar only to higher groups of vertebrates (Amphibia and Reptilia). In 

 connection with what has been said, in order to evaluate the characteristic 

 peculiarities of infection on Teleostomi we will have to analyse first the 

 fauna of separate orders of fishes of this class. The data on this subject 

 are reproduced in Table 17. 



First of all, the nature of the fauna, of mionogenetic trematodes 

 of Acipenseriformes offers interest in connection with the fact that this 

 group is separated into a separate suborder Chondrostei by a large majority 

 of researchers. It is true that L. C. Berg (1940) objects to this, con- 

 sidering that the so-called Chondrostei gradually merge into Holostei and 

 consequently that such a division can be made only artifically. In addition 

 to that, another problem of Acipenseriformes is not without interest. It 

 is a question of their link with Selachii which is now resolved distinctly 

 negatively, whereas A. N. Severtsov (1922a, 1931) considered such links 

 quite probable. At first glance, the considerations of Severtsov expressed 

 in terms of the Monogenoidea are substantiated because representatives of 

 the same order Diclybothriidea are encountered both on Selachii and on the 

 Acipenserids. However, as will be shown later (see page 403 ), the differences 

 between both families of Diclybothriidea are quite large and they show 

 differences in direction in the general way of regressive development of 



354 



