In order to finish with the group of families pertaining to '' 



Monopisthocotylinea and parasitizing Selachii, let us pause on three more 

 which are encountered only among these fishes. Of these, Monocotylidae 

 is more diversified in nnorphological characteristics and comprises a 

 greater nunnber of genera and species; at the sanne time it is very widely | 



distributed both on sharks and on skates. The circumstance that at the 

 present time Monocotylidae is divided into four well-separated subfamilies | 



points to the considerable antiquity of its parasitizing the given group of 



hosts and correspondingly- -a prolonged time of their comnnon evolution. | 



j 



The rather numerous family of Microbothriidae, concerning I 



the systematic status of which we cannot say anything definite (see page | 



385 ), parasitizes exclusively sharks and selachians which are charac- ! 



teristic for this group. I 



Finally, JLoinnoidae parasitize only sharks of two fanailies and 

 are apparently their very ancient parasites (see page 370), , 



Thus, we see that among Monopisthocotylinea which parasitize 

 the Selachii we can easily distinguish two distinct groups. The first of them, 

 phylogenetically primary, i.e., the beginning of its evolution, or more 

 precisely, of the families which enter it, is connected directly with Selachii. 

 Monocotylidae, Microbothriidae, Loimoidae, and Acanthocotylidae enter 

 into the composition of this group. The second group which is phylogenetically 

 secondary for Selachii comprises only the family Capsalidae. | 



The only family of Tetraonchidea parasitizing skates-- p. 30^ 



Amphibdellatidae- -should also be ascribed to the group which is phylo- I 



genetically primary because its representatives are encountered only on 

 Torpedinidae and are undoubtedly their very ancient parasites. 



The representatives of one order from Oligonchoinea, namely 

 Diclybothriidae, parasitize Selachii and one family, Hexabothriidae, en- 

 countered only on sharks and skates, with the single exception indicated , 

 above, is characteristic for them. There is no doubt that this fannily is also i 

 phylogenetically primary for Elasmobranchii. I 



In conclusion, one can say that Elasmobranchii are characterized 

 as a whole by e very specific, phylogenetically primary fauna of monogenetic 

 trematodes which consists almost completely of strongly separated families, 

 the evolution of which has taken place entirely on the given groups of hosts. 

 Only two of these families transfer to other classes of fishes, and in one , 



case this is undoubtedly linked to phylogenetic relations of tne hosts 

 (Monocotylidae) and in the other it is independent of them (Acanthocotylidae). 

 In addition to the primary fauna of Monogenoidea which was indicated, a 

 secondary fauna is also encountered on Elasmobranchii which basically ] 



transferred recently from Teleostei. Among these phylogenetically 



350 



