only on Selachii and the rest, basically not on them but on the Teleostomi. 

 Thus, only two species of Benedenia are kno'i^m from sharks whereas the 

 remaining 21 (the basic mass of species)--from the Perciformes and 

 Tetrodontiformes connected with the latter. Two species of Entobdella 



Possibly also on Mugiliformes related to Perciformes (see page 264). 



are encountered fully authentically on sharks and the remaining on Pleuro- 

 nectiformes. The data concerning the finding of the genus Tristoma , 

 typical for Scombroidei, are very doubtful (see page 261 ); however, the 

 possibility of such findings is not excluded. Thus, one can consider that 

 the representatives of all three genera beyond any doubt either accidentally 

 chance upon the Selachii (which is most probable for Tristoma) or are 

 historically linked secondarily with these fishes and transferred on them p. 302 



from Perciformes, Tetrodontiformes and Pleuronectiformes. This transfer 

 is undoubtedly linked not with phylogenetic consanguinity, which is absent 

 in the present cases, but with the commonness of the biology and the places 

 of habitat. This is distinctly visible on representatives of the genus 

 Entobdella, the hosts of which lead very similar forms of life. With this 

 one must note another important circumstance, namely, that all Capsalidae 

 are freely moving forms which undoubtedly facilitates their existence on 

 hosts quite diversified in structure and surface. This in its turn facilitates 

 the easy transfer of these worms to other hosts including the ones which are 

 very distant from each other phylogenetically. The two genera of Capsalidae 

 encountered only on Selachii are monotypic. They pertain to the same sub- 

 family (Megalocotylinae), basically peculiar to Perciformes. One can also 

 consider that here takes place a secondary adaptation to parasitizing 

 Selachii; however, apparently in the present case the transfer from Perci- 

 formes is more ancient for, after the time of transfer the corresponding 

 species, which undoubtedly separated from Megalocotyle -like ancestors, 

 produced sufficiently distinct genera on new hosts. 



In contrast to Capsalidae, Acanthocotylidae as a rule parasitize 

 Selachii, specifically skates. The monotypic genera Lophocotyle and 

 Enoplocotyle form an exception. As regards the first, its occurrence on 

 Perciformes is doubtful (see page 275), it is possible that it is a parasite 

 of the skates. However, the genus Enoplocotyle , separated into a special 

 subfamily, undoubtedly parasitizes the Anguilliformes. As can be surmised 

 from what will be said further on (see page 385 ), it is quite possible that 

 in the present case we deal with the transfer of parasites in a remote 

 historical period from Selachii to Anguilliformes, with subsequent extreme 

 simplification of structure and with considerable separation from, the initial 

 Anthocotyle-like ancestors. 



349 



