22 Evolution and Distribution of Fishes 



such as occurs more or less from rhabdocoel Turbellarians 

 up to larval amphibian tadpoles, inclined some to a semi- 

 ectoparasitic and later to a purely ectoparasitic freshwater 

 life. That this change took place somewhat recently is 

 suggested by the fact that parasitism is made on teleost 

 fishes, which began to evolve in Jurassic and early Cre- 

 taceous time. As these Teleosts — descended from more 

 ancient freshwater ganoid ancestors — gradually migrated 

 seaward, the ecto-parasitic fishes evidently kept attachment, 

 and in time became modified into the endoparasitic and 

 highly predatory myxinoid group, which now appears to 

 breed in the deep seas. 



From ancient representatives of the above Cyclostomata 

 we would trace the evolution of primitive and later of 

 evolved amphibians, which from present knowledge evi- 

 dently developed from Mid-Devonian to early Permian 

 time. 



A second group, that must early have diverged from 

 common ancestors with the Cyclostomata, gave rise to the 

 main stock of gnathostome or jaw-bearing and toothed 

 fishes. These, as traced in succeeding chapters, had al- 

 ready attained to a fair degree of piscine organization by 

 late Silurian time; and purely amid a freshwater environ- 

 ment. Subsequently this main stock split up into classes of 

 gnathostome fishes, from some of which all of our living 

 representatives have arisen. 



