VI 

 THE VERTEBRATA 



THE Vertebrata are commonly accepted as the dominant 

 phylum of the Animal Kingdom. That position might be 

 disputed by the Arthropoda, for the latter are certainly 

 more all-pervading and have insinuated themselves 

 everywhere where life is found, even into many nooks 

 and crannies which the Vertebrata have left alone. But 

 it is to the credit of the Vertebrata that they have almost 

 completely avoided the dirty corners of parasitism and 

 degeneracy in which arthropods abound, and that their 

 dominance is mainly a matter of power, of high organiza- 

 tion, and finally of intelligence. 



In the most primitive Vertebrata the chief hard parts 

 are external, and though an internal skeleton is always 

 present it was at first of organic material and rarely 

 preserved fossil ; subsequent evolution is marked in 

 general by a steady increase in importance of the endo- 

 skeleton, which becomes a series of articulated bones 

 hardened by calcium phosphate, and a diminution in that 

 of the exoskeleton. 



The first form taken by the exoskeleton is that still 

 found in the sharks constituting the " shagreen ": every 

 point on this rasping surface marks a single " skin- 



