28 HISTORY OF THE HUMAN BODY 



sil, with which we are acquainted, and only suggested in part by 

 the members of the next higher group, the cyclostomes. This 

 group comprises eel-like forms, to be carefully distinguished, 

 however, from true eels or from any of the true vertebrates, 



/ MODERN BIRDS 



FIG. 7. Phylogenetic tree of vertebrates. 



Double underscoring indicates an extinct group; single underscoring one that has 

 but a few living representatives. The boundaries of the Classes are represented by 

 dotted lines. 



since they possess neither jaws nor teeth in the sense of those of 

 the higher vertebrates, but have the mouth surrounded by a 

 circular lip which is capable of being extended so as to re- 

 mind one of the hood possessed by Amphioxus. Within this 

 mouth there are variously shaped spines or plates which serve 



