INTRODUCTION 



1 . The Invertebrate Stage 



I. Invertebrata. — Animals with no skeletal axis and without 

 a central nervous system entirely dorsal of the alimentary 

 canal. Their earliest appearance in geologic time is not 

 known. 



2. The Protochordal Stage 



II. Protochordata. — Small marine forms having, during part 

 of their life at least, a rudimentary skeletal axis and other 

 features marking them as a connecting link between the 



