VERTEBRATE ZOOLOGY 



PART I 

 MORPHOLOGICAL TYPES 



CHAPTER I 



THE VERTEBRATE TYPE AS CONTRASTED WITH THE 

 INVERTEBRATE 



Although Vertebrate animals form the subject of this book, 

 it must be said at once that, strictly, the term Chordate would 

 be more correct as a title. The chorda dorsalis or notochord, 

 from which the name is derived, made its appearance earlier 

 in evolution than the vertebral column. There are therefore 

 some animals which have a notochord but no vertebral column. 

 On the other hand, all animals with a vertebral column also 

 have a notochord at some time in their lives. 



The term Vertebrate is used here partly because it is 

 equivalent in importance to Invertebrate, and the most usual 

 division of the animal kingdom lies between these two, and 

 partly because attention is here paid particularly to the higher 

 groups of " true " vertebrates. The lowly and peculiar 

 Balanoglossids as well as the degenerate Ascidians will be 

 left largely out of account, since they are not of much assistance 

 in tracing the evolutionary history of the higher forms. 

 Amphioxus as representative of the Cephalochordates, how- 

 ever, must be carefully considered on account of the help 

 which it gives in interpreting and understanding various 

 matters in higher forms. 



B 



