TEXT BOOK OF VERTEBRATE ZOOLOGY. 



PART I. 

 MORPHOLOGY OF VERTEBRATES. 



INTRODUCTION. 



NATURALISTS divide all animals into two great groups, the 

 Protozoa, in which all the functions of life are performed by a 

 single cell which constitutes the whole animal, and the Metazoa, 

 in which the body is composed of many cells, and these cells 

 are arranged into layers and organs with a corresponding differ- 

 entiation of functions between the many-celled organs. 



The metazoa in turn are subdivided into several groups or 

 phyla, the highest of which is called Chordata, while the others 

 are frequently spoken of collectively as Invertebrata. The 

 phylum chordata is characterized by the possession of at least 

 three features which occur in no invertebrate, a skeletal axis 

 or notochord arising from the inner germ-layer or entoderm ; the 

 possession of paired gill slits connecting the anterior part of 

 the alimentary canal with the exterior ; and a central nervous 

 system which is entirely on one side of the alimentary canal. 

 Details concerning each of these features will be given on sub- 

 sequent pages. 



The chordata embrace at least three subphyla, the Uro- 

 chordia or Tunicata, the Cephalochordia or Leptocardii, and 

 the Vertebrata, the subject of the present book. It is possible 

 that a fourth phylum, the Hemichordia or Enteropneusti, is to 

 be included here, but as yet there is not agreement upon this 

 point. 



