INTRODUCTION 



13 



which the intestinal (anal) and urinogenital apertures are situated, 

 and posterior to which again is the tail. Head, trunk, and tail 

 constitute the body-axis, as distinguished from the fins or limbs 

 (appendages), which arise from the trunk and of which there are 

 typically two pairs. In addition to the paired appendages, 

 median fins are present in aquatic forms. 



SPINAL CORD 

 BRAIN \ 



NEURAL TUBE (CEREBRO SPINAL CANAL) 



NOTOCHORD 



ORAL CAVITY 



INTERNAL GILL SLITS 



HEART LIVER 



CLOACA 

 ^TEI^ X URINARY BLADDER 



'RILE: nuci 

 PANCREAS 



FIG. 11. DIAGRAMMATIC LONGITUDINAL SECTION OF A VERTEBRATE ( ? ). 



SYSTEMATIC ZOOLOGY is concerned with the classification of 

 animals, on the basis of their relationship to one another, into 

 certain divisions and subdivisions, the chief of which are 

 designated as Phyla, Classes, Orders, Families, Genera, and Species. 

 In this connection, structural resemblance (homology) is taken into 

 account, and not mere functional resemblance (analogy). 



A general classification of the principal Vertebrate groups, 

 sufficient for the purposes of this book, and also a table showing 

 the order of their appearance in past time, are given on the 

 following pages. 



