VERTEBRATE ZOOLOGY 



CHAPTER I 



PRINCIPLES OF VERTEBRATE MORPHOLOGY 



Definition. — The vertebrates may be defined as animals having: — 

 pronounced antero-posterior, dorso-ventral, and bilateral axes; inter- 

 nal metameric segmentation, especially of the mesoblast; a central 

 nervous system dorsal in position and tubular in structure, with a well- 

 defined central canal or neurocoel; a well-defined head, characterized 

 by highly specialized sense organs and by a concentration of nervous 

 tissue into a complex brain; the aUmentary tract opening by anterior 

 mouth and posterior anus, and provided with paired pharyngeal 

 clefts; a notochord derived from the primitive endoderm and situated 

 between the central nervous system and the alimentary tract; an open 

 ccelom, at first segmented, but later the segmental coelomic cavities 

 unite to form the large pericardial, peritoneal, and, in the mammals, 

 thoracic cavities; a closed circulatory system quite distinct from the 

 coelom; a post-anal prolongation of the body into a metamerically 

 segmented tail, without ccelomic cavity; usually paired appendages, 

 pectoral and pelvic. These characters and a few others serve to mark 

 off the vertebrates quite sharply from all other groups. Several of the 

 most fundamental of these characteristics must now be discussed. 

 The diagrams on the following page (Fig. 1) illustrate most of these 

 characters. 



THE FUNDAMENTAL ARCHITECTURAL PLAN OF VERTEBRATES 

 The Three Morphological Axes 



The Axis of Polarity (Primary Axis). — ^A typical vertebrate has an 

 elongated form, with head and tail ends clearly defined. An imagi- 

 nary line drawn from the extreme anterior to the extreme posterior 

 end indicates the primary structural and functional axis of the body, 

 which is designated the antero-posterior or apico-basal axis. The or- 



1 



