16 



INTRODUCTION. 



any skeleton, this is external (forming an " exo-skeleton "), 

 and it is really nothing more than a hardening of the skin. 

 The limbs, when present, are turned toward the neural aspect 

 of the body. 



In the Vertebrate, on the other hand, the body, if trans- 

 versely divided, exhibits two tubes. In one (Fig. 1, B) is placed 

 the main mass of the nervous system (the brain and spinal 



FIG. 1. Diagrams representing transverse sections of one of the higher Invertebrata, A 

 and one of the Vertebrata, B. a Wall of the body ; b Alimentary canal ; c Haemal or 

 blood-vascular system ; n Nervous system ; n' Cerebro-spinal axis, or brain and spinal 

 cord of the Vertebrata, enclosed in a separate tube : ch Noto-chord or chorda dorsalis. 

 (Slightly altered from Huxley.) 



cord). In the other tube are the alimentary canal, the haemal 

 or blood-vascular system, and certain other portions of the 

 nervous system, which are known as the " sympathetic " sys- 

 tem of nerves, and which correspond to, or are homologous 

 with, the entire nervous system of the Invertebrata. Further, 

 in the Vertebrata there is always an internal skeleton (or 

 endo-skeleton), the central stem of which is usually consti- 

 tuted by a true backbone or " vertebral column." When this is 

 not present, there is always a structure which will be after- 

 ward described as the " noto-chord " or " chorda dorsalis." 

 Lastly, the limbs of the Vertebrata^ when present, are never 

 more than four in number, and they are always turned away 

 from the neural aspect of the body away, that is, from the 

 side on which the main masses of the nervous system are 

 placed. 



Subjoined is a short tabular view of the main existing 

 divisions of the Animal Kingdom, the characters and smaller 

 divisions of which will be considered- hereafter at length : 



