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 Vertebrata, 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 Mglier Invertebrata, A — 

 and one of the Vertebrata, B. a Wall of the body ; 6 Alimentary cansl ; e Hfflmal or 

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

 cord of the Vertebrata, enclosed in a separate tube ; cA 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 lenscth : 



