100 



CHAPTER XXIX. 



SUB-KINGDOM VERTEBRATA. 



The sub-kingxlom Vcrtcbrata may l:)e shortly defined as in- 

 cluding animals in tchich the body is comjwscd of a succession 

 of definite segments, arranged along a longituclincd axis ; tlie 

 main masses of the nervous system (brain and sjnnal cord) are 

 situated along the dorsal surface of the body, and are completely 

 shut off froin the general body-cavity. The limbs are never 

 more than four in number, and are cdways turned CLVXiy from 

 that aspect of the body tqjon which the main masses of the ner- 

 vous system are situated. In all, the nervous axis is lorimi- 

 tively supported, by a celhdar rod, which is termed the " noto- 

 chord ; " but in most the notochord is replaced in the adult by 

 the bony axis knoivn as the " spine " or " vcrtehrcd column." 



The past existence of Vertebrate animals is chiefly recog- 

 nised by the preservation of their hard structures. These 

 hard structures are of two kinds — some belonging to the 

 internal or true skeleton (endoskeleton), others being of the 

 nature of horny or bony plates, scales, or appendages of 

 various kinds, developed in the integument (exoskeleton). 

 The nature of the exoskeleton in the Vertebrates differs 

 very much in different cases, and it will be considered when 

 treating of the separate groups. It will be well, however, to 

 give an extremely general and brief view of the structure of 

 the endoskeleton, taking for this purpose a ]\Iammal as a 

 typical form. In this way the student will be enabled 

 readily to trace the modifications of the skeleton in the 

 lower forms, and will without difficulty comprehend the 



