CHAPTER II. 



THE BRANCH OF VERTEBRATA, OR VERTEBRATES. 



THE Vertebrata embrace all animals which have a 

 bony or cartilaginous axis, called the spinal column, 

 with an elongated cavity above it, containing the great 

 nervous centre, the brain and spinal cord, and an- 

 other below it, containing the organs of respiration, di- 

 gestion, circulation, and reproduction. From the brain 

 and spinal cord branch the nerves, in the form of threads, 

 to every part of the body. In all the Vertebrates the 

 skeleton is internal, and constitutes the frame upon 

 which the muscles are placed, the skin, with its appen- 

 dages, covering the whole. The axis of the 

 skeleton is made up of parts, which are more 

 or less movable upon one another. Each of 

 these parts is called a vertebra, and hence 

 the axis is often called the vertebral axis, or 

 A vertebra vertebral column, as well as spinal column and 



backbone. 



All Vertebrates have red blood, which is propelled 

 through the system by a muscular heart. The mouth 

 is furnished with two jaws, usually armed with teeth, 

 which are more or less bony, and often enamelled. 

 Vertebrates exhibit perfect bilateral symmetry, that is, 

 the organs are arranged in pairs on the two sides of the 

 body. The eyes are two, ears two, and the locomotive 

 appendages never exceed four. The animals of this 



