General Characters of Vertebrates. 3 



The chain of these rings or disks, around or replacing 

 the notochord, which forms the axis in the adult stage 

 of all but the lowest of the vertebrates, is called the 

 vertebral column, and each disk, with the parts im- 

 mediately joined to it, is called a vertebra (fig. 2). 



Each vertebra has attached to it behind a ring or 

 arch (made up of two lateral projections or processes) 

 which surrounds the spinal marrow, and forms the 

 wall of the neural cavity. This arch is called the 

 neural arch. 



The mouth opens at the foremost end of the body 

 in all vertebrates, and communicates internally with a 

 cavity called the pharynx, on whose walls, directly or 

 indirectly, the blood-vessels are arranged for the 

 purposes of respiration. This part of the digestive 

 canal ! is pierced by slits at some period hi the life of 

 each vertebrate. 



Below the pharynx is a narrow part of the diges- 

 tive canal, called the oesophagus, which passes between 

 the spinal column above and the heart below, and 

 leads into the stomach, from whence the intestinal 

 canal is continued, to open at the posterior end of 

 the body ; directly below the stomach the duct of the 

 liver opens in all vertebrates, and this organ is 

 peculiar in this sub-kingdom, in that the vein which 

 conveys the impure blood back from the digestive 

 organs enters this gland and breaks up within it into 

 a network of fine vessels, which, reuniting, pass back 

 from hence to the heart. The vessel which thus 



1 The digestive or alimentary canal is a tube traversing 

 the whole length of the body, in which the food is digested, 

 and its nourishing part taken into the blood. 

 B 2 



