HOW ANIMALS ARE CLASSIFIED 



Of all the bone-structures in the body, the most important is 

 the spine, or backbone. This is in part hollow, 

 and through it run the nerves connected with all 

 the lower parts of the body. If you were to break 

 your spine, you would no longer be able to walk 

 or stand, or use your limbs at all. And you 

 would lose all sense of feeling in them too, so that 

 they might be wounded, or burned, or even cut off, 

 without paining you in the least. 



The spine is not a single bone, but is made 

 up of a number of little bones, which are fastened 

 to one another in such a manner that the whole 

 can be curved or bent. In your own spine there 

 are thirty-three of these little bones, which are 

 called "vertebrae". 



As every animal which possesses bones must 

 have a spine, or spinal or vertebral column, as it 

 is called, and as the spine is always made up of 

 these little vertebrs, those animals that have bones 

 are called VERTEBRATES, while those that have 

 not are called INVERTEBRATES. 



These two divisions are separated into smaller 

 groups, which we call classes. The classes, there- 

 fore, correspond with the brigades in an army. 



In the Vertebrates there are five of these 

 classes, namely: 



(i) Mammals. These suckle their young, 

 and have hearts containing four chambers. 



(2) Birds. These are covered with feathers, 

 and do not suckle their young, which they rear 

 from eggs. 



(3) Reptiles. These are cold-blooded, and possess hearts with 

 only three chambers. 



(4) Batrachians. These resemble reptiles, but pass through 

 a tadpole stage before attaining to their perfect form; and 



(5) Fishes. These live in the water, breathe by means of gills, 

 and have only two chambers in their hearts. 



Human Spine or 

 Backbone 



