VERTEBRATES AND INVERTEBRATES 9 



All animal life falls into two great divisions : the Verte- 

 brate and the Invertebrate. 



The Vertebrate Animals (Animalia Vertebrata) form the 

 first sub-kingdom of the animal world. They are easily 

 distinguished by the possession of an internal skeleton, 

 or bony framework. Their body consists of a head, trunk, 

 and limbs. The head is composed of the skull, which 

 incloses and protects the brain ; and the face, in which 

 are the organs of taste, smell, sight, and hearing. The 

 head rests upon, or is attached to, the vertebral column, 

 which is built of a number of separate bony rings, movable 

 one upon another, and forming a canal for the spinal cord, 

 which is the great nerve centre of the body. A man has 

 twenty-six separate bones in his vertebral column, while a 

 python has no less than four hundred. The limbs, which 

 never exceed four, are in pairs. The blood of the verte- 

 brates is warm and red ; all, with the exception of fishes, 

 breathe air through lungs ; and all of them possess a heart. 



The Invertebrate Animals include all beings of lower 

 organisation. They possess neither cranium nor brain, 

 no internal skeleton, and no spinal cord. With the 

 exception of the earthworm their blood is colourless and 

 cold. Not a single invertebrate uses the mouth in re- 

 spiration ; they breathe through holes or slits in the neck, 

 sides, and even the tail. The jaws move horizontally 

 instead of vertically. No vertebrate has more than four 

 true limbs, but invertebrates seldom possess so few. All 

 insects have six ; the spider, which is not an insect, has 

 eight ; crabs and lobsters have ten. Many insects are 

 fitted with wings in addition. Scientific men do not 

 always agree exactly how to classify some of the Inver- 

 tebrates, if only because they include the insects, which 

 in variety of structure and appearance, and still more so 

 in the numbers of their individuals, far surpass all the 

 larger branches of the animal world. 



All animal life can be arranged in two great groups : the 

 warm-blooded and the cold-blooded. Man, the four-footed 

 beasts, and birds fall into the first group ; snakes, frogs, 

 fishes, &c., are contained in the second. It is customary, 

 however, to divide the Vertebrates as set out overleaf. 



