HOW ANIMALS MOVE 



373 



walk. Lizards are light and agile, but progression is 

 aided by a wriggling of the body. 



The locomotive organs of the mammalian quadrupeds 

 are much more highly organized. The bones are more 

 compact ; the vertebral column is arched and yet elastic, 

 between the shoulder and hip, and the limbs are placed 

 vertically underneath the body. The bones of the fore 

 limb are nearly in a line ; but those of the hind limb, 

 which is mainly used to project the body forward, are 

 more or less inclined to one another, the angle being 



^ji^ 



FIG. 325. Feet of Carnivores : A, Plantigrade (Bear); B, Pinnigrade (Seal); C, Digiti- 

 grade (Lion). 



most marked in animals of great speed, as the horse. 

 Some walk on hoofs, as the ox (ungulate) ; some on the 

 toes, as the cat (digitigrade); others on the sole, touch- 

 ing the ground with the heel, as the bear (plantigrade). 

 In the pinnigrade seal, half of the fore limb is buried 

 under the skin, and the hind limbs are turned backward 

 to form a fin with the tail. The normal number of toes 

 is five ; but some may be wanting, so that we have one- 

 toed animals (as horse), two-toed (as ox), three-toed (as 

 rhinoceros), four-toed (as hippopotamus), and five-toed 



