CLASS MAMMALIA. 99 



CHAPTER XXIII. 



CLASS MAMMALIA. 



The class Mammalia (Latin, mamma, the breast) includes 

 the ordinary Quadrupeds, all of which suckle their young 

 for a longer or shorter time after birth. As an example 

 of this class we may select the domestic Dog (Ganis fami- 

 liaris). 



The Dog is a " quadruped " in the strict sense of this 

 term, since it possesses four limbs, two fore-legs and two 

 hind-legs, and all of these are employed in supporting the 

 weight of the body. Unlike many quadrupeds, however, 

 the Dog does not place the soles of his feet upon the 

 ground, but on the contrary walks upon the tips of the 

 toes, thus obtaining his light and springy gait. The soles 

 of the feet, therefore (fig. 46, m m), are raised above the 

 ground, and are covered with hair. The fore-feet have 

 five toes each, the hind-feet (as a regular thing) only four 

 toes each ; and all the toes are furnished with strong claws. 

 The claws of the Dog, however, unlike those of the cat, 

 are always exposed to view, and cannot be withdrawn 

 into sheaths. 



In place of the feathers which are so distinctive of Birds, 

 the skin of the Dog is covered with the pecvdiar ajipen- 

 dages known as "hairs ;" and these appendages are char- 

 acteristic of the Mammals, in which they are never and 

 at all times of Life altogether wanting. 



The body of the Dog is supported internally by a com- 

 plicated skeleton (fig. 46), or series of bony structures, 

 which protects the internal organs from injury, and serves 

 for the attachment of the muscles by which all the move- 

 ments of the animal are effected. In briefly examining 

 some of the more im]3ortant points connected with this 

 skeleton, it may be considered as consisting of the skull, 

 the backbone and its appendages, and the bones of the 



