84 STRUCTURE OF THE MAMMALIA. 



rels, and the sloth, which spend most of their time 

 in trees. A few, as, the mole and the hamster, 

 dwell chiefly underground ; others, as the beaver, 

 the water-rat, the white bear, and the seal, are 

 sometimes on land, and sometimes in the water, 

 and a few live entirely in the sea. 



Such of these animals as live partly on land and 

 partly in water have their fingers and toes con 

 nected by a membrane or web, which is of great 

 use to them in swimming. Bats have the long 

 finger-like toes of their fore-feet united by a 

 delicate web, which, when the toes are opened, 

 expands and forms wings, and -are by this means 

 enabled to fly. They are the only mammalia 

 which have this power, though there is one species 

 of squirrel, called the flying-squirrel, which can 

 support itself for considerable distances through 

 the air when springing from tree to tree. 



Many animals in this class have horny and un- 

 divided hoofs, as the horse and the ass ; others 

 have cloven feet, as the sheep and the cow. Most 

 of them walk on their toes, only a very few species 

 planting the sole of the foot on the ground. 



The bodies of most of the mammalia are covered 

 with hair. Some have it scattered very thinly 

 over the body, as the elephant and the rhinoceros; 

 others are very thickly covered, as the sheep. 

 This hair varies greatly, both in length and colour. 

 In some species it is curled and woolly, as in many 

 varieties of dogs ; in others it is stiff and straight, 

 forming bristles ; and sometimes it is in the shape 



