THE VERTEBRATES: MAMMALS 239 



asses, oxen, camels, reindeer, elephants, and llamas are 

 beasts of burden and draught; swine, sheep, cattle, and 

 goats furnish flesh, and the two latter milk for food; the 

 wool of sheep, the furs of the carnivores, and the leather of 

 cattle, horses, and others are used for clothing, while the 

 bones and horns of various mammals serve various purposes. 



Body form and structure. — The mammalian body varies 

 greatly. Its variety of form and general organization is 

 explained by the facts that, although most of the species live 

 on the surface of the earth, some are burrowers in the ground, 

 some flyers in the air, and some swimmers in the water. 

 Mammals never have more than two pairs of limbs; in most 

 cases both pairs are well developed and adapted for terres- 

 trial progression. In the aerial bats the fore limbs are 

 modified into organs of flight; among the aquatic seals, sea- 

 lions, .walruses, and whales both sets are modified to be 

 swimming flippers or paddles. In many of these aquatic forms 

 the hind limbs are greatly reduced or even completely wanting. 



Most mammals are externally clothed with hair, which 

 is a peculiarly modified epidermal process. Each hair, 

 usually cylindrical, is composed of two parts, a central pith 

 containing air, and an outer more solid cortex; each hair 

 rises from a short papilla sunk at the bottom of a follicle 

 lying in the true skin. In some mammals the hairs assume 

 the form of spines or "quills," as in the porcupine. The 

 hairy coat is virtually wanting in whales and is very sparse 

 in certain other forms, the elephant, for example, which has 

 its skin greatly thickened. The claws of beasts of prey, the 

 hooves of the hoofed mammals, and the outer horny sheaths 

 of the hollow-horned ruminants are all epidermal structures. 



The bones of mammals are firmer than those of other 

 vertebrates, containing a larger porportion of salts of lime. 

 Among the different forms the spinal column varies largely 

 in the number of vertebrte, this variation being chiefly due 

 to differences in length of tail. Apart from the caudal ver- 



