BRANCH CHORD AT A: CLASS MAMMALIA 381 



Body form and structure. The mammalian body 

 varies greatly. Its variety of form and general organiza- 

 tion 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 terrestrial 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. 



tThe bones of mammals are firmer than those of other 

 vertebrates, containing a larger proportion of salts of lime. 

 Among the different forms the spinal column varies largely 

 in the number of vertebrae, this variation being chiefly 

 due to differences in length of tail. Apart from the 

 caudal vertebrae their usual number is about thirty. The 

 mammalian skull is very firm and rigid, all the bones 

 composing it, excepting the lower jaw, the tiny auditory 

 ossicles, and the slender bones of the hyoid arch, being 



