THE STRUCTURE AND ACTIVITIES OF MAMMALS 399 



outstretched mem- 

 branes. The bats, 

 however, possess 

 wings and are as much 

 air inhabitants as the 

 birds. 



A few species, like 

 the mole, pass almost 

 their entire existence 

 underground, and the 

 ground squirrels, 

 woodchucks, prairie 

 dogs, and similar spe- 

 cies live part of the 

 time in burrows. 



Protection. — Mam- 

 mals, like birds, are 

 warm-blooded animals 

 and must be protected 

 not only from their 

 natural enemies and 

 from the ordinary 

 hard knocks of life, 

 but also from weather 

 conditions, such as 

 extreme cold, which 

 would not injure such 

 cold-blooded creatures 

 as the frog and turtle. 

 Heat is kept in the 

 body in various ways. 

 Mammals that live in 

 cold water, like the 

 whale, possess a very 

 thick layer of fat, the 



Fig. 271. — Longitudinal section through a hair 

 in its follicle. 



Ap, muscle; Co, dermis; F,F', fibrous layers 

 of follicle ; Ft, fat ; GH, membrane ; HBD, se- 

 baceous gland; HP, hair papilla; M, pith; O, 

 cuticle; R, cortical layer; Sc, horny layer of 

 epidermis; Sch, hair shaft; SM, epidermis; 

 WS, WS', layers of root-sheath. (From Wie- 

 dersheim.) 



