EPIDERMAL STRUCTURES. 



99 



with the hair follicles, while a system of smooth muscle fibres 

 (especially strong in porcupines) serves to erect the hairs. 



Closely related to hair are the nails, 

 claws, and hoofs of mammals, and the 

 horn of sheep, goats, and cattle ; in fact, 

 these structures may be regarded as com- 

 posed of agglutinated hairs. 



Somewhat different in character are 

 the scales which cover the body in the 

 pangolins (manids), and are found on the 

 tail of the rodent Anomalurus, although 

 these are both of epidermal origin. True 

 dermal bones in the skin occur only in 

 the armadillos among recent forms, where 

 they form an armor upon the dorsal sur- 

 face of the body. In the fossil glypto- 

 dons the body was enclosed in a similar 

 bony case, while some extinct cetacea possessed dermal bones. 



Fig. io8. Hair tracts 

 on early cat embryos, after 

 Maurer. 



