504 VERTEBRATE LIFE AND ORGANIZATION 



Epiclermis 

 Dermis 



Epidermis 

 Dermis 



B 



-"Horny scale. 



Epid-ermis 

 Dermis 



"Bony plate Q 



Figure 25.2. Vertical sections through the skin of vertebrates to show the rela- 

 tionship of the various types of scales. A, Bony scales of a fish; B, horny scales of a 

 snake; C, horny scales and bony plates as in the skin of certain lizards, crocodiles and 

 turtles. 



aquatic animals, for its thermal insulation depends on air trapped within 

 it, and it has been lost on most of the body surface of adult whales. 



Though the skin itself is relatively simple, its derivatives are nu- 

 merous and complex. These may be grouped into bony structures, horny 

 structures, glands and pigment. The bony structures develop within the 

 dermis, though parts of them may become exposed if the overlying 

 epidermis wears off. Thick bony scales and plates were prominent in 

 ancestral vertebrates, and have been retained in reduced form in most 

 groups of living fishes (Fig. 25.2 A). Certain of the dermal plates in the 

 head region early in evolution became associated with the skull and 

 pectoral girdle, and these have been retained by later vertebrates as 

 integral parts of the skeleton. Most of the primitive bony scales have 

 been lost in tetrapods, but the dermis retains the ability to form bone 

 and becomes heavily ossified in certain species. The shell of a turtle is 

 composed of dermal plates covered by large horny laminae; a compar- 

 able condition is found in the skin of certain lizards and crocodiles and 

 in the shell of the armadillo. The antlers of deer (Fig. 25.3) are also 

 composed of derm.al bone. During its development, the antler is covered 

 by skin, the velvet, but this sloughs off when the antler is fully formed. 

 Antlers branch, are shed annually and, with the exception of the rein- 

 deer and caribou, are found only on males. The horns of sheep and 



