THE INTEGUMENT 



669 



and the lungless salamanders respire largely by the skin, the corium is 

 richly supplied with blood vessels, which, at the time of the metamorpho- 

 sis of anura, penetrate into the epidermis. It is at this time that the 

 lungs are not yet functioning, and the gills are being absorbed. The 

 stratum corneum is shed periodically, either as a whole, as in urodeles, 

 or in patches. The "warts" of toads are partially cornifications of the 

 epidermis. A similar hardening of the skin at the ends of the toes results 

 in claws. 



REPTILES 



All these have horny scales and sometimes bony plates though some 

 of the fossil groups have a naked skin. 



Glands are rare, though some "turtles have scent glands beneath 

 the lower jaw along the line between carapace and plastron ; snakes and 

 crocodilians have them connected with the cloaca, while the latter have 

 others, of unknown function, between the first and second rows of plates 

 along the back, as well as protrusible musk glands on the lower jaw." 



Fig. 396. 



Comparison of human finger nail (A) 

 and hoof of horse (B). 



d.pt 



Fig. 398. 



Feather tracts of the pigeon. A, ventral ; B, 

 dorsal, al.pt, alar pteryla or wing tract ; c.pt, 

 cephalic pteryla or head tract; cd.pt, caudal 

 pteryla or tail tract ; cr.pt, crural pteryla ; cr.apt, 

 cervical apterium or neck-space ; fm.pt, femoral 

 pteryla ; hu.pt, humeral pteryla ; lat.apt, lateral 

 apterium ; sp.pt, spinal pteryla ; v.apt, ventral 

 apterium; v.pt, ventral pteryla. (From Parker 

 and Haswell, after Nitzsch.) 



These latter are not true glands, as they produce no secretion, but cast 

 out the living cells. Color changes are not remarkable except in a few 

 snakes and lizards. Claws are common on the toes. 



The so-called "femoral pores" on the under surface of the legs of 

 lizards are not true glands. They are epidermal structures composed of 

 horny cells, possibly having a sexual function. 



BIRDS 



The distinguishing characteristic of birds is that they possess 

 feathers. Both layers of skin are quite thin, there being both scales and 



