30 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



ing season into hair-like structures, supplied with nerves and apparently 

 sensory in character. 



REPTILES. All living reptiles are characterized by the extensive 

 development of horny scales and frequently of bony plates in the skin, 

 but some of the fossil groups (ichthyosaurs, pterodactyls, some dino- 

 saurs, possibly plesiosaurs) had a naked skin. Correlated with this 

 cornification of the epidermis, glands are rare. Some turtles have scent 

 glands beneath the lower jaw and 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. These latter are not true glands as they 

 produce no secretion but cast out the lining cells. 



The corium presents two layers, the outer rich in chromatophores, but, aside 

 from some snakes and lizards, the color changes are not remarkable. The femoral 

 pores of lizards are not connected with glands but with branching tubes filled with 

 cast cells. Claws are common on the toes. 



BIRDS have both layers of the skin very thin, the epidermis develop- 

 ing both scales and feathers. Correlated with this extensive develop- 



FIG. 22. Diagram of base of contour feather, a, aftershaft; b, barbs; &/, barbules; 

 h, hooks on ends of barbules; lu, lower umbilicus; q, quill; s, shaft; u, umbilicus; v, vane. 

 A, portion of a barb showing the barbules and hooks. 



ment of cornified structures is a striking paucity of glands. There are 

 none in the ostriches, but others have the familar oil (uropygial) glands 

 at the base of the tail, the secretion of which is used in dressing the 

 feathers. The only other dermal glands in birds are modified sebaceous 



