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- 
Fic. 22.—Diagram of base of contour feather. a, aftershaft; b, barbs; bl, barbules; 
h, hooks on-ends of barbules; Ju, lower umbilicus; g, quill; s, shaft; «, 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 
