STRUCTURE OF THE VERTEBRATES 113 



Ectodermal scales are an identifying character of the reptiles. 

 The lizard skin is typical of the primitive type. In this group 

 the scales are usually sharply pointed at the posterior tip, over- 

 lapping the one behind. Under each scale is a mesodermal 

 papilla, and so intimately connected are the two that the epi- 

 dermis may be removed without destroying the appearance of 

 the skin. In the turtles the scales are wide and flat, but are 

 essentially like those of the lizard. The snakes have dorsal scales 

 like the lizard, but the ventral scales, or scutes, are wide with a 

 rib attached at either side. 



Claws arose first in the reptiles as modified areas of epi- 

 dermis on the tips of the toes. They are present in all reptiles 

 with feet. The development of claws is discussed under the 

 mammal (page 115). 



Structures of particular interest to the evolutionist are sen- 

 sory organs in the skin of lizards. Under each scale there is a 

 minute invagination with a nerve ending at its base, and from 

 the slight pit a hair-like sensory thread projects. The evidence 

 indicates that this is the reptilian homologue of the mammalian 

 hair. 



Birds. Like the reptiles, the birds are almost devoid of skin 

 glands. Some have scent glands around the anus, and most of 

 them have oil glands at the base of the tail feathers. Scales and 

 claws were carried over to the bird, the legs of most birds be- 

 ing encased in typical reptilian scales, with their toes terminated 

 by claws. The horny beak is also an epidermal derivative. 



Feathers are modified scales, although highly different in the 

 full grown condition. There are two major types of feather: (1) 

 down feathers, found in the young and scattered under the larger 

 feathers of the adult; and (2) contour feathers. The latter are 

 the typical feathers, the flat vanes spreading on either side of 

 the central quill. These vary in size and structure from the hair- 

 like filoplumes to the large quills of the wing and tail. 



Mammals. The mammalian skin and its derivatives are so 

 highly modified that only a few of the latter can be thought of 

 as directly inherited from the reptiles. The claws, nails and 

 hoofs; the scales of the primitive orders; and the pigment are 



