STRUCTURE OF THE VERTEBRATES 



115 



1. Scales. The earliest mammals were midoubtedly supplied 

 with scales. Among the primitive orders today there are many 

 illustrations of their survival, particularly in the tail region which 

 is most liable to retain primitive characters. A casual examina- 

 tion of the tail of a rat or mouse will demonstrate the condition. 

 Manis, an edentate (page 87), is completely covered with large 

 scales. The hairs, in groups of three, grow out from the under 

 side of the scale, in the same relative position as the sensory 

 threads of the lizard. Scales are absent in the human, such 

 freaks as Barnum's "Alligator Boy" being diseased conditions 

 and not reversions to a primitive type. 



Monkey 



Man 



Echidna Carnivore 



Fig. 52. Sections of Claws and Nails, showing the forward growth of the 

 ball of the finger, and the shortening of the ventral plate of the claw. 



2. Claws, Nails, and Hoofs. The primitive ending of the 

 mammal's digits is the claw, similar in origin and structure to 

 that of the reptile. Claws are retained by the insectivores, ro- 

 dents, carnivores and others. During the course of evolution 

 modification has proceeded in two directions: (1) in the her- 

 bivorous, running animals the claws have become rounded 

 hoofs, and (2) in the primates the claws have gradually become 

 flattened into nails, which reach their culmination in the human. 



3. Pigment. Although the pigment of the lower vertebrates 

 is more often located in the dermal layer as pigmented con- 

 nective tissue cells, pigment is extensively scattered through the 

 epidermis of reptiles and birds. In the mammals the pigment 

 layer is apparently limited to the epidermis of the skin. The 

 pigment granules are located within the cells and give the skin 



