TEGUMENTARY ORGANS— SKIN STRUCTURES. 



459 



Fig. 317. — Diagram of an ostrich plume, con- 

 sisting of shaft (sh), barbs, and barbules, but 

 no hooks. 



through the body feathers, down, and plumes, to simple 

 hairs. The plumes of the ostrich have the structure 

 of feathers, except 

 they have no hook- 

 lets, and therefore 

 the barbules do 

 not cohere (Fig. 

 317). Plumes like 

 those of the heron, 

 egret, etc., have no 

 barbules; they are 

 essentially slender, 

 branching hairs 

 (Fig. 318). Again, 

 about the beaks of 

 many birds we find 

 simple hairs (Fig. 

 319). This would 

 seem to indicate a 

 close relation in 

 this regard be- 

 tween birds and 

 mammals. But 

 birds undoubtedly 

 came from reptiles, 

 and therefore feathers are probably some modifica- 

 tion of scales. But the gradations here have not been 

 found. 



Scales are characteristic of reptiles and fishes, espe- 

 cially the latter. 



Fish Scales. — We take these as the type. They cover 

 the whole body. They are formed much like nails — 

 i. e., on the surface of the mucous layer, and especially 

 in pocketlike infoldings. The manner in which from 

 those pockets they grow backward, shingling over one 



Fig. 318. — Diagram of an egret plume, consist- 

 ing only of shaft and barbs. 



Fig. 319. — Hair about the beak of a bird, con- 

 sisting of shaft only. 



