BODY COVBKINGS OP VERTEBRATES 



377 



becomes so thick and horny as to form a true exoskeleton, 

 namely, the scales of the crocodile and the horny layers of 

 the turtle. Again, in the amphibians it may be so thin that 

 respiration takes place through it with readiness. 



The skins of vertebrates differ mainly in the structures 

 produced by modifications of the 

 epidermis and the forms of glands. 

 For example, the scales of the fishes 

 and the bony horn-covered plates 

 of the turtle are of like origin. 

 Both consist of a bony plate de- 

 veloped by the dermis, and covered 

 with a horny portion of the outer 

 layer of epidermis. The feathers of 

 the bird and the hairs of man are 

 likewise identical in origin, being 

 merely modifications of the epider- 

 mal layers. 



In like manner, by infolding of 

 the epithelial cells of the lower epi- 

 dermis, glands of various forms 

 are built; such are the sebaceous 

 and sweat glands of man, the mam- 

 mary glands or milk glands of all 

 mammals, the various scent glands 

 of different forms, and the slime 

 glands of the amphibians and some 

 of the fishes. Some of these glands 

 are purely excretory in function, 

 while others are protective (scent 

 glands) or produce secretions of value in animal opera- 

 tions; for example, the slime glands of the amphibians 



Fig. 174 — A feather; a, quill; 

 6, sliaft; (,', vane; £?, down; 

 fij/) upper and lower orifice. 



