AN AGENT OF EVOLUTION THE BODY COVERING 



133 



Feb. 2 



%^^ 



March 20 



June 22 



Fig. 8.6. Antlers of male mule deer. A, usual annual growth: Feb. 2, March 20, 

 June 22. B, structure and shedding; diagrams of sections. 1, growing prong in the 

 velvet, i.e., covered with hairy skin; 2 and 3, skin worn off and antler shed; 4, 5, 6, 

 regrowth and mature condition in which the bone is bare. Each successive breed- 

 ing season is marked by new antlers; to a certain limit older animals have more 

 prongs. (A, redrawn from Hamilton: American Mammals. New York, McGraw- 

 Hill Book Co., 1939. B, redrawn from Walter and Sayles: Biology of the Verte- 

 brates, ed. 3. New York, The Macmillan Co., 1949.) 



velvet stage throughout life (Fig. 8.7). Antlers of deer, reindeer, moose, and 

 elk are not composed of horn at any time. 



Functions o£ Skin 



Skin is a protection from heat and cold: by pigment in cells (frog); by 

 coverings of feathers (birds) and hair (mammals), with few apparent excep- 

 tions — whale, armadillo, et al.; by erection of feathers and hairs securing 

 greater insulation from cold because of the increase of air space between them; 

 by fat associated with the deep layer (dermis) — the blubber of whales and 

 other marine animals. 



The amount of water in the body is regulated by the control of its entrance 

 through the skin (frog), resistance to its passage through the skin by chitinous 

 coverings (many insects) and by cornified layers and fat (mammals), by 

 scales (fishes and reptiles), by feathers and hair, by oil or wax glands (in birds 

 especially water birds, cockroaches, certain beetles, bees, ants, and aphids). 



Skin resists the entrance of parasites and diseases by special thickened areas, 



