8 



An Agent or Evolution— 

 T lie Body Covering 



Skin is a meeting place, the frontier between an animal and its surround- 

 ings, a region of come and go, of shutting in and out. 



The body coverings of animals are strikingly different: tenuously delicate 

 in a jellyfish, tough enough to stop bullets in a rhinoceros. They include such 

 contrasts as the ectoplasm of an ameba, the ciliated pellicle of paramecium, 

 the simple slimy skin of earthworms, the thin skin of birds, the leathery 

 skin of mammals. The multiplicity of structures that have developed from 

 skin is a record of its many functions that usually help and sometimes hinder 

 animals that live surrounded by shifting climates and shifty neighbors. Skin 

 glands secrete the shells of oysters, the chitinous exoskeletons of grasshop- 

 pers, the scales of butterflies, the slippery mucus of fishes and frogs, the 

 watery sweat of mammals, and the oil that waterproofs the feathers of birds. 

 Cellular outgrowths of skin form the claws of owls and tigers, horns of cattle, 

 beaks of birds and turtles, and hair — bent and crinkled in the wool of sheep 

 and straight on a monkey. Although less significant than the kidneys, the sweat 

 glands are also excretory organs. Sweat is similar to very dilute urine; in man 

 it contains about 99 per cent water, about 0.08 per cent urea and some other 

 salts. Skin is more or less resistant to disease and to the entrance of bacteria 

 and parasites. The mucus secreted from the skin glands of fishes and the 

 cornified layers in the skin of land animals are among its defenses. 



Pigment is deposited in skin cells making patterns — the spots on leopard 

 frogs, the stripes of zebras, which disguise their owners against the back- 

 ground of their homes. Certain cells of the skin are sensitive to touch, others 

 to temperature, to chemicals, some of them to light. Animals, human and 

 nonhuman, learn much about their surroundings through their skins. 



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