690 EVOLUTION OF ANIMALS Part V 



Structure and Function. Form, Covering, and Color. The flattened head 

 of a frog still suggests that of its ancient ancestors. A frog's neck is short, and 

 like those of other aquatic animals, the fishes and whales, is not marked off 

 from the rest of the body. The body is short. Its complete lack of external tail 

 is rare among lower vertebrates. As in all vertebrates, the skin consists of an 

 outer epidermis and inner dermis (Fig. 34.9). Throughout their lives frogs 

 molt the outermost dead part of the epidermis, casting it off in a whole piece 

 every few weeks except in winter when living processes are slowed and molting 

 is almost or entirely absent. The shed skin is pulled over the head like a sweater 

 and swallowed. The comparable layer of human skin is constantly shed, but 

 less dramatically in scalelike bits. Numerous mucous and poison glands orig- 

 inate in the epidermis but project down into the dermis where they are nour- 

 ished by the blood. Mucus becomes a lifesaver by moistening the skin and 

 slowing evaporation. The deeper-lying dermis includes blood vessels that func- 

 tion in skin respiration, many small nerves, smooth muscle cells, connective 

 tissue and pigment cells. 



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Fig. 34.8. Life history of the green frog, Rana clamitans, that transforms and 



goes onto the land in its second year. 



