CHAPTER 23 



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«%P^** Ojl 



A Representative 



Vertebrate. 



Frog 



326 



HE frog is used for laboratory study more 

 often than any other animal as a vertebrate 

 type. A knowledge of its structures and 

 physiologic processes helps in understanding 

 the vertebrates in general and gives a back- 

 ground for the study of more complex forms, 

 including man. The following account of 

 the structure, physiology, and development 

 of the frog applies to any common species 

 such as the leopard frog, Rana pipiens, or 

 the bullfrog, Rana catesbeiana. The prin- 

 cipal subjects considered are the external 

 features, histology and physiology of the 

 skin, structure of the systems of organs, 

 processes of digestion, respiration, internal 

 transport by blood and lymph, disposal of 

 wastes, mechanical support and movement 

 by bones and muscles, control of the body 

 exercised by nerves and sense organs, ecol- 

 ogy, behavior, reproduction, and embryonic 

 development. 



Many structural and physiologic charac- 

 teristics of the frog are similar to those of 

 man, hence it is suggested that comparisons 

 be made from time to time. Chapters 31 to 

 34, which contain general considerations of 

 nutrition, skeletal structures and movement, 

 coordination and behavior, and reproduction 

 and development, might profitably be in- 

 cluded with the study of the frog. 



The leopard frog lives in or near fresh- 

 water lakes, ponds, and streams, and is dis- 

 tributed over the North American continent 

 except on the Pacific slope. The frog leaps 

 on land and swims in water. The hind- 

 limbs are large and powerful; when the frog 

 is on land they are folded up, but to propel 

 the body through the air they are suddenly 

 extended. Likewise in swimming, the hind- 

 limbs are alternately folded up and ex- 

 tended; and during their backward stroke, 

 the toes are spread apart, increasing the 

 webbed surface so as to offer more resistance 

 to the water. Frequently frogs float on the 

 surface with just the tip of the nose exposed 

 and with the hindlegs hanging down. When 

 disturbed in this position, the frog dives 

 under water, the hindlimbs are flexed, a 



