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EVOLUTION OF ANIMALS 



Part V 



lations of the fluid (Fig. 34.20). In the common species of Rana, there are four 

 lymph hearts, each of them a two-chambered pump which forces lymph into 

 the blood stream through openings in the vessels. 



Respiration, Breathing, and Voice, Properly speaking, breathing is ex- 

 ternal respiration and the chemical changes in the cells constitute an internal 

 respiration. 



Breathing. Floating with only its nostrils above the surface, a frog breathes 

 air and takes oxygen from it by way of its mouth and lungs. It also takes 

 oxygen from the water through its skin. In winter, when there is less oxygen 

 demand, skin breathing alone is sufficient for life. The breathing organs of the 

 adult frog, lungs, skin, and lining of the mouth cavity, are abundantly supplied 

 with blood vessels. The lungs are thin elastic sacs with low internal folds that 

 greatly increase the surface between which the capillaries extend (Figs. 34.16, 

 13.9). The lining of the lungs is continuous with that of the larynx into the 

 alimentary canal. The lungs branch from a hardly perceptible trachea. Their 

 outer covering is continuous with the lining of the body cavity. Nerves, con- 

 nective tissue, and pulmonary arteries, veins, and connecting capillaries are 

 outspread between the covering and lining of the lungs. 



As a frog breathes, the floor of its mouth rhythmically rises and falls, a 

 throat-breathing in which the capillaries of the lining of the mouth and throat 



Caudal 

 Lymph 

 Heart 



Cranial 

 Lymph- 

 Heart 



Fig. 34.20. Frog's lymphatic system Sacs for the lymph which creates a fluid 

 coat about the frog's body. The skin has been removed from this frog. The dark 

 lines represent the boundaries of lymph sacs. Lower, lymph hearts in the frog 

 (Rana); these are pulsating lymph pumps which keep the lymph moving. (Cour- 

 tesy, Walter and Sayles: Biology of Vertebrates, ed. 3. New York, The Macmillan 

 Co., 1949.) 



