BULLFROG AS TYPICAL VERTEBRATE ANIMAL 81 



hibernation, practically all respiration of the bullfrog is of this 

 nature. Even at other times, the skin releases more carbon dioxide 

 than do the lungs. The functions of respiration are discussed in the 

 chapter on Physiology. 



Excretory System and Excretion 



The two kidneys lie betv^een the parietal peritoneum and dorsal 

 body wall in the posterior region of the body cavity. The space in 

 which they are enclosed is called the cisterna magna. They are 

 dark red in color, flattened and elongated. They are made up of 

 a very great number of uriniferous tubules. A mesonephric duct 

 runs from the posterior lateral border of each kidney and empties 

 into the dorsal side of the cloaca. The urinary bladder also opens into 

 the cloaca but does so on its ventral surface, and the ducts do not join 

 the bladder. The bladder is a two-lobed sac with very thin walls 

 which stores the urine collected from the cloaca. When filled, the 

 bladder contracts and forces the urine back through the cloaca and 

 outside through the anus. Embedded in the ventral surface of each 

 kidney is a yellowish red patch, the adrenal gland, which will be dis- 

 cussed in the chapter on Endocrine Glands and Their Functions. 



The waste products resulting from the vital processes of destruc- 

 tion, repair, and growth in the body must be removed if the organism 

 lives. These are taken from the tissues by the blood and more espe- 

 cially by the lymph. We have already mentioned the expulsion of 

 carbon dioxide and water through the skin and lungs. Another prod- 

 uct of protein metabolism is urea. This soluble crystalline substance, 

 formed to a large extent in the liver from the nitrogen of protein 

 metabolism, enters the blood stream and is' removed by the kidneys, 

 The kidneys also remove foreign substances from the blood and pass 

 these to the outside through their mesonephric ducts and the cloaca. 



Frogs and toads excrete considerably more urine per day propor- 

 tionally than does man. It has been estimated that, while man ex- 

 cretes about one-fiftieth of his weight per day, the frog excretes 

 about one-third of its weight. During the winter, how^ever, in com- 

 mon with the slowing down of its other body functions, the kidney 

 function of the frog is practically stopped. 



The kidney is not only concerned with the elimination of waste 

 products but also has other functions. One of these is the reabsorp- 

 tion by its tubules of useful substances, such as some of the salts 



