CLASS AMPHIBIA 515 



cal movements of the throat. Oxygen is taken np by blood vessels in 

 the lining of the month by diffusion. 



The skin of the bullfrog plays a large part in its respiration, and 

 frogs that are not protected from drying out soon die. Gaseous ex- 

 change of carbon dioxide and oxygen can take place through the 

 moist vascular skin, and, since its area is large, it serves effectively 

 as a respiratory organ. This type of respiration is known as cutane- 

 ous respiration. During 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 respira- 

 tion are discussed in the chapter on The Vertebrate Animal. 



Excretory System and Excretion 



The two kidneys lie between the parietal peritoneum and dorsal 

 body wall in the posterior region of the body cavity. They are 

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

 very great number of uriniferous tuhules. 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 Internal Regulators. 



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, although this may vary considerably, for in 

 some forms the bladder may act as a filter for water which is used 



