THE METABOLIC .MACHINERY OF ANIMALS 321 



possibly to reabsorb any nutrient materials which escaped in wastes 

 from the fluid in the body cavity (see figure, page 192). 



In the insects still another type of excretory system is composed of 

 special tubules called Malpighian tubules. The cavity of each tubule 

 is surrounded by large cells covered by a peritoneal lining, emptying 

 into the intestinal canal. The free ends of the tubules lie in the body 

 cavity, where they are bathed in blood. The waste products pass 

 into the Alalpighian tubules from the blood. This interpretation is 

 supported by the detection of considerable quantities of nitrogenous 

 material in the tubules (see figure, page 210). 



Excretory Devices of Vertebrates — Kidney 



The excretory organs of vertebrates are known as kidneys. While 

 several different forms of kidneys are known to exist, they are all 

 derived embryologically from paired segmented structures, which in 

 many of the lower types may be connected with the body cavity 

 by a series of ciliated funnels reminiscent of the earthworm. Along 

 with the complex changes of the various systems of organs found 

 in the higher forms, especially of the circulatory system, there is a 

 much more intimate association of the circulatory and excretory 

 systems and a decrease in the importance of the part played by the 

 body cavity in the removal of wastes. 



The Mammalian Excretory System 



A typical mammalian excretory system is a complex affair, for it 

 involves not only the kidneys and their associated duets, but also 

 the bladder and portions of the circulatory system as well. This 

 does not tell the entire story, for the lix'er, lungs, skin, and alimentary 

 tract also play an important part in the excretion of wastes. 



The Liver. The liver, which was considered in connection with 

 the digestive system, also plays a vital role in the elimination of cer- 

 tain wastes from the body. Proteins are absorbed from the digestive 

 tract in the form of amino acids. Too heavy a protein diet results in 

 the absorption of more nitrogen-containing material than can be 

 utiUzed by the cells of the body for tissue building. The cells of the 

 liver have the ability to split off the nitrogen-containing radical and 

 in some instances resynthesize the remaining materials to carbohy- 

 drates and even fat. The nitrogen which is thus left behind may 

 have been removed as ammonia (NH3) which is quite toxic to the 



