206 GENERAL ZOOLOGY 



Metabolism. Before discussing the last of the series of 

 physiological processes, it will prove helpful to consider all 

 that has been said as being descriptive of processes that are 

 merely stages of the one great sum-process of living. In 

 scientific terminology we call this sum-process metabolism. 

 Considered as a single process, we recognize a series of events 

 during which things are coming into the body. These things 

 are in the course of making up a part of the body of the 

 organism ; this is called constructive metabolism, anabolic 

 metabolism, or simply anabolism (Gr. ana, up ; ballein, to 

 throw). Upon the occurrence of oxidation, food stored in 

 the muscle-cells, and protoplasm, when it is oxidized, are 

 broken up and reduced to simpler chemical compounds ; 

 this is destructive metabolism, katabolic metabolism, or katab- 

 olism (Gr. kata, down ; ballein, to throw). The products of 

 katabolic metabolism are the wastes of the organism. The 

 undigested food and the indigestible substances that pass 

 through the length of the intestine are not considered 

 wastes, a fact that has already been stated in the chapter 

 on the locust. 



Excretion. The last of the stages of metabolism is excre- 

 tion. In the broadest sense of the term excretion includes 

 all those activities which result in ridding the body of wastes. 

 Carbon dioxide is given off only from the skin ; water prob- 

 ably in part from the skin ; uric acid and water are discharged 

 from pairs of small contorted funnel-shaped tubes in the lateral 

 portions of the cavity of the somites of the earthworm. These 

 excretory organs are called nephridia (Fig. 100, 25, 26). They 

 correspond in function to the kidneys of the higher animals. 

 Each nephridium has the mouth of its funnel extending 

 through a septum (Fig. 100, 4) into the cavity immediately 

 anterior to the somite which contains the greater portion of 

 the organ. The external opening of a nephridium is on the 

 ventral surface. 



