EARTHWORM 91 



the external environment. There is also good evidence that the 

 highly vascular calciferous glands relieve the blood of considerable 

 quantities of carbon dioxide which they excrete into the alimentary 

 canal. It should be clearly understood that the essential part of 

 respiration is the interchange of oxygen and carbon dioxide by 

 every cell in the body. The carrying of these gases to and from 

 the body surface by the blood is a necessary mechanism of respi- 

 ration. The blood plasma of the Earthworm is colored red by a 

 complex, oxygen-carrying compound, hemoglobin, which is dis- 

 solved in it. 



4. Excretory System 



The Earthworm has an important system for the excretion of 

 the liquid metabolic wastes of the body, consisting of numerous 

 tiny coiled tubes, known as nephridia. 1 Except in the first three 

 or four segments there are two nephridia present in each segment 

 of the body. They he in the coelom, below the alimentary canal, 

 and close to the ventral body wall through which they open. In 

 each segment one nephridium lies to the right and one to the left 

 of the mid-ventral line. (W. fs. 59, C; 60.) 



Each nephridium consists of the following parts : (a) A ciliated, 

 coelomic opening, the nephrostome ; a long, greatly coiled tube 

 which lies in three loops and consists of (6) a long, very narrow, 

 thin-walled portion leading from the nephrostome, and (c) a larger, 

 tubular region, of considerable length, which continues through 

 the ventral body wall to the external opening, or nephridtopore. 

 A nephridium does not lie entirely in one segment, but the 

 nephrostome and a small portion of the fine connecting tubule 

 penetrate the posterior septum of the segment just in front. 

 (W. f. 128, a, b, c.) 



It is probable that most of the metabolic wastes of the tissues 

 accumulate in the coelomic fluid. These wastes may be drawn 

 into the lumen of a nephridium by the action of the ciliated nephro- 

 stomal cells and then passed the length of the tubular portion and 

 through the nephridiopore to the exterior. Or, again, the wastes 

 may be removed from the coelomic fluid, which bathes the ne- 

 phridia, by the direct absorptive action of specialized cells located 

 in the middle region of the nephridia, which then secrete the 

 wastes so removed into the central lumen for passage to the exterior. 



1 Singular, nephridium. 



