EXCRETION 183 



However, the interpretation of the analysis is of first importance 

 in every case because the urine rapidly reflects normal changes in 

 the physiological condition as well as those that are abnormal: 

 even the nervous tension of an examination may well be evidenced 

 by the appearance of grape sugar. Again, the amount of urine 

 excreted depends upon many factors. Under normal conditions 

 with a given intake of water, the volume of urine varies chiefly 

 with the temperature and moisture of the atmosphere. For in- 

 stance, when the atmosphere is hot and dry, a relatively large 

 amount of water is eliminated as perspiration. Accordingly the 

 intake of water should be greater in order to carry off readily the 

 waste products of metabolism through the kidneys. One is apt to 

 think of the kidneys as essentially passive organs that merely 

 drain materials from the blood. But, as a matter of fact, a 

 grain of kidney tissue consumes, on the average, more oxygen 

 per unit of time than the same weight of the beating heart. The 

 kidneys work. 



2. Evolution of Kidneys 



Aside from their functional importance, the kidneys are of 

 considerable interest to the comparative anatomist because of 

 their complicated evolutionary his- 

 tory throughout the Vertebrate se- 

 ries. Indeed the basic elements of the 

 vertebrate kidney may be most 

 readily interpreted with the excre- 

 tory system of the Earthworm in 

 mind. (Figs. 59-61, 128.) 



The chief excretory organs of the 

 Earthworm consist of pairs of coiled 

 tubes, or nephridia, segmentally 

 arranged in the coelom on either Fig. 128. — Diagram to show 

 side of the alimentary canal. Each the general structural plan of a 



nephridium begins as an open fun- ™P Mdi ™ of an , Earthworm, an- 

 ^ ^ tenor end toward the right, a, ln- 

 nel in the coelom of one segment, ternal opening of nephridium; 6, 

 passes through the partition to the external opening; c, capillary net- 

 next posterior segment and there, wo * abou t V he c ? iled ' g landular 

 _ . . ' portion in the coelom. 



after coiling, passes to the ventral 



surface and opens to the exterior by a pore. Thus, reduced to its 



simplest terms, a nephridium is a tube communicating between 



