METABOLISM, EXCRETION 



215 



contraction and secondarily by the production of heat, but at 

 the same time, substances are formed and set free, which show 

 that chemical processes are at work and which give a clue as to 

 the nature of those processes. The oxygen and food substances, 

 C 6 Hi 2 O 6 , let's say, make their appearance again, but in an 

 altered form. Carbon dioxide is produced in large quantities 

 together with other substances which must be eliminated from 

 the body, or else serious disorders occur. Nitrogen waste 

 compounds are also formed, and like the CO2, they have a lower 

 energy value than the substances from which they were derived. 



EXCRETION 



489. The waste matters produced by metabolism are soluble 

 in the body fluids and in water. Hence, small animals like 

 amoeba and hydra can eliminate them from the body surface by 



-TV 



FIG. 119. Diagram of a nephridium of an Annelid, b, b f , blood-vessels; 

 c, coelom; d, duct; e, opening through the epidermis; /, funnel; gl, glandular 

 portion; s, mesentery; W, wall of body; iv, wall of intestine. (From Galloway.) 



diffusion. In larger animals the substances excreted by the 

 cells pass out into the body fluid or the blood current and are 

 thus carried to the place of elimination. The CO2 being a gas 

 is chiefly given off from the organs of respiration, following the 

 path of the oxygen, but in the reverse direction. The volume of 

 oxygen absorbed in the human lung'is about 5 per cent, of the 

 inspired air. The volume of C0 2 given off is a little over 4 



