60 



PHYSIOLOGY 



water and urea. Carbon dioxide and water vapor, by 

 osmosis through the lung tissue, pass from the blood 

 to the air in the lungs and are removed from the body. 

 Solids are not able to do this. Special organs therefore 



Fig. 39.— Excretory organ of the earthworm. IF, body wall ; 0, opening to the out- 

 side ; S, septum. The tube passes through the septum and opens through a 

 funnel-shaped ciliated mouth into the body cavity. 



prepare them for removal and they leave the body 

 in solution in water (Fig. 39). 



The Kidneys. — The kidneys, two reddish brown, 

 bean-shaped bodies, lie on each side of the spinal column 

 under the lowest ribs (Fig. 40). Large arteries enter 

 them and become subdivided into small capillaries which 

 penetrate every part of the tissue. Later these reunite 

 into veins which carry the blood from which the waste 

 has been removed back to the heart. The capillaries 

 gather together in small masses, each of which becomes 

 surrounded by a cup which opens into a tube closely 

 wrapped about by capillaries (Fig. 41), Through the 

 thin walls of these capillaries water carrying wastes, 

 principally urea in solution, passes into the cups or 

 directly into the tubes. The tubes unite and empty 

 eventually into the bladder through a large tube called 



