Chap. 14 THE BY-PRODUCTS OF METABOLISM — EXCRETION 243 



breakdown of proteins is usually eliminated as ammonia, urea, and uric acid. 

 Besides these there are other substances in very small amounts. 



The Simple Excretory Organs and Their Functions 



Vacuoles. In most protozoans there is no hint of a special excretory organ. 

 In fresh-water amebas, paramecia and others, the water constantly entering 

 the animal collects in contractile vacuoles along with the metabolic waste 

 products (Fig. 14.1). A vacuole is in no sense empty. It fills until the sur- 

 rounding protoplasm will stretch no more then suddenly contracts and dumps 

 the contents outside. Through the lower power of the microscope the vacuole, 

 as it were, winks at the observer. A contractile vacuole is primarily a water- 

 regulator that disposes of extra water diffusing into an animal because its 



Fig. 14.1. The simpler excretory organs. The contractile vacuole of Amoeba 

 verrucosa. A, Vacuole that has reached full size and is near the surface of the body. 

 B, The vacuole, about to empty, is pressed against the outer covering which 

 stretches momentarily and forms a cone before it breaks. C, A living ameba is held 

 in place by a minute rod. A slightly blunt microneedle is inserted into the animal 

 and pushed against the contractile vacuole indenting it like a transparent rubber 

 ball pushed in from one side. This and other experiments have shown that a 

 contractile vacuole is enclosed by a transient but definite membrane. (Redrawn 

 after Howland: "Experiments on the contractile vacuole of Amoeba verrucosa." 

 J. Exp. Zool. 40:251-270, 1924.) 



protoplasm is saltier than the water outside. Fresh-water fishes would have 

 the same trouble if they did not have means of preventing it. Marine amebas 

 do not have vacuoles, and when fresh-water species are kept in salt water their 

 vacuoles disappear or work very slowly since the salt content of protoplasm 

 and sea water nearly balance. 



Association of Kidneys and Blood. Except for the vacuoles all excretory 

 organs are tubes, always intimately associated with blood or other body fluid. 

 In lower animals the kidney is called a nephridium and many words associated 

 with this term are used in connection with all kidneys, such as nephritis, a 

 disease of the kidney. 



Fresh-water planarians have no circulating blood to transport waste and 

 the excretory system is a series of minute tubes whose closed ends, the flame 



