STRUCTURE AND PHYSIOLOGY OF ANIMALS 



13 



supply is being constantly received. This new supply of energy is 

 derived from the substance of the food-particles ; and this at the 

 same time maintains the bulk of the Amoeba, which, if food par- 

 ticles are absent from the water, gradually diminishes. 



Accompanying the degradation, or destructive metabolism as it 

 is termed, of the protoplasm, and intimately connected with it, is 

 the passage inwards of oxygen from the air dissolved in the water, 

 and the passage outwards of carbonic acid gas. Oxygen is a 

 necessary agent in the process of destructive metabolism, and 



FIG. 2. Amoeba polypodiain successive phases of division- The light spot is the contractile- 

 vacuole ; the dark the nucleus. (From Lang's Text-Book, after F. E. Schulze.) 



carbonic acid is a constant waste-product of such action. This 

 interchange of oxygen and carbonic acid is the essence of the pro- 

 cess of respiration observable in all living things. In addition to 

 the carbonic acid given off in this process, other waste-products are 

 formed and have to be got rid of. In all probability the contractile 

 vacuole already referred to has to do with this process the process 

 of excretion since uric acid, which in higher animals is the typical 

 form assumed by such waste-products, is said to have been detected 

 in the interior of the contractile vacuole in the case of certain near 

 relatives of Amoeba. 



When food is abundant the Amoeba increases in bulk more 



