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 hulk of the Amoeba, which, if food par- 

 ticles are absent from the water, gradually diminishes. 



Accompanying the degradation, or destructive metabolism ^,s 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 



3&ary agent in the process of destructive metabolism, and 



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Fi<;. 2. -Amoeba polypodia in successive phases of division The light spot is the contractile 

 vacuole ; the dark the nucleus. (From Lang's Text-Book, after P. 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- 

 cesa of resp iration 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 Anioeba increases in bulk — more 



