I STRUCTURE AND PHYSIOLOGY OF ANIMALS 13 



falls to deal out working power so as to cause the movement of the 

 machinery of the clock, so the protoplasm is able, by the degra- 

 dation or decomposition of its complex compounds, to deal out 

 working power enabling the Amoeba to move. In the case of the 

 clock-weight there comes a time when all the potential energy is 

 expended ; the weight reaches its lowest limit, and unless it is 

 wound up again the clock stops. The like holds good of the 

 Amoeba ; the protoplasm is continually being used up decomposed 

 into compounds of a lower order and, in course of time, the whole 

 potential energy would become exhausted, were it not that a new 

 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 or 

 catabolism as it is termed, of the protoplasm, and intimately con- 

 nected with it, is the passage inwards of oxygen from the air dissolved 

 in the water, and the passage outwards of carbonic acid gas (carbon 

 dioxide). Oxygen is a necessary agent in the process of destructive 

 metabolism, and carbonic acid is a constant waste-product of such 

 action. This interchange of oxygen and carbonic acid is the essence 

 of the process of respiration observable in all living things. 

 When Amoebae are placed in water from which all atmospheric 

 oxygen has been removed, all movement soon becomes arrested, the 

 pseudopods become withdrawn, the body becoming rounded off 

 and enclosed in a thin covering or cyst, and if the deprivation is made 

 to last long enough death and disintegration follow. Similar 

 results follow the presence of excess of carbon dioxide in the water. 

 In such an environment the Amoeba is unable to get rid of the carbon 

 dioxide which it is itself producing, and becomes poisoned by the 

 accumulation. 



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 proba- 

 bility 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 

 food being ingested than is required for simply maintaining the 

 size unaltered and soon a remarkable change takes place. The 

 processes are withdrawn, and a fissure appears dividing the 

 Amoeba into two parts (Fig. 2). This fissure grows inwards, and. the 

 two parts become more and more completely separated from one 

 another, till eventually the separation is complete, and we 

 have two distinct Amoebae resulting from the division of the one. 

 While the protoplasm has been undergoing this division into 



