8 INTRODUCTION TO ZOOLOGY chap. 



The protoplasm in a living body is in a constant 

 ■ state of change, new protoplasm is continually 

 being built up by the assimilation of food, and then this very 

 complex compound breaks down again into simpler compounds, 

 which are finally excreted from the body in gaseous or liquid 

 form. One of the chief gaseous excreta is carbon dioxide. 

 This is formed by the union of the carbon of the disintegrating 

 protoplasm with the free oxygen which has been dissolved 

 by the surrounding water from the air above, and lias passed 

 from the water into the protoplasm of the body. The carbon 

 and oxygen combine in the proportion of one part of carbon 

 to two of oxygen to form carbon dioxide, the chemical 

 composition of which is expressed by the formula CO^. This 

 gas is given off from the surface of the protoplasm and 

 disperses, dissolving in the surrounding water. A change is 

 therefore continually taking place in the gaseous contents of 

 the water in which the Amoeba lives, the amount of dissolved 

 oxygen lessening and being replaced by carbon dioxide. 

 This oxidation of the carbon of the disintegrating protoplasm 

 by the oxygen from the surrounding medium (be it air or 

 water), and the resultant excretion of carbon dioxide, is the 

 process known as Eespiration : the essential nature of the 

 process is the same in all animals and plants, though — 

 owing to the complexity of the bodies of the higher animals 

 — special organs have been evolved in them for facilitating 

 the interchange of gases. 



The simpler nitrogenous substances, which are also formed 

 when protoplasm disintegrates, probably accumulate in liquid 

 form in the contractile vacuole, and are expelled from the 

 body when this suddenly contracts. 



Growth I^ the assimilation of food is more rapid than 



and Bepro- the disintegration of the protoplasm and the ex- 

 duction. cretion of waste matter, then Growth results, but 

 this is always limited in extent, the limit being probably 

 reached when the bulk of the body is so great, compared with 

 its surface, that any further enlargement of it would demand 

 a greater increase in the absorption of oxygen and of food than 

 is possible to the absorbent superficial layer. Having reached 

 its full limit of size, the Amoeba proceeds to divide into two ; 

 the nucleus fiist divides, and then the whole body lengthens 

 out, the two daughter nuclei moving apart ; finally a constric- 



