RESPIRATION AND EXCRETION 



been assi; 

 unknown. 



31 



been assumed to be excretory, but their chemical nature is 



THE CONTRACTILE VACUOLE 



The contractile vacuole gradually increases in size up to a 

 maximum, and then bursts, shedding its clear, watery contents 

 to the exterior ; a new vacuole then grows again in the same spot 

 and the process is repeated. There is no true contraction, but 

 from analogy with the contraction and swelling of the vertebrate 

 heart the small phase is sometimes called systole and the large, 

 diastole. The vacuole removes water, and this is its only known 

 function. The pellicle of Amoeba has differential permeability for 

 water and for dissolved substances, so, since the osmotic pressure 

 of the cytoplasm is higher than that of fresh water, water must 

 enter osmotically. If this were not eliminated the creature 

 would swell. Marine amoebae, living in an environment of osmotic 

 pressure approximately the same as that of protoplasm, have no 

 contractile vacuole. The passage of water from the cytoplasm into 

 the contractile vacuole is against the direction of osmotic flow, 

 and must mean that work is being done by the animal. In fact, 

 in an amoeba deprived of oxygen, swelling of the contractile 

 vacuole ceases. 



THE NUCLEUS 



It is possible, in various ways, to remove the nucleus from 

 Amceha proteus. Individuals thus amputated continue living for 

 some days, and then die. They can move, but their pseudopodia 

 are fewer and unusual in shape ; they can in jest and kill small 

 prey, but cannot digest it ; they give normal responses to 

 stimuli, but their oxygen consumption is greatly reduced. All 

 this suggests that the nucleus is in some way responsible for the 

 formation or activity of enzymes. 



DEPRESSION 



Unfavourable conditions of life may bring about a condition 

 known as depression, in which the nucleus of the amoeba is 

 enlarged and the various functions become deranged. This 

 condition, however, is more famihar and has been more closely 



