50 ORGANISMS OF ONE CELL 



the next until finally the elements derived from the protein food 

 are added to the protoplasmic molecules. 



Knowledge of the anabolic or building processes is much more 

 hypothetical than that in regard to the katabolic or breaking 

 down processes, the latter taking place whenever energy is 

 expended. These processes take place in the cell body or cyto- 

 plasm and are due to enzymes which probably come from the 

 inter-action of cell nucleus and cytoplasm. They are oxidizing 

 enzymes which bring about the union of oxygen with receptors 

 of the protoplasmic molecules. The ultimate result of such 

 combustion is the formation of simple compounds, the hydrogen 

 leaving the protoplasm molecule to unite with the oxygen, form- 

 ing water ; carbon with oxygen, forming carbon dioxide, and the 

 ammonium combination (NHs), forming with carbon and oxy- 

 gen the compound urea (NH^CO, which still contains some 

 energy. This urea, however, cannot be used by the animal pro- 

 toplasm as a further source of energy but is voided to the outside 

 as waste matter. The energy, however, is not wasted in nature 

 for, as we have seen, bacteria have the power of breaking urea 

 into free ammonia, carbon dioxide and water, and of converting 

 the contained energy into the energy of their own vital 

 processes. 



As urea is of no use to the animal but rather a menace in case 

 of its undue accumulation, it is necessary for the animal to get 

 rid of it. In all animals this is accomplished by means of 

 special organs which form the excretory systems. In mammals 

 and vertebrates generally, the kidney and associated organs 

 are set apart for this purpose; in lower animals like worms, 

 Crustacea, etc., special funnel-like organs termed "nephridia" 

 perform this function. In all types of animals in short, there 

 is some structure or structures of more or less complicated type 

 which are devoted almost exclusively to this end. 



In Amoeba proteus there is a special organ which has the func- 

 tion of disposing of waste matters and is analogous, therefore, 

 to the kidney of higher types. This is the " contractile vacuole" 

 which pulsates with a regular or rhythmic contraction, the 

 rate of pulsation varying with the temperature. It is not im- 



