20 ESSENTIALS OF ZOOLOGY 



protoplasm, and it is gradually built up into, and takes its 

 place in, the colloid material of the living cell. During its 

 progress from the state of food to living material it may be 

 oxidised to provide the energy required in carrying on the 

 life processes. The residue represents the material available 

 for growth. This goes on until full growth is being approached, 

 when an equilibrium is reached between digestive capacity 

 and the energy required, and this is the prelude to cell divi- 

 sion. It is evident that in a state of encystment enzymotic 

 action is practically suspended together with nitrogenous 

 consumption ; the small amount of energy required in this 

 state of inanition is contributed by the oxidation of the 

 protoplasm. 



These changes so characteristic of life are called metabolic 

 changes (metabolism), the building-up processes associated 

 with enzymotic action are called anabolic (anabolism), and the 

 breaking- down processes mainly associated with oxidation, 

 katabolic (katabolism). The anabolic processes are directly 

 related to the digestive processes and the presence of 

 secretions, and the katabolic to the development of material, 

 some of which may be used as loci for the development of 

 higher compounds, but most of which is deleterious and has 

 to be discharged as excretion. Excretion should not be con- 

 fused with egestion. 



The oxygen dissolved in the water is essential to the life 

 of the Protozoa, for the energy derived from the food is brought 

 into use by oxidation, and it has been proved by experiment 

 that Protozoa cannot live in water from which oxygen has 

 been removed. Carbon dioxide is a common product of 

 oxidation in all cells, and with other similar products is 

 easily passed out of the cells at the surface in exchange for 

 oxygen, and this is respiration. 



The Protozoa lead a simple life and exhibit in their life 

 an epitome of the essential physiological processes. They 

 behave as animals ; that is, they are holozoic. The energy they 

 obtain and utilise is probably in the main derived from food. 

 The Protophyta obtain energy from the sun's rays in holo- 

 phytic existence, and otherwise the group exhibits a wonder- 

 ful capacity of forming enzymes in response to different kinds 



