PROTOZOA TO WORMS 35 



The amoeba eats food as we do, but the food is di- 

 gested directly in the internal protoplasm instead of in 

 a stomach ; and once digested it diffuses to all parts of 

 the cell ; here it is built up into compounds of a more 

 complex structure, and forms an integral part of the 

 animal body. The dead food particle has been trans- 

 formed into living protoplasm, the continually repeated 

 miracle of life. But it does not remain long in this 

 condition. In contact with the oxygen from the air it is 

 soon oxidized, burned up to furnish the energy neces- 

 sary for the motion and irritability of the body. We 

 are all of us low-temperature engines. The digestive 

 function exists in all animals merely to bring the food 

 into a soluble, diffusible form, so that it can pass to all 

 parts of the body and be used for fuel or growth. In 

 our body a circulatory system is necessary to carry food 

 and oxygen to the cells and to remove their waste. For 

 most of our cells lie at a distance from the stomach, 

 lungs, and kidney. But in a small animal the circula- 

 tory system is often unnecessary and fails. Breathing 

 and excretion take place through the whole surface of 

 the body. The body of the frog is devoid of scales, so 

 that the blood is separated from the surrounding water 

 only by a thin membrane, and it breathes and excretes 

 to a certain extent in the same way. 



But another factor has to be considered. If we 

 double each dimension of our amceba, we shall in- 

 crease its surface four times, its mass eight-fold. Now 

 the power of absorbing oxygen and excreting waste is 

 evidently proportional to the excretory and respiratory 

 surface, and much the same is true of digestion. But 

 the amount of oxygen required, and of waste to be 

 removed is proportional to the mass; for every par- 



