RESPIRATION 2OQ 



air, specifically the oxygen of the air. Otherwise the combus- 

 tion will not continue, the fire will die out, and the engine fin- 

 ally come to a standstill. An animal also, when deprived of air, 

 soon goes into a quiescent state, and when active, the amount 

 of air required varies with the energy expended. The living 

 animal is also continually evolving COi, and that, too, in pro- 

 portion to the energy expended. It is evident, therefore, that 

 there is combustion, or oxidation of carbon, going on in the 

 organism. It is known that this process takes place in the 

 tissues, i. e., in the cells, and we must, therefore, account for 

 the presence of oxygen in the tissues. 



FIG. 115. Part of the body of nereis, showing the respiratory organs. The 

 broad superior ligula of the dorsal ramus of each parapodium has a thin integu- 

 ment and is richly supplied with blood-vessels. 



476. Amoeba and many other organisms can absorb enough 

 oxygen through the general surface of the body. Even com- 

 paratively large animals, because of their form and peculiarity 

 of structure, can obtain enough oxygen in this way. The sea- 

 anemone, for example, though comparatively large, exposes 

 not only the external surface of the body and tentacles, but the 

 much larger folded surface of the gastro-vascular cavity is 

 exposed to the water, which is being continually renewed by 

 currents passing in and out of the mouth. Even the frog, when 

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