CHAPTER VII 



RESPIRATION 



The process of respiration involves a multitude of factors. At the 

 morphological level there is the necessity for providing a surface 

 across which diffusion of gases can occur e.g. lungs and gills, and 

 the necessity for providing, even in the most arid situations, a con- 

 tinuous layer of moisture into which the gases can dissolve is 

 essential. In many cases it is also necessary to provide a current of 

 water or air over the exchange surface as is accomplished by 

 ventilation. Internally there needs to be, in the more highly 

 organized animals, a means of transporting the dissolved gases to 

 the appropriate sites for utilization and removal. This involves the 

 dissolution in fluid circulating in vessels, or passage of air in 

 trachea. The process may be assisted by the occurrence of a 

 pigment that combines readily with oxygen to carry it to the tissue, 

 there to release it as required. The removal of carbon dioxide may 

 also be accomplished by special devices. Last, but not least, at the 

 cellular level there are all the many enzyme reactions that go to pro- 

 vide energy for metabolism synthesis and excretion and which rely 

 ultimately on the provision of oxygen and removal of carbon dioxide. 



It is obvious that there will be many variations upon these things, 

 dependent upon the complexity, activity and ecology of the species 

 in question. What suffices for the amoeba, a straightforward 

 gaseous diffusion across the cell surface and no need for a transport 

 system, will obviously not do for a fish that swims actively through- 

 out its life e.g. Scomber scomber^ and must maintain a current over 

 its gills for gaseous exchange followed by a highly organized vascu- 

 lar system involving a pumping mechanism, valves and a respiratory 

 pigment. 



The majority of oligochaetes fall midway between these two 

 extremes. For while there is often no specialized gas exchange 

 organ there is a transport system and a respiratory pigment. 



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