378 ANIMAL RESPIRATION 



breathing or respiratory organs, the special duty of which is to 

 take in oxygen and get rid of carbonic acid gas and water. Nor 

 must we underestimate the importance of this kind of work, since 

 it may be stated broadly that the activity and intelligence of an 

 animal are proportionate to the efficiency of its breathing organs. 

 For rapid movement means quick wasting of the muscles, and this 

 is not possible unless abundant oxygen is supplied and waste 

 products speedily removed. And great intelligence is associated 

 with rapid oxidation of brain-substance, and similar quick re- 

 moval of waste substance. It may also be said that, as a general 

 rule, efficient breathing organs are associated with efficient cir- 

 culatory organs. The most active and most intelligent groups of 

 animals now existing are undoubtedly mammals, birds, and insects, 

 all of which are particularly well off in the matter of breathing 

 arrangements. 



One very important principle in regard to such organs must be 

 carefully borne in mind. Great efficiency as regards breathing 

 organs means the existence of a large surface in close proximity to 

 the blood, so that on the one hand oxygen may pass in, and on 

 the other carbonic acid gas and water pass out. And we shall 

 find that a large surface may, by various devices, be packed away 

 in a comparatively small space, and further, that it is often asso- 

 ciated with complicated arrangements by which constant and rapid 

 renewal of air is provided for. 



BREATHERS IN WATER AND BREATHERS 



IN AIR 



All animals, without exception, are breathers or intakers of air, 

 or rather of the oxygen gas which it contains, and it is incorrect to 

 speak, as is sometimes done, of " air-breathers " as contrasted with 

 "water-breathers". Some animals, such as whales, thoueh living- 

 in water, have from time to time to come to the surface to obtain 

 air for breathing, but a host of aquatic creatures, such as most 

 fishes, crustaceans, marine molluscs, &c., do not find it necessary to 

 do this. The oxygen which they need is not, however, obtained 

 from the water (H2O) itself though this consists of oxygen united 

 or combined with hydrogen. But such animals do not possess the 

 power of decomposing water for the sake of the oxygen which 

 enters into its composition. If they did, free hydrogen gas would 



