292 THE VITAL FUNCTIONS. 



stances, soon swarm with millions of these micro- 

 scopic beings, ever exhibited a single animal- 

 cule ; although these soon made their appearance 

 in great numbers, if the smallest quantity of air 

 was admitted into the receiver. 



Animals which inhabit the waters, and remain 

 constantly under its surface, such as fishes, and 

 the greater number of mollusca, are necessarily 

 precluded from receiving the direct action of 

 atmospheric air in its gaseous state. But as all 

 water exposed to the air soon absorbs it in large 

 quantities, it becomes the medium by which that 

 agent is applied to the respiratory organs of 

 aquatic animals ; and the oxygen it contains may 

 thus act upon the blood with considerable effect ; 

 though not, perhaps, to the same extent as when 

 directly applied in a gaseous state. The air 

 which is present in water is, accordingly, as 

 necessary to these animals as the air of the 

 atmosphere is to those which live on land : hence 

 in our inquiries into the respiration of aquatic 

 animals, it will be sufficient to trace the means 

 by which the surrounding water is allowed to 

 have access to the organs appropriated to this 

 function ; and in speaking of the action of the 

 water upon them, it will always be understood 

 that I refer to the action of the atmospheric air 

 which that water contains. 



Respiration, in its different modes, may be 

 distinguished, according to the nature of the 



