RESPIRATION. 265 



cu instances, soon swarm with millions of these 

 microscopic beings, ever exhibited a single animal- 

 cule ; although they 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 inollusca, are necessarily pre- 

 cluded from receiving the direct action of atmos- 

 pheric air in its gaseous state. But as all water 

 exposed to the air soon absorbs it in large quan- 

 tities, it becomes the medium by which that agent 

 is applied to the respiratory organs of aquatic ani- 

 mals ; 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 pre- 

 sent 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 dis- 

 tinguished, according to the nature of the medium 

 which is breathed, into aquatic or atmospheric; 

 and in the former case, it is either cutaneous, or 

 branchial, according as the respiratory organs are 

 external or internal. Atmospheric respiration, 

 again, is either tracheal, or pulmonary, according 



