166 THE VITAL FUNCTIONS. 



is principally effected by the action of atmospheric air in 

 certain organs, hereafter to be described, where that ac- 

 tion, or aeration as it may be termed, in common with an 

 analogous process in vegetables, takes place. In all verte- 

 brated animals the blood has a red colour, and it is also red 

 in most of the Annelida; but in all other invertebrated ani- 

 mals, it is either white or colourless.* We shall, for the 

 present, then, consider it as having undergone this change, 

 and proceed to notice the means employed for its distribu- 

 tion and circulation throughout the system. 



* Vauquelin has observed that the chyle has often a red tinge in animals. 



