226 THE VITAL FUNCTIONS. 



aquatic respiration the air contained in water is made to 

 act on the blood circulating in vessels which ramify on the 

 external surface of the filaments of the gills; while in at- 

 mosph'eric respiration the air in its gaseous state is always 

 received into cavities, on the internal surface of which the 

 blood vessels, intended to receive its influence, are distri- 

 buted. It is not difficult to assign the final cause of this 

 change of plan; for in each case the structure is accommo- 

 dated to the mechanical properties of the medium respired. 

 A liquid, being inelastic and ponderous, is adapted, by its 

 momentum alone, to separate and surround the loose float- 

 ing filaments composing the branchiae; but a light gaseous 

 fluid, like air, is, on the contrary, better fitted to expand di- 

 latable cavities into which it may be introduced. 



Occasionally, however, it is found that organs constructed 

 like branchiae, and usually performing aquatic respiration, 

 can be adapted to respire air. This is the case with some 

 species of Crustacea, of the order Decapoda, such as the 

 Crab, which, by means of a peculiar apparatus, discovered 

 by Audouin, and Milne Edwards, retain a quantity of water 

 in the branchial cavity so as to enable them to live a very 

 long time out of the water. It is only in their mature state 

 of development, however, that they are qualified for this 

 amphibious existence, for at an early period of growth they 

 can live only in water. 



There is an entire order of Gasteropodous Mollusca which 

 breathe atmospheric air by means of pulmonary cavities. 

 This is the case with the Limax, or slug, and also with the 

 Helix, or snail, the Testacella, the Clausilia, and many 

 others, which, though partial to moist situations, are, from 

 the conformation of their respiratory organs, essentially 

 land animals. The air is received by a round aperture near 

 the head, guarded by a sphincter muscle, which is seen to 

 dilate or contract as occasion may require, but which is 

 sometimes completely concealed from view by the mouth 

 folding over it. The cavity, to which this opening leads-, 

 is lined by a membrane delicately folded, and overspread 



