286 THE VITAL FUNCTIONS. 



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 floating filaments composing 

 the branchiae ; but a light gaseous fluid, like air, is, 

 on the contrary, better fitted to expand dilatable 

 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 Crabs, 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 developement, however, 

 that they are qualified for this amphibious exist- 

 ence, for at an early period of growth they can live 

 only in water. • 



There is an entire order of Gasteropodous Mol- 

 lusca which breathe atmospheric air by means of 

 pulmonary cavities. This is the case with the 

 Litnax, 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, essen- 

 tially 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 occa- 

 sion may require, but which is sometimes com- 

 pletely concealed from view by the mantle folding 

 over it. The cavity, to w hich this opening leads, 

 is lined with a membrane delicately folded, and 



