RESPIRATION IN MOLLUSCA. 317 



are distributed. It is not difficult to assign the 

 final cause of this change of plan ; for in each 

 case the structure is accommodated to the me- 

 chanical properties of the medium respired. A 

 liquid, being inelastic and ponderous, is adapted, 

 by its momentum alone, to separate and sur- 

 round 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 perform- 

 ing aquatic respiration, can be adapted to respire 

 air. This is the case with some species of Crus- 

 tacea, of the order Decapoda, such as Crabs, 

 which, by means of a peculiar apparatus, dis- 

 covered 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 de- 

 velopement, 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 Ctausilia, 

 and many others, which, though partial to moist 

 situations, are, from the conformation of their 



