AMnilBIOUS MOLLUSCA. 



191 



the intestine or even into the air-duct leading to the air-bladder, 

 and thus both organs might become organs of respiration, since, 

 fundamentally, every growing or living cell must breathe as soon 

 as it comes into contact with a highly oxygenated medium. 



Even among the Invertebrata we know of animals which may, 

 in this sense, be designated as true Amphibians. The opercu- 



Fia 55— Gill lungs of Ampnllarisu a, Ampnllaria tnmlixrtim (II'OTh.). n, Itmgrespira- 

 tory siphon ; 6, section in the direoiion of tbe arrow b ; h, the upper lang-cavity ; «, 

 branchial cavity witii the right ana left gills ; the cavities communicate bya passage 

 in the centre of the dividing wall. 



lated snail Ampullaria (fig. 55) has a well-developed branchial 

 cavity and gills, and above these, and separated from them, it 

 has a well-developed lung-cavity, of which the structure is pre- 

 cisely similar to that of the lungs of our common land-snails. 

 The Ampullaria uses both organs in rapid alternation ; lying 

 not far from the surface of the water, it protrudes above it a 

 breathing siphon, and inhales aii- through it ; then it closes its 



