VITALITY OF MOLLUSCS. 339 



ently provided. It is true the molluscan gills are formed of 

 leaflets heavier than the air ; but when we take him from 

 the water he closes his shell, and in that shell a reasonable 

 supply of water remains. But this is not his chief safeguard. 

 A constant exudation from the surface keeps the gills moist, 

 and this moisture permits the exchange of gases, on which 

 respiration depends. It is this cause which enables the land- 

 crabs to live in the atmosphere, although their gills are 

 formed on the same plan as those of the marine crabs. Milne 

 Edwards has shown that a special reservoir exists which pre- 

 serves the humitlity of their gills.* In those Molluscs which 

 have no supply of water in their shells to keep the branchige 

 floating, there is always a constant moisture to keep them 

 fit for respiration ; and although the respiration must neces- 

 sarily be feebler under such circumstances, yet we must re- 

 member that the vital changes are not so rapid in its lethar- 

 gic and comjjaratively simple organism as in that of the fish. 

 The molluscs do not rank high in the scale of intelligence, 

 yet even the Oyster seems to be educable to a small degree. 

 Milne Edwards relates, that in the great oyster establishments 

 on the coasts of Calvados, he learned that the merchants 

 teach these succulent molluscs to keep their shells closed when 

 out of the water, by which means they retain the water in 

 their shells, keep their gills moist, and arrive lively in Paris. 

 The process is this : No sooner is an oyster taken from the 

 sea than it closes its shells, and opens them only after a cer- 

 tain time — from "fatigue," it is said, but more probably 

 because the shock it received, and which caused its muscles 



* See his Legons sur laPhi/s. et CAnat. Comparie, 1857, vol. i. p. 519. 



