356 - SEA-SIDE STUDIES. 



lapse of the former : the leaflets, instead of floating free 

 in the air, are pressed together, so that only the exter- 

 nal surface of the two outer leaflets are in contact with 

 the air, and this is obviously too small a surface to 

 suffice for the whole aeration of the blood ; and the 

 fish dies of asphyxia. Add to this cause, the rapid des- 

 sication which ensues on exposure to the air, and which 

 we know is an obstacle to respiration. 



The Mollusc — our Cockle for instance — is somewhat 

 difi*erently 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 per- 

 mits the exchange of gases, on which respiration de- 

 pends. 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 reservok exists which 

 preserves the humidity of their gills.* In those Mol- 

 luscs which have no supply of water in theii^ shells to 

 keep the branchiae floating, there is always a constant 

 moisture to keep them fit for respiration ; and although 

 the respiration must necessarily be feebler under such 

 circumstances, yet we must remember that the vital 

 changes are not so rapid in its lethargic and compara- 

 tively simple organism as in that of the fish. 



The molluscs do not rank high in the scale of intelli- 



* See his Legons stir la Phys. el VAnat. Comparee, 1857, vol. i. p. 519. 



