THE SHELLS OF MOLLUSCS. 301 



beautiful things alike in form and colour, they grow larger 

 by month and year, and mark their progress by rings of 

 growth and changing tints of colour, they afford their bearers 

 efficient protection which the hermit-crabs who steal whelk 

 and buckie shells evidently appreciate. More precise obser- 

 vation shows us that the shell consists in great part of 

 carbonate of lime ; that it has a thin outer " horny " layer, 

 a thick median " prismatic " stratum of lime, and an internal 

 mother-of-pearl layer with the usual iridescence produced by 

 fine markings and repeated, according to Brewster's famous 

 experiments, on its impress on a piece of wax. On the 

 dorsal surface of almost every mollusc embryo, there is a 

 little shell-sac in which an embryonic shell is begun ; the 

 adult shell, however, begins on a separate area of the skin, 

 and it is always lined and increased by the mantle. If the 

 increase of the shell be carefully watched on young molluscs, 

 or if chemical analysis be made, it becomes plain that the 

 shell is no mere deposition of carbonate of lime. Like other 

 cuticular products, it has an organic basis (called conchiolin), 

 along with which, in a manner that we do not clearly under- 

 stand, the lime is associated. 



Mr. Irvine's experiments at Granton Marine Station suggest 

 that the lime salt originally absorbed is not the carbonate 

 (of which there is a scant supply in sea-water), but the 

 sulphate (which is abundant), and that the internal transfor- 

 mation from sulphate to carbonate is perhaps associated 

 with the diffuse decomposition of nitrogenous waste-pro- 

 ducts. Thus carbonate of ammonia, which seems to occur 

 abundantly in the mantle of perfectly fresh mussels, would, 

 with calcium sulphate, yield carbonate of lime and am- 

 monium sulphate. I do not suppose that shell-making is 

 expressible in a chemical equation of this simplicity, but it 

 is time that we ceased to think that Molluscs simply absorb 

 carbonate of lime from the sea-water, and sweat it out on 

 their skins. It is reasonable to inquire how far shell-making 

 may express a primitive mode of excretion to which a 

 secondary significance has come to be attached, and in 

 what way carbonate of lime shells are associated with pre- 

 ponderant sluggishness of habit. The thickness of the shell 

 seems often to bear some relation to the external and internal 

 activities of the mollusc, for it is thin in the active scallop 



