GENERAL NOTES ON MOLLUSCS. 389 



The shell is a very characteristic molluscan structure, 

 but in spite of all the years of conchology, we cannot 

 answer the fundamental questions about shell making. 

 Mollusc shells are very 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. That 

 they afford their bearers efficient protection, is shown by 

 the appreciation which some hermit crabs exhibit for stolen 

 whelk or buckie shells. More precise observation shows us 

 that the shell consists in great part of carbonate of hme ; 

 that it has a thin outer " horny " layer, a thick median 

 " prismatic " stratum of lime, and an internal mother-of- 

 pearl layer. This last exhibits iridescence, produced by 

 the fine lines which mark successive deposits. The mark- 

 ings, fine as they are, were repeated, according to Brewster's 

 famous experiments, on a piece of wax, which in consequence 

 became iridescent. 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 on 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 deposi- 

 tion of carbonate of lime. Like other cuticular products, it 

 has an organic basis (called conchioHn), along with which, 

 in a manner that we do not clearly understand, 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 reaction 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 



