24 Shell Life 



that it is closely lined with a very soft material called 

 the mantle. The name suggests that this organ 

 invests the whole body of the Snail when it is with- 

 drawn into the shell ; it does this and more — the 

 mantle is the immediate producer of the shell. 



The porcelain of our Snail's shell is composed mainly 

 of carbonate of lime, and a microscopical section 

 reveals the fact that it is made up of three distinct 

 layers, each formed separately by different portions 

 of the mantle. The material is obtained from the 

 tissues of the plants upon which the Snail feeds, and 

 has been absorbed from the soil by the roots. It 

 appears to be separated from the Snail's blood as this 

 circulates through the mantle, which has the power to 

 pour it in a fluid state upon its outer surface. By 

 exposure to the atmosphere the fluid lime mixed with 

 animal matter hardens into shell. This animal matter 

 is conchiolin, a substance not differing greatly from 

 chitin, of which the hard parts of insects are con- 

 structed. The calcareous particles appear to be held 

 together by the animal matter, and the whole sets 

 into a hard stony formation. When a portion of shell 

 is soaked in a weak acid, the carbonate of lime is dis- 

 solved and a very delicate gelatinous film is left. A 

 similar result is obtained in the case of Whelk and 

 Top-shells to which certain sea-anemones attach them- 

 selves. The base of the anemone appears to pour out 

 some acid which dissolves the lime and leaves only a 

 soft papery shell-form behind. When the mollusk is 

 dead and the empty shell is left exposed to the atmo- 

 sphere, the animal matter soon perishes and leaves the 

 shell without gloss and exceedingly brittle. 



All parts of the mantle have power to excrete one 



