THE HOUSE THAT THE OYSTER BUILT. 9 1 



appears in relief. When it is intended that this should be 

 cut out, this can be done with a fine frame-saw, or a knife- 

 edged tool. Should several devices be required of the 

 same shape and size, a number of films of shell are glued 

 together into a compact mass. The ornament is then 

 drawn on one of the two exterior films, and the saw is 

 employed to cut through the whole at one time. The 

 mass is then put into warm water, which softens the glue, 

 and the various pieces are separated one from another. 



But the enlargement and decoration of its dwelling is 

 not the limit of the oyster's curious power. The shell 

 becomes gradually much thicker, and on the agency that 

 produces this change we are happily left in no doubt. 



Professor R. Jones observes : "While the margin of 



r O 



the mantle is the sole agent in enlarging the circumference 

 of the shell, its growth in thickness is accomplished by a 

 secretion of a kind of calcareous varnish, derived from the 

 external surface of the mantle generally ; which, being 

 deposited layer by layer over the whole interior of the 

 previously existing shell, progressively adds to its weight 

 and solidity. ' : () 



The fact is equally deserving of notice that, as glands 

 are situated only in the circumference of the mantle, no 

 colouring matter is ever mixed with the layers that increase 

 the thickness of the shell. And why ? Because, were it 

 otherwise, there would be a loss of the secreted substance ; 

 and economy is one of the laws of creation. As it is, the 

 substance always continues of a delicate white hue, and 

 forms what is called nacre, or mother-of-pearl. 



" In structure, the nacre is very dense, hard, and firmly 

 laminated ; but the superficial outer layer is made up of 



"Animal World." 



