250 THE MECHANICAL FUNCTIONS. 



original shell is quite buried in this vitreous sub- 

 stance. 



The forms of the Cone and Olive shells are such 

 as to allow but a small space for the convolutions 

 of the body of the animal, which accordingly 

 becomes, in the progress of its enlargement, ex- 

 cessively cramped. In order to obtain more 

 space, and at the same time lighten the shell, 

 the whole of the two exterior layers of the inner 

 whorls of the shell are removed, leaving only 

 the interior layer, which is consequently very 

 thin when compared with the other whorl, that 

 envelopes the whole, and which, retaining its 

 original thickness, is of sufficient strength to 

 give full protection to the animal. That this 

 change has actually been effected is very dis- 

 tinctly seen in the Conus (Fig. 115) by examining 

 a vertical section of that shell, as is repre- 

 sented in Fig. 116. All the inner partitions of 



115 116 117 



the cavity thus laid open are found to be ex- 



