XVIII. 

 GROWTH OF SHELLS. 



"Here's a pretty palace with a good stout 

 front door," announced Tom, displaying a 

 pearly Trochus. 



" The name of that ' front door,' " said his 

 cousin, " is the operculum. Many shells are fur- 

 nished with opercula, but not all are so strong 

 and homy as this. You see it is even thicker 

 than the walls of the shell itself. It is devel- 

 oped upon a part of the foot of the animal and 

 moved by strong muscles, which enable the 

 little householder to shut his door quickly at 

 the approach of an enemy. 



" In the baby Trochus this operculum can 

 be discerned, and grows with the growth of the 

 animal and the other shell covering." 



"I don't see how shells grow, anyway!" 

 exclaimed Tom. 



"Very likely you don't," said Miss Breme- 

 ly. "Nature does not make a parade of her 

 fine work, but if we care enough about her and 



12 " 155 



