XVIII. 
GROWTH OF SHELLS. 
“HERE'S a pretty palace with a good stout 
front door,” announced Tom, displaying a 
pearly Zrochus. 
“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 horny 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. 
“Tn the baby Zrochus this operculum can 
be discerned, and grows with the growth of the 
animal and the other shell covering.” 
“T 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 
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