62 THE HALL OF SHELLS. 
“ Another theory has been that pearls were 
always the result of a grain of sand or some 
irritating substance entering the shell; the ani- 
mal, unable to discharge it, converted it into a 
pearl. Hence we are told: 
“Learn from yon Orient shell to love thy foe, 
And strew with pearls the hand that brings thee woe. 
This is beautifully suggestive, yet seems to 
be only haif the story. While irritating sub- 
stances are known to be covered by nacre it 1s 
believed all pearls are not the result of irrita- 
tion, but are secreted by the mollusk and held 
ready to be dissolved by powerful acids, which 
are also of the animal’s secretion, for spreading 
over openings made in their shells by the borer. 
“Injured shells are often found with their 
points of irritation covered with thin lamina- 
tions of this nacreous matter. 
“ Pearl fishers tell us the little pearl maker 
is, sometimes at least, able to expel his jewel 
at will, and often does so when captured ; un- 
derstanding this, the fisher places his hand over 
the shell so as to close its valves or secure the 
pearl if ejected. 
“ After possessing himself of all the pearls 
in old mussels—the old being the most produce- 
tive—the gatherer sometimes deposits the mol- 
