OF THE GULF OF MANAAR. 
Ill 
ness in its own shelly and if any foreign body which causes 
friction finds lodgment in the pearl oyster it coats it also, 
in self-protection, with this same Nacre. So sensitive is the 
animal of the least friction that it cannot endure the presence 
of even its own microscopically minute ova without coating 
them with Nacre. These ova which have escaped by rupture 
from the over-distended ovarium of the parent pearl oyster, 
and the ova of Filaria, of Circaria, and of three other para- 
sitic worms, form, when coated with Nacre, the valued pearl 
of commerce. When first covered they are so exceedingly 
small as to be valueless seed pearls, but coating after coating 
of Nacre increases their size, and so the pearl continues to 
grow in size as long as the oyster is in vigorous health, and 
the varying size of pearls probably depends, as in the varying 
size of stags' antlers, on the vigor of the individual oyster. 
But it is a pretty general rule with shell-fish to absorb or 
re-utiHze portions of their own shells that can be thus advan- 
tageously economized, and following this rule it is believed 
that as the reproducing powers of the pearl oyster wane with 
the failing vigor of age, it falls back on its own pearls for a 
supply of Nacre, and absorbing it utilizes it in fresh places 
in economy of the labor of producing fresh Nacre. And thus 
it is believed that there comes an age in the oyster when the 
pearls not only cease to grow but actually diminish in size. 
Their form depends on the accident of the form of the 
foreign body coated. If the foreign body be a single ovum, 
then the pearl following its form is of sequence perfectly 
round, but if the nucleus of the pearl consists of a number of 
ova, then the form of the pearl is necessarily irregular, and 
when by such accident of origin the pearl assumes the form 
of a pear it is most highly prized in the market. When the 
Nacre is deposited over sand adhering to the shell the pearl 
becomes an excresence on the shell, and is called a blister 
pearl. Such pearls are ordinarily very much larger than 
any others, but their iridescence is inferior and they have only 
