136 SEA-SHORE LIFE 
which is deposited in layers by the mantle as the shell grows. Con- 
trary to the general opinion, pearls appear not to be formed around 
grains of sand but around minute parasites, or even an egg of the 
oyster itself. However, some foreign body causes an irritation of 
the mantle, and the nacre is then deposited in layers around the 
disturbing substance. The best pearls are spherical and are not 
attached to the shell itself, for if they become fastened to the shell 
they grow irregularly, and their value is lessened. They are always 
of the same color as the nacre of the shell, and as individual oysters 
vary considerably, this may be a steely black, a brilliant iridescent 
white, delicate pink or yellow. Inthe Gulf of California only about 
one oyster ina thousand contains a pearl. Pearls are most abund- 
ant in diseased oysters, or those which are attacked by boring 
sponges and other parasites, and are best developed in oysters 
about four years old. Great as may be the value of individual 
pearls, the pearl fisheries are mainly dependent upon the sale of the 
shells themselves. Immense numbers of shells are annually used 
in the manufacture of buttons and ornaments. 
The pearl oysters attach themselves, when young, by a strong 
byssus-thread to rocks in water from 25 to 250 feet deep, and they 
are abundant in some of the lagoons of the coral islands of the Pa- 
cific in water about 100 feet in depth. In the Paumotos Islands the 
natives obtain them by the primitive method of diving to the bottom 
without the aid of diving suits or other apparatus. Having dis- 
covered the situation of a pearl shell by means of the water glass, 
which is merely a glass-bottomed bucket, the diver proceeds to 
whistle shrilly, filling his lungs repeatedly to their fullest capacity. 
He then jumps in feet foremost but immediately turns and swims 
head down to the bottom, carrying with him a half pearl shell 
with which to cut the living pearl shells off from the rocks below. 
The writer observed one experienced diver who went down in 90 
feet of water and remained below two minutes and five seconds, 
bringing up two pearl shells. The largest known pearl belongs to 
Mr. Hope, of England, and has long been on exhibition in the South 
Kensington Museum. It is nearly four and a half inches in diam- 
eter, but is somewhat irregular. The iridescence of pearls is an 
optical phenomenon and is due to the interference of light caused 
by minute corrugations over the surface of the pearl, 
