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. In the Gulf of California only about 

 one oyster in a 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, 



