MOLLUSCA 6 1 



mantle around some small foreign body as a nucleus. This foreign 

 body may be a grain of sand, a dead egg, a dead parasite, or a small 

 shot or other object introduced by man between the shell and the 

 mantle. The irritation of this foreign body probably causes an active 

 secretion by the mantle glands of the nacreous material in concentric 

 layers over the offending object, the result being a pearl, whose value 

 depends upon its size, shape, color, etc. If the pearl be attached to the 

 shell it will, of course, have to be cut free, which will leave a scar on one 

 side and render it fit only for a setting where this side will be hidden. 

 In some places, it is said, fine shot are introduced between the mantle 

 and shell to act as nuclei for pearls; the oyster is then replaced in the 

 water to be examined for pearls at a later date. 



This characteristic of the mantle has been made use of in producing 

 the sacred clam shells that the Chinese Buddhists believe to have a mi- 

 raculous origin, and that are sometimes seen in collections. Brooks 

 (79) says: 



"The inside of the shell has a beautiful pearl lustre, and along it is a row 

 of little fat images of Buddha, squatting with his. legs crossed under him, 

 and his elbows on his knees. They are formed of pearl precisely like that 

 which lines the rest of the shell, a little raised above its surface and outlined 

 in faint relief, but they are a part of the shell, with no break or joint. In 

 the process of manufacturing them, the shell of the living animal is wedged 

 open, and thin images, punched out of a sheet of bell-metal, are inserted. 

 The animal is then returned to the water, and is left there until enough new 

 shell has been formed to cover them with a varnish of pearl thick enough to 

 fasten them, and to hide the metal, while permitting the raised outline to be 

 seen." 



Although pearls of considerable size are often found in the edible 

 oysters and other bivalves they have little value, as the glands of 

 the mantle do not secrete the proper nacreous material. 



The precious pearls are found chiefly in the pearl oysters of the 

 family Amculidce and in the pearl-bearing mussels of the family Union- 

 ida. The true pearl oysters are found in the Indian Ocean, Red Sea, 

 Gulf of California and elsewhere, though the value of the pearls and 

 mother-of-pearl varies greatly in different localities. In some places 

 the*shells are heavy and of fine quality and may be worth 50 cents 

 each; in other places they are thrown away and only the pearls are 

 sought. 



