PEARLS 207 



merely a thin casket, for the noisome mud had not 

 solidified. The care with which the impurity had been 

 rendered innocuous demonstrated the correct ideas of 

 the oyster on sanitation. No doubt the germ of the 

 special form of tape-worm which troubles oysters, 

 irritates to pearl-making, and passes through other trans- 

 formations in other hosts, and completes its cycle in 

 the body of a shark, would be too minute for inexpert 

 detection. The fact that molluscs do intern foreign 

 and obnoxious substances is testimony to their decency 

 and love of cleanliness, and so may the pearl be still 

 accepted as the embodiment of purity. Though all 

 its little soul be dirt, the pearl is pure, and but for the 

 dirt or the germ of a filthy ailment it would not be pearl. 



So many molluscs produce pearls that it would be 

 absurd for the great oyster family to set up exclusive 

 rights. They do not, for your oyster is ever humble 

 even when tenanted with a rivalless pearl. On the coast 

 of North Queensland, within the Great Barrier Reef, 

 pinnas of at least two species are among the producing 

 agents, which, covering a wide range, seem to meet in 

 two distinct genera, far apart in appearance and habit. 

 There is the frail, flat, translucent "window-shell" 

 (Placuna), the valves of which fit so closely that the 

 poor little inhabitant is squeezed to a wafer, a film, a 

 fragment of muscle. Yet in some localities nearly every 

 individual has a pearl, pretty in tint, but too minute to 

 be of value. An allied species is common on the coast 

 of China, where the pearls are collected for export to 

 India, to be reduced to lime by calcination for the use 

 of luxurious betel-nut chewers. These almost micro- 

 scopic pearls are also burnt in the mouths of the dead 

 who have been influential and wealthy. 



Coal-black pearls occur in one of the pinnas, the 

 interior of which is sooty, shot with iridescent purple, 



