MOLLUSCS 99 



surrounding land must have at one time sunk below sea 

 level and later risen again. 



The Pearl-wing and Hammer Oysters are nearly allied 

 forms mostly from tropic seas. Of the Pearl Oyster 

 much has already been written. All shelled molluscs, 

 even whelks, are capable of providing pearls of a kind, 

 the pearl being merely a foreign body which has become 

 coated over with layers of the material with which the 

 shell happens to be lined. The true Pearl Oysters (Mar- 

 garetifera) abound off the coasts of India, Ceylon and 

 North- West Australia. The Ceylon Pearl Fishery alone 

 entails the gathering of some 80 million shells per annum, 

 the animals being obtained by native divers or the dredge. 

 Oriental pearls, the most valuable, are often caused by a 

 parasitic worm that finds its way into the tissues of the 

 animal, whilst the seed or mussel pearl is generally formed 

 round small particles of nacre which serve as a nucleus. 

 The Japanese " culture pearls," produced in such abun- 

 dance of late years, are encouraged by providing the animal 

 with a nucleus in the shape of a scrap of nacre wrapped 

 in a piece of tissue taken from another oyster. " Blister " 

 pearls, which have no commercial value, are formed on 

 the inner wall of the shell itself. Small fish and crabs 

 are sometimes incarcerated within them. In Asia they 

 often introduce small metal images of Buddha, which in 

 time become covered with mother of pearl. 



The Thorny Oysters (Spondylus) are amongst the most 

 beautiful of all bivalve shells. The massive valves are 

 joined by such firmly interlocking teeth that only great 

 force can sever them, whilst the outer walls bear a 

 forest of long spines often richly coloured and grace- 



