PRODUCTS FROM THE SEA 347 



formed inside the mantle tissue and so unconnected with 

 the shell, may be produced in this manner. Parasites can 

 certainly cause the formation of pearls, a view which was 

 first advanced in 1554 and has been proved by the discovery 

 of mummified remains of different parasites in the centre 

 or " nucleus " of many pearls. The parasite which 

 most frequently causes pearl formation in the Ceylon 

 pearl oyster is the early stage of a tapeworm which becomes 

 adult in a large ray which, as we saw in Chapter IX, 

 habitually feeds on these oysters thereby infecting itself 

 with the tapeworm. After becoming mature, the tape- 

 worms lay eggs which pass into the sea where they may find 

 a temporary home in an oyster. A temporary home if the 

 oyster is swallowed by the ray shortly after, but a grave 

 if the oyster should be provoked to protect itself from the 

 irritating parasite by coating it with pearly substance. 

 It may be that pearls are also formed round tiny granules 

 of waste matter produced by the oysters themselves. 



The process of pearl formation is perfectly straight- 

 forward. The nucleus, of whatever irritating substance 

 that may happen to be, becomes enclosed within a tiny 

 chamber in the tissues from the walls of which is produced 

 the pearly substance which is composed of the same nacreous 

 material which lines the inside of the shell and forms 

 " mother-of-pearl." This is applied in the regular layers 

 around the nucleus so that a round bead is gradually formed, 

 the lustre of which is due to the concentric layers of the 

 crystalline nacre (Plate 123 and text figure 64). 



The most famous pearl fisheries in the world are those 

 off the coast of Ceylon. The Government controls the 

 oyster beds and only allows a fishery when a previous 

 inspection has shown there are suitable numbers of oysters, 

 which is by no means frequent for there have only been 

 thirty-nine fisheries since the beginning of the last century 



