THE PEAEL FISHERY OF 1904. 



33 



Fi 



negatived this idea and showed that these animals have little or no influence upon 

 pearl production ; the sponge, Cliona margaritifercs, causes occasional roughening, or 

 even rugged tuberculatum (fig. 10), at the insertion scar of 

 the adductor muscle, and the annelid, Polydora, may 

 produce an occasional discoloured internal excrescence, 

 usually sharp-pointed : more rarely a parasitic nematode, 

 free or encysted, is cemented to the nacre and sealed up in 

 a pearly sarcophagus. 



" Bv far the larger numher of shell-pearls, fully 90 per 

 cent, of the whole number, are due to the attachment of 

 cyst- and muscle-pearls to the nacreous lining of the shell, 

 consequent upon fusion of the pearl-forming sac with the 

 epidermal layer outside, and rupture of these tissues caused 

 by the pressure of the growing pearl, thus placing the pearl 

 in actual contact with the nacre and converting its closed 

 pearl-secreting sac into a pouch or ampulla, the open neck directed outwards and in 

 continuity with the general epidermal layer upon the exterior of the mantle. 



" Shell-pearls originating in this manner are at first pedunculated, and at this 

 stage may be detached readily. The peduncle tends, however, to become obliterated, 

 and eventually the pearl may be entirely lost in the substance of the shell nacre. 

 Such shell-pearls are in most cases affixed singly, without definite arrangement. 

 Examples may be located anywhere, the largest, however, being usually either in the 

 peripheral region of the nacre, or in the central region, corresponding with the lateral 

 surfaces of the visceral mass. 



10. Inside of pearl oyster 

 shell, showing adductor im- 

 pression affected by Cliona 

 borings on the outside, x h 



Fig. 11. Diagram showing comparative frequency 

 of cyst pearls in the various parts of the mantle. 



Fig. 12. Diagram showing the positions most 

 frequently occupied by muscle pearls. 



" Smaller attached pearls are not infrequently either singly or in a serial line of 

 from two to five along the pallial line, each pearl coinciding in position with a muscle 



p 



