221 



NOTES ON PEARL FORMATION AND JAPANESE CULTURE PEARLS. 



By T. H, Haynes. 



Bead 10th June, 192L 



(PLATES VII AND VIII.) 



The study of pearl oysters and the problems presented by the 

 formation of pearls demand an acquaintance with the distinction 

 between pearls and " blisters ", or excrescences on the shell surfaces. 

 All pearls are formed within the tissues of the mollusc, but they 

 frequently pass out and become included between the animal and 

 the shell, when they are treated in the same manner as any foreign 

 object in that position. They are joined to and embedded in the 

 shell by successive layers of nacre, or other sb ell-substance. These 

 excrescences are gradually reduced in height by the successive layers 

 of nacre covering the foreign body, being thinner at the top than at 

 the base or plane of the shell (PI. VIII, f. 5). " Decreasing blisters " 

 are represented on PI. VII, Figs. Ij 2, and 3. Fig. 4 represents 

 different features and represents an " increasing blister " or barrier 

 raised as a defence against penetration by a boring bivalve from the 

 outside ; and the layers of nacre are thicker at the top than at the 

 base. In the event of the borer working vertically the shell is usually 

 pierced ; but in most cases the borer works at an angle, and the 

 pearl oyster is able to increase the height of the blister quickly 

 enough to prevent complete penetration. When the borer is com- 

 pletely within the substance of the shell the entrance it has left 

 behind itself often becomes choked with mud, which prevents the 

 entry of sea-water, in which case the borer perishes and the pearl 

 oyster then reduces the height of the blister in the same way as 

 described in the three preceding instances. Small blisters are thu.8 

 often completely levelled down in the growing shell. 



If there is a hole at the back of the shell opposite to a blister 

 the pearler takes no further interest in it ; but if there has been no 

 borer at work the pearler punches out the blister by a series of holes 

 round it with a steel punch, and by smart but careful tapping with, 

 a hammer laterally the blister will disclose the nature of its contents 

 by splitting in two at the point where the foreign substance which 

 lodged against the shell surface was attached and covered by the 

 successive layers of nacre. These true blisters may contain a crab 

 (Fig. 2) or stray shell (Fig. 1) or stone or a pearl (Fig. 3) escaped 

 from the tissues of the oyster, or they may be hollow and foul- 

 smelling when opened — in which case an animal or vegetable object 

 was entombed and has decomposed and left no trace of its identity. 



There is no direct evidence as to the frequency of shell secretion. 

 When young the lateral growth of the animal is so rapid that the 

 secretion of layers of nacre, each overlapping the other outwards 

 towards and over the horny lip of the shell and increasing the 



