Ill] THE PEARL OYSTER 25 



which is of the greatest importance in regard to 

 theories of pearl formation. 



The shell is repaired, but the repair substance is 

 not, as was once thought, only nacre. The mantle 

 cells, which normally would be secreting nacre, now 

 secrete at the area of breakage, first periostracum, 

 then prismatic layer, and finally mother-of-pearl. 



Thus, although certain parts of the mantle surface 

 secrete normally certain definite parts of the shell, 

 the cells which secrete the nacre are capable of 

 secreting all the other layers if necessary. This 

 extraordinary property is made use of in the 

 formation of pearls, as will be seen later. 



Let us now leave the dead shelly covering and 

 look at the animal itself which has been responsible 

 for the secretion of the valves. The figure (fig. 4) 

 is a view of the right side of the animal. In the 

 centre there is to be seen the large adductor muscle, 

 and this seems to take the place of a central scaf- 

 fold. Dorsal to this, that is towards the hinge-line, 

 is a soft unsegmented body in which lies the heart 

 and the viscera. Ventral to the muscle are the 

 flat plate-like gills hanging down as curtains into 

 the cavity that is enclosed by the two valves. On 

 the left the foot is to be seen with fibres of byssus. 

 The mouth is situated above the byssus. This figure, 

 with the rough description given above, represents 

 what is seen when a flap of tissue (very important, as 



