26 PEARLS [CH. 



we shall see, from the point of view of pearls) has 

 been removed. This flap, known as the mantle or 

 pallium, is a lateral fold of the wall of the body of 

 the animal. It is similar in shape to the valve of 

 the shell; in fact it has formed the latter. 



The cavity between the two mantle flaps into 

 which one looks when a mollusc's shell is opened, 

 is of course nothing else but part of the external 

 world, and is known as the pallial cavity. The sea- 

 water enters this space and bathes the gills which 

 hang down into it and also the internal faces of 

 the mantle flaps. The mantle lobes are therefore 

 separated anteriorly, ventrally and posteriorly, but 

 continuous dorsally underneath the hinge-line. The 

 free edge of the mantle is thickened and darker in 

 colour than the rest. It also bears numerous short 

 tentacles. In the common scallop the mantle edge 

 bears numerous complex eyes as well as the 

 tentacles. 



It is usual to find the edge of the mantle thickened 

 very considerably in the majority of bivalves, and, as 

 we have seen, the edge is of great importance, for it 

 secretes tiro of the layers of the shell, the external 

 periostracum and the prismatic layer. These two 

 layers are renewed by the mantle edge, and growth 

 of the shell, so far as these layers are concerned, can 

 only take place at the margin. 



The inner nacreous layer is, on the other hand, 



