2 PEARLS AND PARASITES 



gives the inside of the shell of so many molluscs a 

 beautiful iridescent sheen. 



An oyster-shell consists of three layers, the outer- 

 most termed the periostracuni, the middle the pris- 

 matic layer, and the innermost the nacreous layer. 

 Everywhere the shell is lined by the mantle, consist- 

 ing of a right and left fold or flap of the skin, which is 

 in contact with the nacreous layer all over the inside 

 of the shell. The edge of the mantle is thickened 

 and forms a ridge or margin ; and it is this edge 

 which secretes the two outer layers. This permits the 

 shell to grow at its edge whilst the rest of the mantle 

 secretes all over its surface the nacreous or pearly 

 layer. The relative thickness of these three layers 

 varies very greatly. In the fresh -water mussel 

 (Unio) the nacreous layer is many times thicker than 

 the two outer layers put together ; and such nacreous 

 shells are usually associated with molluscs which are 

 known to represent very ancient or ancestral species. 

 It is also the layer which disappears most readily as 

 the specimens become fossilized ; and in fossil Mollusca 

 it is often represented by mere casts, which fill the 

 position it once occupied. 



The fact that the nacre is deposited by the whole 

 surface of the mantle has been appreciated by the 

 Chinese. By inserting little flattened leaden images 

 ,of Buddha between the mantle and the shell, and 

 leaving the oyster at rest for some time, the image 

 becomes coated with mother-of-pearl and incorporated 

 in the substance of the shell ; and in this way certain 

 little joss figures are produced. This industry is said 

 to support a large population in some coast districts 

 of Siam. 



The nacre, then, is produced by the outermost layer 

 of the mantle or fleshy flap that lines the shell the 

 external epithelium ; and, if a foreign body gets be- 

 tween this epithelium and the shell, the mantle will, 



