668 The Outline of Science 



water mussel) and the underlying skin or mantle, which lines the 

 shell and is always adding to it (see colour plate facing p. 650). 

 Round the intrusion the skin secretes layer after layer of mother- 

 of-pearl, or nacre, and a sort of pearl is formed. But this is not a 

 true pearl. It is usually adherent to the shell and has a solid 

 foreign body, sometimes opaque, at its core. 



A finer result is obtained when a tiny hole is bored in the shell 

 and a rounded granule of mother-of-pearl is slipped in between 

 the shell and the skin. The hole is sealed up and the "pearl" is 

 left to grow. In this case the result is more homogeneous, for the 

 core is of mother-of-pearl. But this never yields what is called a 

 "fine pearl." 



An improvement on the previous procedure consisted in 

 introducing into the skin of mussels little fragments of living tis- 

 sue. In the course of time these were found encased in concentric 

 layers of mother-of-pearl, yielding reputable pearls. It is prob- 

 able that this method is capable of great improvement. 



What happens in natural conditions is rather different. In 

 the common edible mussel a pearl may be formed from a sac of 

 skin-cells surrounding an intruding fluke-larva. The skin-cells 

 secrete a sepulchre for the fluke delicate concentric layers of 

 mother-of-pearl. As the core is the tenuous remains of a very 

 delicate and minute parasite, the pearl that results has con- 

 siderable opalescence. Similarly, some naturalists believe that 

 the fine pearls of the Oriental pearl-oyster are formed 

 round intruding larva? of a tapeworm, but conclusive proof is 

 wanting. 



There is considerable reason for believing that pearls often 

 arise round nuclei of the substance called conchin, which is secreted 

 by the skin as the organic foundation of the calcareous shell. A 

 little blob of this clear secretion, formed in a skin-sac as the result 

 of some slight disturbance in the ordinary routine of shell-making, 

 may form the centre of a really fine pearl, which is built up of 

 numerous concentric layers of nacre. 



