PEARLS AND PARASITES 89 



34. The Nature of Pearls 



Pearls have been lately studied by zoologists, and 

 their true history made known. They are a disease, 

 caused, like so many other diseases, oy an infecting 

 parasite. It is common knowledge that they are 

 found much as we see them in jewellery, as little 

 lustrous spheres embedded in the soft bodies of various 

 shellfish, such as mussels, oysters, and even some kinds 

 of whelks. They are not found in the shellfish like 

 crabs and lobsters, called Crustacea, but only in those 

 like snails, clams and oysters, called Mollusca. Pink 

 pearls are found in some kinds of pink-shelled whelks, 

 A pearl-mussel or pearl-oyster has a pearly lining to its 

 shell, which is always being laid down layer by layer by 

 the surface of the mussel's or oyster's body, where it 

 rests in contact with the shell, which consequently in- 

 creases in thickness. If a grain of sand or a little fish 

 gets in between the shell and the soft body of its 

 maker, it rapidly is coated over with a layer of pearl, 

 and so a pearly boss or lump is produced, projecting on 

 the inner face of the shell, and forming part of it. 

 These are called " blister-pearls," and are very beautiful, 

 though of little value, since they are not complete all 

 round, but merely knobs of the general "mother-of- 

 pearl" surface. These blister-pearls can be produced 

 artificially by introducing a hard body between the 

 shell and the living oyster or mussel. 



It used to be thought that the true spherical pearls 

 were caused by a hard granule of some kind pressing 

 its way into the soft substance of the shell-fish, pushing 

 a layer of the pearl-producing surface like a pocket in 

 front of it. But it is now known that this " pushing 

 in " is the work, not of an inanimate granule, but of a, 

 minute parasitic worm, which becomes thus enclosed by 

 a pocket of the outer skin. The pocket closes up at its 



