230 PEARLS. 



are, therefore, not properly " the unhandsome excretion of 

 an oyster," nor " a distemper in the creature that produces 

 them," and cannot, under any view, be compared with cal- 

 culi in the kidneys of man ; * for, though accidental for- 

 mations, and, of course, not always to be found in the 

 shell-fish which are known usually to contain them, still 

 they are the products of a regular secretion, applied, how- 

 ever, in an unusual way, either to avert harm or allay irri- 

 tation. That in many instances they are formed by the 

 oyster to protect itself against aggression, is evident ; for, 

 with a plug of this nacred and solid material it shuts out 

 worms and other intruders which have perforated the softer 

 shell, and are intent on making prey of the hapless inmate ; 

 and it was apparently the knowledge of this fact that sug- 

 gested to Linnaeus his method of producing pearls at plea- 

 sure, by puncturing the shell with a pointed wire, -j- But 

 this explanation, it is obvious, accounts only for the origin 

 of such pearls as are attached to the shell ; while we know 

 that the best and the greatest number, and, indeed, the 

 only ones which can be strung, have no such attachment, 

 and are formed in the body of the animal itself. " The 

 small and middling pearls," says Sir Alexander Johnston, 

 " are formed in the thickest part of the flesh of the oyster, 

 near the union of the two shells ; the large pearls almost 

 loose in that part called the beard." J Now, these may be 

 the effect merely of an excess in the supply of calcareous 

 matter, of which the oyster wishes to get rid ; or they may 

 be formed by an effusion of pearl, to cover some irritating 

 and extraneous body. The reality of the latter theory is, 

 perhaps, proved by a practice of the modern Chinese, who 

 force a large coarse species of swan-mussel to make pearls, 

 by throwing into its shell, when open, five or six minute 

 mother-of-pearl beads strung on a thread : in the course 

 of one year these are found covered with a crust which per- 

 fectly resembles the real pearl. § The extraneous body 



* List. Hist. An. Ang. p. 150. Dr. Turton has adopted this notion of 

 Lister's (Edit. Gmel. iv. 176) ; and the learned Mr. Turner, receiving it as a 

 truth, infers, from the abundance of pearls in the mussels of the Conway, 

 that the water of this river "had some quality or substance which gave the 

 Mya a sickly tendency." — Sac. Hist. i. 303. 



t Lin. Corresp. by Smith, ii. 429, Pearls formed somewhat in this man- 

 ner, by the fresh-water mussel, are preserved in the Hunterian Museum. — 

 Home's Lect. Comp. Anut. vi. 296. See also Edinb. Phil. Journ. xi. 40. 



% Home's Comp. Anat. v. 308. 



§ The Chinese appear to have more ways than one of getting artificial 

 pearls. Sir E. Home says their method is this : — " They take the substance 

 of the clamp-shell, turn it in a lathe into hemispheres of different sizes, and 



