THE MYA TRUNCATA. 267 



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. 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. i 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.' * 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 the swan 

 muscle (Anodon cygneus) 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 perfectly resembles the 

 real pearl. The extraneous body which naturally serves for 

 the nucleus appears to be very often, or, as Sir E. Home 

 says, always, a blighted ovum. Christophorus Sandius, in 

 1673, on the authority of Henricus Arnoldi, i an ingenious 

 and veracious person,' asserted that the ova left unexpelled 

 from the shell became the nuclei on which pearls, in the 

 fresh water muscle, were formed. ' Sometimes,' he says, 

 ' it happens that one or two of these eggs stick fast to the 

 sides of the matrix, and are not voided with the rest. These 

 are fed by the oyster against her will ; and they do grow, 

 according to the length of time, into pearls of sufficient big- 

 ness, and imprint a mark both on the fish and the shell, by 

 the situation, conforming to its figure.' This theory has been 



t Sir Everard Homes Lectures on Comparative Anatomy, vol. v. p. 308. 



