314 THE WONDER OF LIFE 



that the larvae cause the pearls. It may be a case of two 

 parallel diseases, comparable to the case of a dog infected 

 simultaneously with tapeworms and mange. Mr. Jameson 

 maintains that pearls arise round nuclei of some variety 

 of shell- substance formed when the normal rhythm of 

 secretion is disturbed. 



A very careful study of the formation of pearls has been 

 made by A. Rubbell in the case of a freshwater mussel 

 Margaritana margaritifera, which is common, for instance, 

 in some of the mountain streams of Bavaria. His obser- 

 vations are quite against the theory that pearls are sepulchres 

 of flukes or any other parasites. He finds that they arise 

 around minute particles of a yellowish substance, which 

 resembles the outermost layer of the shell (the periostracum). 

 The pearls are formed in closed, single-layered sacs of 

 epithelium, which are constricted off from the external 

 epithelium of the mantle, that is to say, the fold of skin 

 which hangs down like a flap on each side of the bivalve, 

 lining and making the shell. Growth takes place by the 

 deposition of layer after layer around the yellowish centre. 

 The coalescence of several pearl sacs may give rise to 

 curious compound pearls. What are called ' shell-pearls ' 

 begin in the mantle and become secondarily attached to 

 the shell ; they are to be distinguished from shell-concre- 

 tions which are formed around intruded bodies, and do not 

 show any concentric layering. According to Rubbell, the 

 innermost, or mother-of-pearl layer of the shell, is divided, 

 at certain places at least, into an inner and an outer stratum 

 by a clear intermediate layer, which is also seen inside 

 the pearls. 



It appears, then, that there are pearls and pearls. 

 Keeping to those which are formed in ' pearl-sacs ' of the 



