386 Voyage of tlie Novara. 



oyster is but a produce of disease in the animal, has long 

 been refuted by scientific research, and although the great 

 German poet, Henry Heine, in his " Romanzero," sings, 



" Those world-famed pearls, 

 They are but the wan mucus 

 Of a sad oyster, 

 Dimly sickening in the depth of the sea!" 



it is rather a poetic fancy than a scientific fact. We have lat- 

 terly been especially indebted to the German naturalist, Theo- 

 dore von Hessling, for a very circumstantial and thoroughly 

 exhaustive memoir on the natural history of the pearl oysters 

 and their pearls,* in which the learned author seeks to 

 establish that the enveloping matter of the germ of the pearl 

 is identical with the covering of the animal, and that in the 

 process of growth two influences are at work, an external 

 and an internal. The first is called into play by the property 

 peculiar to the hinge system that unites the double shell, of 

 gaping wide open, in consequence of which extraneous sub- 

 stances rush in with the current of water, such as minute frag- 

 ments of quartz, molecules of plants, &c., which, being detained 

 either circling in the cavity, or eddying round the hinges, are 

 seized on in the course of their revolutions, and entangled in 

 the parenchyma of the various organs, which is specially 

 secreted from the mantle, till it becomes enveloped by layers 

 of solid shell. On the other hand, the internal development 



* Die Perlen-Mnschel, und ilire Perlen, Natm-wisscnschaftHch und Gescliichtlich 

 mit Beriicksichtigung der Perlen-gewiisscr Bayerns, beschrieben von Thcodor von 

 Hessling, Leipzig, 1859. 



