THE SHELL. 17 



the common river mussels {Blargaritana margaritifera). They 

 are also louiid occasionally in the common oyster, in Anodonta, 

 Unio, Pinna nobilis, Mytilus edidis, or common mussel, and in 

 S2:>ondylus gaederopus. In these they are generally of a green or 

 rose color. The pearls found in Area Nose are violet, and in 

 Anornia cepa purple. They are similar in structure to the shell, 

 and, like it, consist of three layers ; but what is the innermost 

 layer in the shell is placed on the outside in the pearl. The 

 iridescence is due to light falling upon the out-cropping edges of 

 partially transparent corrugated plates. The thinner and more 

 transparent the plates the more beautiful is the iridescent lustre; 

 and this is said to be the reason why sea pearls excel those obtained 

 from fresh-water mollusks. Besides the furrows formed by the 

 corrugated surface there are a number of fine dark lines (y yVo i'^ch 

 apart), which may add to the lustrous effect. In some pearls 

 these lines run from pole to pole like the longitudes on the globe ; 

 in others they run in various directions ; and in a few the lines 

 on the same pearl have different directions, so that they cross 

 each other. The nucleus frequently consists of a fragment of a 

 brownish-yellow organic substance, which behaves in the same 

 way as epidermis when treated with certain chemical reagents. 

 Sand is generally said to be the nucleus ; but this is simply a 

 conjecture which has gradually become regarded as a fact ; it is 

 quite the exception for sand to be the nucleus ; as a general rule 

 it is some organic substance. In some districts one kind of 

 nucleus seems to be more common than another ; at least, this 

 is how the different results obtained by observers in different 

 localities may be explained. FiVipin {SuP origine delle Perle, 

 translated in Miiller's Archiv, 1856) found the parasitic helminth 

 Distoma to be the nucleus in many Anodontas ; Kuchenmeister 

 found that the pearls were most abundant in the mollusks living 

 in the still parts of the River Eister, where the water-mites 

 {Limnochares anodontse) existed most numerously. The most 

 generall}^ prevalent nucleus appears to be the bodies or eggs of 

 minute internal parasites, such as Filaria, Distoma, Buchephalus, 

 etc. Completely spherical pearls can only be formed loose in 

 the muscles or other soft parts of the animal. The Chinese 

 obtain them artificially by introducing into the living mussel 

 foreign substances, such as pieces of mother-of-pearl fixed to 

 wires, which thus become coated with a more brilliant material. 

 The relief figures of idols in pearl which ornament the interior 

 of Chinese species of Dipsas (i, 10) are a deposit of nacreous 

 material made upon metal forms introduced between the shell and 

 mantle; they are produced in about a year. The manufacture of 

 pearls is a large industry in China ; the process was discovered 

 by a native of Hutchefu in the thirteenth century. European 



