MOLLUSCA. 



535 



in fig. 249 ; and all transverse sections of pearls from 

 Oysters sIioav the same successive rings of growth or 

 deposit. 



In a segment of a transverse section of a small purple 

 pearl from a species of Mytilus (fig. 250), all trace of 

 prismatic structure has disappeared, and only a series of 

 fine curved or radiating lines is seen. This pearl consists 

 of a beautiM purple-coloured series of concentric lamina? ; 

 many of which have a series of concentric zones, and are 

 of a •yellow tint. The most beautiful sections for micro- 

 scopic examination are obtained from Scotch pearls. 



£rachio2>oda, " Lamp-shells,' ' or, as the name literally 



Fus, 250. 

 •1, A. transverse section of a small Pearl-- from a species of Mytilus. 2, Hori- 

 zontal section of same Pearl magnified 260 diameters, to show prismatic 

 structure and transverse striae. 



signifies, arm-footed, is intended to express a most re- 

 markable characteristic of these animals, the presence of a 

 pair of arms, often of great length, rolled up in a spiral 

 form, and believed by Cuvier to replace the foot in other 

 bivalves. Professor Owen has shown that these organs 

 are tubes closed at each end, and contain a fluid, which by 



