126 



COLOURS OF SHELLS. 



madrepore. When the mollusca rarely move from the 

 place of their abode, they are of the same colour as 

 the sites they occupy, or the parti- coloured stones or 

 sea-weeds to which they cling. 



Many shells exhibit, on several parts of their inner 

 surface, a glistening or silvery appearance. It is caused 

 by the peculiar thinness, transparency, and regular ar- 

 rangement of the outer layers of the membrane, which, 

 in conjunction with the particles of lime, enter into the 

 formation of that part of the surface of the shell. This 

 has been dignified by the name of " mother-of-pearl," 

 it being supposed to be the material of which pearls are 



formed. It is true, indeed, that pearls are actually com- 

 posed of the same materials, and have the same kind of 

 structure ; but it has been proved that these bright 

 colours are the effect of the parallel grooves that arise 



