MOLLUSC A: THEIR TONGUES 61 



unite to form white light; and this takes place with the 

 greater perfection in proportion as the furrows are lost, 

 and become converted into a surface of fine elevations and 

 depressions. 



"For their lustre, pearls are indebted to their being 

 composed of fine layers, which allow light to pass through 

 them, while the numerous layers, lying one under the 

 other, disperse and reflect the light in such a manner that 

 it returns and mixes with that which is directly thrown 

 back from the outer surface. It is the co-operation of 

 light reflected from the surface, with light dispersed and 

 reflected in the interior, that gives rise to lustre; for this 

 reason the knots of window-glass exhibit pearly lustre, and 

 the membranes of pearls deprived of their lime are almost 

 as lustrous as solid pearls, except that their whiteness is 

 destroyed. 'The two masses of light entering the eye act 

 upon it from different distances. Now, as it adapts itself 

 to the body seen through the transparent layer, it cannot 

 distinctly see the light reflected from the surface, and the 

 consciousness of this infinitely perceptible reflection pro- 

 duces the phenomena of lustre.' * The thinner and the 

 more transparent the layers of which the pearl consists, 

 the more beautiful is its lustre; and in this respect the 

 sea-pearls excel those of our river-mollusks. " 9 



We will pass now, by an easy transition, from the shells 

 of the Mollusca to their tongues. Who that looks at the 

 weather-worn cone of the Limpet, as he adheres sluggishly 

 to the rock between tide-levels, would suspect that he car- 

 ries coiled up in his throat a tongue twice as long as "his 

 shell? And that this tongue is armed with thousands of 



1 Dove, "Farbenlehre," 117. * "Ann. & Mag. N. H.," Feb., 1858. 



