52 EVENINGS AT THE MICEOSCOPE. 



tinctly see the light reflected from the surface, and 

 the consciousness of this infinitely perceptible reflection 

 produces 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-mol- 

 lusks." t 



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 sus- 

 pect that he carries coiled up in his throat a tongue 

 twice as long as his shell ? And that this tongue is 

 armed with thousands of crystal teeth, all arranged 

 with the most consummate art in a pattern of perfect 

 regularity ? It sounds almost like a fable to be told 

 that the great Spotted Slug, which we sometimes find 

 crawling in damp cellars, carries a tongue armed with 

 26,800 teeth ! Yet there is no doubt of the fact. 



Ton see on this slip of glass a very slender band 

 about two inches in length. This is the tongue of the 

 common Periwinkle. While in the living animal, its 

 fore-part occupied the floor of the mouth, whence it 

 passed down below the throat, and turning towards 

 the right side, formed a close spire of many whorls, 

 exactly like a coil of rope, which rested on the gullet. 

 Here we have it extracted, uncoiled, cleansed, and 

 affixed to a slip of glass for microscopical examination. 



Only a small portion of the ribbon is visible at a 



time with such a power as is necessary to display the 



structure, but by means of the stage-movement we can 



bring the whole in succession under the eye, and dis- 



* Dove. Farbenlehre, 117. f Ann. & Mag. N. H. ; Feb. 1858. 



