52 EVENINGS AT TUE MICKOSCOPE. 



tiiictlj 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 weathei'-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 lono; as his shell ? And that this tonome is 

 armed with thousands of crystal teeth, all arranged 

 with the most consummate art in a pattern of perfect 

 •egularity ? 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. 



You see on this slip of glass a very slender band 

 about two inches in lensfth. Tliis is the ton2:ue 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 

 aSixed to a slip of glass for microscopical examination. 



Only a small portion of the ribbon is visible at a 

 time ^-\t\\ such a power as is necessary to display the 

 structure, but by means of the stage-movement m'c can 

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



• Dove. Farbenlehrc, 117 + Ann. & Mag. X. H. ; Feb. 1868 



