46 EVENINGS AT THE MICROSCOPE. 



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 

 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." f 



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 

 carries coiled up in his throat a tongue twice as long 

 as his shell ? And that this tongue is armed with thou- 

 sands of crystal teeth, all arranged with the most consum- 

 mate 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. 



You see on this slip of glass a very slender band about 

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

 Periwinkle. While it was 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 ex- 

 tracted, uncoiled, cleansed, and affixed to a slip of glass 

 for microscopical examination. 



Only a small portion of the band 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 discover that, with 

 some modifications of form, the same essential plan of 



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



