62 EVENINGS AT THE MICROSCOPE 



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. 



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 in the living animal, its fore-part oc- 

 cupied the floor of the mouth, whence it passed down be- 

 low the throat, and turning toward 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 mi- 

 croscopical 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 discover that, with 

 some modifications of form, the same essential plan of 

 structure, and even the same elements, exist throughout. 

 Concentrating our attention on a single transverse series 

 of the numerous curved lines that at first sight "bewilder 

 the mind, we perceive by delicate focusing that the ob- 

 ject before us consists of a number of hooks projecting 

 from the surface of the translucent ribbon, and arching 

 downward. In this case a single row consists of seven 

 such hooked plates or teeth; one in the centre and three 

 on each side. Each hooked plate has its arching tip cut 

 into five toothlets, of which the central one is the largest; 

 and its base is united with the cartilaginous substance of 

 the ribbon. Only the middle plate is symmetrical; the 



