PHYTIYOROUS MOLLUSCA. 327 



brought to press against a mammillary eminence on the floor 

 of the mouth ; it is of a semilunar shape, hard and corneous, 

 and either serrated on the cutting edge, or armed with a 

 single obtuse knob in the centre. The tongue is a mem- 

 brane roughened with minute prickles, set in the most re- 

 gular array, either in close transverse lines, or on the angles 

 of a network of the most minute delicacy. These prickles, 

 by pointing backwards, prevent any regurgitation of the 

 food ; and, as they are capable of being raised and depressed 

 at pleasure, they must tear and rasp the vegetable fibre into 

 shreds, and prepare it for an easier digestion. The shape of 

 the tongue, and the pattern in which the prickles are set, are 

 very variable ; and I know few objects which are more in- 

 teresting to the microscopical observer. It is always, as 

 Swammerdam remarked of that of Paludina vivipara, " so 

 elegantly formed, that it can scarce be exactly described, 

 and as difficultly be represented in a figure ;"* for, indeed, 

 the figures which have been given of it convey not the 

 slightest idea of the extreme beauty and delicate organisa- 

 tion of this wonderful organ ; nor am I able to supply this 

 deficiency. It is sometimes broader than long, as in *Tri- 

 tonia and Doris ; at other times elliptical or spoon-shaped, 

 as in the snails ; and in others it is lengthened out in the 

 most extraordinary manner, so as not merely to equal, but 

 greatly to exceed the length of the body ! ) In the latter 

 cases it lies reversed along the gullet, and reaches the in- 

 terior of the stomach, where it is convoluted or twisted into 

 spiral bendings, like a serpent closely rolled together. The 

 Periwinkles (Littorina) and Limpets afford familiar examples 

 of this remarkable modification ; and I enclose a figure of it 



of the snail's tooth, p. 180. tab. 25, fig. 1. See also Swammerdam, Book of 

 Nature, p. 49 ; Lister, Exer. Anat. de Cochleis, tab. 2, fig. 2, and tab. 3, fig. 

 9 j List., Conch. Anat. tab. 1, fig. iv., and tab. 4, fig. ii. ; Cuvier, Mem. 

 pi. ii. fig. 4. 



* Book of Nature, p. 79. Cuvier asserts that the tongue of the slugs 

 and snails is not spinous, and that the food is introduced into the gullet by 

 a sort of peristaltic movement of the tongue and the buccal mass on which 

 it lies. Mem. xi. 17. Swammerdam says, " On the tip of the tongue of 

 the snail, there is a little horny bone, cut as it were, into two or three very 

 tender little teeth ; with which, as with a hook, the snail, when it is about to 

 eat, first lays hold of the small herb, and immediately after, suddenly 

 snatches and pulls the piece into its mouth ; afterwards it nips them pretty 

 fast with its teeth, so that the noise it makes in biting and eating may be 

 sometimes heard very distinctly." p. 49. 



f For a rude and unfinished figure of the tongue of Tritonia, see Cuvier's 

 Mem. tab. 2, fig. 8, 9 ; of Aplysia, tab. 2, fig. 6 b ; of Paludina, tab. 1, fig. 

 8, 9 ; of Patella, tab. 2, fig. 18, 19. The figures of the tongue of the Nudi- 

 branches, given by Alder and Hancock in their Monograph, are the best of 

 any hitherto published. 



