TONGUES OF GASTEROPODS. 517 



the thickness of the plates ; and these intervals are in great part 

 filled up by what ■ appear to be fibres or slender pillars, passing 

 from one plate or floor to another. A more careful examination 

 shows, however, that instead of a large number of detached 

 pillars, there exists a comparatively small number of very thin 

 sinuous laminae, which pass from one surface to the other, wind- 

 ing and doubling upon themselves, so that each lamina occupies 

 a considerable space. Their precise arrangement is best seen by 

 examining the parallel plates, after the sinuous laminae have 

 been detached fi'om them ; the lines of junction being distinctly 

 indicated upon these. By this arrangement, each layer is most 

 efiectually supported by those with which it is connected above 

 and below ; and the sinuosity of the thin intervening laminse, 

 answering exactly the same purpose as the " corrugation" given 

 to iron plates for the sake of diminishing their flexibility, adds 

 greatly to the strength of this curious texture ; which is at the same 

 time lightened by the large amount of space between the paral- 

 lel plates that intervenes between the sinuosities of the laminae. 

 The best method of examining this structure, is to make sections 

 of it with a sharp knife in various directions, taking care that 

 the sections are no thicker than is requisite for holding together; 

 and these may be mounted on a black ground as opaque objects, 

 or in Canada balsam as transparent objects. 



344. The structure of Shells generally is best examined by 

 making sections in different planes, as nearly parallel as may be 

 possible to the surfaces of the shell ; and other sections at right 

 angles to these ; the former may be designated as horizontal, the 

 latter as vertical. ISTothing need here be added to the full direc- 

 tions for making such sections, which have already been given 

 (§§ 108-110). Much valuable information may also be derived, 

 however, from the examination of the surfaces presented by 

 fracture. The membranous residua left after the decalcification 

 of the shell by dilute acid, may be mounted in weak spirit or in 

 Goadby's solution.' 



345. Tongue of Gfasteropod MoUusks. — The organ which is 

 commonly known under this designation, is one of a very singu- 

 lar nature ; and we should be altogether wrong in conceiving of 

 it as having any likeness to that on which our ordinary ideas of 

 such an organ are founded. Por instead of being a projecting 

 body, lying in the cavity of the mouth, it is a tube that passes 

 backwards and downwards beneath the mouth ; its hinder end 

 being closed, whilst in front it opens obliquely upon the fioor ot 

 the mouth, being (as it were) slit up and spread out, so as to 

 form a nearly flat surface. On the interior of the tube, as 

 well as on the flat expansion of it, we find numerous transverse 



' For fuller details on the minute structure of tlie shells of the Mollusca, see the 

 Author's memoirs on that subject in the "Reports of the British Association" for 1844 

 and 1847: also Mr. Bowerbank's memoir on the same subject in " Transact, of Micro- 

 scopical Society." Ser. 1, vol. i; and Mr. Queitett's "Lectures on Histology," voL ii, 

 Chaps, xvi-xxii. 



