520 MOLLUSCOUS ANIMALS GENERALLY. 



of the best guides to the natural affinities of the species, genera, 

 and families of this group, since any important alteration in the 

 form or position of the teeth must be accompanied by some 

 corresponding peculiarity in the habits and manners of the ani- 

 mal.^ Hence a systematic examination and delineation of the 

 structure and arrangement of these organs, by the aid of the 

 Microscope and Camera Lucida, would be of the greatest service 

 to this department of Natural History. The short thick tube 

 of the Limax and other terrestrial Gasteropods, appears adapted 

 for the trituration of the food previously to its passing into the 

 oesophagus ; for in these animals we find the roof of the mouth 

 furnished with a large strong horny plate, against which the flat 

 end of the tongue can work. On the other hand, the flattened 

 portion of the tongue of Buocinum and its allies is used by these 

 animals as a file, with which they bore holes tlu-ough the shells 

 of the moUusks that serve as their prey ; this they are enabled 

 to elFect, by everting that part of the proboscis-shaped mouth 

 whose floor is formed by the flattened part of the tongue, which 

 is thus brought to the exterior, and by giving a kind of sawing 

 motion to the organ, by means of the alternate action of two 

 pairs of muscles, — a protractor, and a retractor, — which put 

 forth and draw back a pair of cartilages whereon the tongue is 

 supported, and also elevate and depress its teeth.^ Of the use of 

 the long blind tubular part of the tongue in these Gasteropods, 

 however, scarcely any probable guess can be made ; unless it be 

 a sort of "cavity of reserve," from which a new toothed surface 

 may be continually supplied, as the old one is worn awa_y, some- 

 what as the front teeth of the Rodeuts are constantly being regene- 

 rated from the surface of the pulps which occupy their hollow 

 conical bases, as fast as they are rubbed down at their edges. 



347. The preparation of these tongues for the Microscope, 

 can, of course, be only accomplished bj' carefully dissecting 

 them from their attachments within the head ; and it Avill be 

 also necessary to remove the membrane that forms the sheath of 

 the tube, when this is thick enough to interfere with its transpa- 

 rency. The tube itself should be slit up with a pair of fine 

 scissors, through its entire length ; and should be so opened out, 

 that its expanded surface may be a continuation of that which 

 forms the floor of the mouth. The mode of mounting it will 

 depend upon the manner in which it is to be viewed. For the 

 ordinary purposes of Microscopic examination, no method is so 

 good as mounting in fluid; either weak spirit or Goadby's 

 solution answering very well. But many of these tongues, 

 especially those of the marine Gasteropods, become most beauti- 



I "Annals of Natural History,'' Ser. 2, vol. x, p. 413. 



^ For additional details on the organisation of the tongue and teeth of the Gasteropod 

 MoUusks, see Mr. W. Thomson, in "Cyclop, of Anat. and Physiol." vol. iv, pp. 1142, 

 ] 143 ; and in " Ann, of Nat. Hist." Ser. 2, vol. vii, p. 86. 



