TONGUES OF GASTEEOPODS. 



609 



Fig 



viously to its passing into the (Esophagus ; for in these animals 

 we Unci 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 Buccinwni and 

 its allies is used hy these animals as a file, with which they hore 

 holes through the shells of the Mollusks that serve as their prey ; 

 this they are enabled to effect by everting that part of the pro- 

 boscis-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-away, 

 somewhat as the front Teeth of the Kodents are constantly being 

 regenerated from the surface of the pulps which occupy their 

 hollow conical bases, as fast as they are rubbed-down at their 

 edges. 



474. The preparation of these Tongues for the Microscope can, 

 of course, be only accomplished by 

 carefully dissecting them from 

 their attachments within the head; 

 and it will be also necessary to 

 remove the membrane that forms 

 the sheath of the tube, when this 

 is thick enough to interfere with 

 its transparency. 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 Micro- 

 scopic 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 beautiful objects for the Polariscope when they are 

 mounted in Canada Balsam ; the form and arrangement of the 



R R 



Palate of Buccinum undatum, as 

 seen under Polarized Light. 



