930 



MOLLUSCA AND BEACHIOPODA 



it is connected above and below, and the sinuosity of the thin 

 intervening laminae, 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 open 

 space between the parallel plates that intervenes among the sinu- 

 osities 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 ; these may be mounted on a black ground as 

 opaque objects, or in Canada balsam as transparent objects, under 

 which last aspect they furnish very beautiful objects for the polari- 

 scope. 



Palate of Cephalophorous Molluscs. The organ which is 

 sometimes referred to under this designation, and sometimes 



as the ' tongue,' is one of a 

 very singular nature, and 

 cannot be likened to either 

 the tongue or the palate of 

 higher animals ; it is best to 

 call it by its distinctive name 

 ' odontophore.' For it is a 

 tube that passes backwards 

 and downwards beneath the 

 mouth, closed at its hinder 

 end, whilst in front it opens 

 obliquely upon the floor of 



the mouth, being (as it were) 

 FiG.705.-Portionofthelefth a lfofthepaJate t go as 



of Helix liorteit.'iis, the rows of teeth near " L ,. 



the edge separated from each other to show to form a nearly flat surtace. 



their form. Q n the interior of the tube, 



as well as on the flat expan- 



sion of it, we find numerous transverse rows of minute teeth, which 

 are set upon flattened plates, each principal tooth sometimes 

 1 uwing a basal plate of its own, whilst in other instances one plate 

 carries several teeth. Of the former arrangement we have an 

 example in the palate of many terrestrial (Jastropods. such as the 

 snail (HeUx) and slug (L/nii/.>-). in which the number of plates in 

 each row is very considerable (figs. 70-1, 706), amounting to 180 

 in the large garden slug (Limn.'- HHLI-UHHS) ; whilst the latter prevails 

 in many marine Gastropods, such as the common whelk (lim-cin mn 

 itiiiliitu in), the palate of which has only three plates in each row, one 

 bearing the small central teeth, and the two others the large lateral 

 teeth (fig. 709). The length of the palatal tube and tin- number of 

 rows of teeth vary greatly in different species. ( Jem-rally speaking, 

 the tube of the terrestrial < lastropods is short, and is contained 

 entirely within the nearly globular head; but the rows of teeth 

 being closely set together are usually very numerous, there being 

 frequently more than 100, and in some species as many as lu'Oor 

 170; so that the total number of teeth may mount up. as in //<//. > 

 pomatia, to 21,000, and in L'm>.<- inn,<-linnx to _'(>. 800. The trans- 



