TONGUES OF GASTEROPODS. 



607 



many Marine Gasteropods, such as the common Whelk (Buccinum 

 undatum), the tongue of which has only three plates in each row, 

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

 lateral teeth (Fig. 321). The length of the Tongue, and the num- 

 ber of rows of Teeth, vary greatly in different species. Generally 

 speaking, the Tongue of the Terrestrial Gasteropods 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 160 or 170 ; so that the total number of Teeth may mount-up, 

 as in Helix pomatia, to 21,000, and in Limaxmaximus, to 26,800. 

 The transverse rows are usually more or less curved, as shown in 

 Fig. 318, whilst the longitudinal rows are quite straight ; and the 

 curvature takes its departure on each side from a central longitu- 

 dinal row; the Teeth of which are symmetrical, whilst those of the 

 lateral portions of each transverse row present a modification of 

 that symmetry, the prominences on the inner side of each tooth 

 being suppressed, whilst those on the outer side are increased ; this 

 modification being observed to augment in degree, as we pass from 

 the central line towards the edges. 



472. The Tongue of the Marine Gasteropods is generally longer, 

 and its teeth larger ; and in many instances it extends far beyond 

 the head, which may, indeed, contain but a small part of it. Thus 

 in the common Limpet {Patella), we find the principal part of the 

 Tongue to lie folded 

 up, but perfectly free, 

 in the Abdominal 

 cavity, between the 

 intestines and the 

 muscular foot ; and 

 in some species its 

 length is twice or 

 even three times as 

 great as that of the 

 entire animal. In a 

 large proportion of 

 cases, these Tongues 

 exhibit a very marked 

 separation between 

 the central and the 

 lateral portions (Figs. 

 319, 321) ; the Teeth 

 of the central band 

 being frequently 

 small and smooth at 

 their edges, whilst 

 those of the lateral 

 are large and ser- 



Fig. 319. 



Tongue of Trochus zizyphinus. 



