518 



MOLLUSCOUS ANIMALS GENERALLY. 



rows of minute teeth, which are set upon flattened plates ; each 

 principal tooth sometimes having a basal plate of its own, whilst 

 in other instances one plate carries several teeth. Of the for- 

 mer arrangement we have an example in the tongue of many 



FIG. 265. 



FIG. 266. 



Fig. 205. Portion of the left half of the Palate of the Helix hortensis; the rows of teeth near the 

 edge separated from each other to show their form. 

 Fig. 266. Palate of Zonites cellarius. 



terrestrial Gasteropods, such as the Snail (Helix) and Slug (Li- 

 max), in which the number of plates in each row is very con- 

 siderable (Figs. 265, 266), amounting to 180 in the large garden 

 Slug (Limax maximus) ; whilst the latter prevails in many marine 

 Gasteropods, such as the common Whelk (Buccinum undatum\ 

 the tongue of which has only three plates in each row, one bear- 

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

 teeth (Fig. 269). The length of the tongue, and the number of 

 rows of teeth, vary greatly in different species. Generally speak- 

 ing, 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, they 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 Limax 

 maximus, to 26,800. The transverse rows are usually more or 

 less curved, as shown in Fig. 266, whilst the longitudinal rows 

 are quite straight; and the curvature takes its departure on each 

 side from a central longitudinal row, the teeth of which are 

 symmetrical, whilst those of the lateral portions of each trans- 

 verse row present a modification of that symmetry, the pro- 

 minences 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. 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 



