MOLLUSCOUS ANIMALS GENERALLY. 



181 



other instances one plate carries several teeth. Of the former arrange- 

 ment we have an example in the palate of many terrestrial Gasteropods, 

 such as the snail (Helix) and Slug (Limax), in which the number of 

 plates in each row is very considerable (Figs. 397, 398), amounting to 180 

 in the large garden Slug (Limax maximus); whilst the latter prevails in 

 many marine Gasteropods, such as the common Whelk (Buccinum unda- 

 tum), 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. 



FIG. 397. 



FIG. 398. 



Portion of the left half of the Palate of 

 Helix hortensis ,' the rows of teeth near 

 the edge separated from each other to show 

 their form. 



Palate of Zonites cellaring 



401). The length of the palatal tube, and the number of rows of teeth, 

 vary greatly in different species. Generally speaking, the tube 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 



FIG. 399. 



FIG 400 



Palate of Trochus zizyphinus. 



Palate of Doris tuberculata. 



some species as many as 160 or 170; so that the total numer 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. 398, whilst the longitudinal rows are quite straight: and 



