182 THE MICROSCOPE AND ITS REVELA.TION8. 



the curvature takes its departure on each side from a central longitudi- 

 nal 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. 



577. The palatal tube 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 tube to lie 

 f olded-up, but perfectly free, in the abdominal cavity, between the in- 

 testine 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 pro- 

 portion of cases, these palates exhibit a very marked separati6n between 

 the central and the lateral portions (Figs. 399, 401); the teeth of the 

 central band being frequently small and smooth at their edges, whilst 

 those of the lateral are large and serrated. The palate of Trochus zizy- 

 phinus, represented in Fig. 399, is one of the most beautiful examples 

 of this form; not only the large teeth of the lateral bands, but the deli- 

 cate leaf -like teeth of the central portion, haying their edges minutely 

 serrated. A yet more complex type, however, is found in the palate of 

 Haliotis ; in which there is a central band of teeth having nearly straight 

 edges instead of points: then, on each side, a lateral band consisting of 

 large teeth shaped like those of the Shark; and beyond this, again, 

 another lateral band on either side, composed of several rows of smaller 

 teeth. Very curious differences also present themselves among the dif- 

 ferent species of the same genus. Thus in Doris pilosa, the central band 

 is almost entirely wanting, and each lateral band is formed of a single row 

 of very large hooked teeth, set obliquely like those of the lateral band in 

 Fig. 399; whilst in Doris tuber culata, the central band is the part most 

 developed, and contains a number of rows of conical teeth, standing 

 almost perpendicularly, like those of a harrow (Fig. 400). 



578. Many other varieties might be described, did space permit; but 

 we must be content with adding, that the form and arrangement of the 

 teeth of these ' palates' afford characters of great value in classification, 

 as was first pointed out by Prof. Loven (of Stockholm) in 1847, and has 

 been since very strongly urged by Dr. J. E. Gray, who considers that 

 the structure of these organs is one of the best guides to the natural 

 affinities of the species, genera, and families of this group, since any im- 

 portant alteration in the form or position of the teeth must be accom- 

 panied by some corresponding peculiarity in the habits and food of the 

 animal. 1 Hence a systematic examination and delineation of the struc- 

 ture and arrangement of these organs, by the aid of the Microscope and 

 Camera Lucida, would be of the greatest service to this department of 

 Natural History. The short thick tube of Limax and other terrestrial 

 Gasteropods, appears adapted for the trituration of the food previously 

 to its passing into the oesophagus; for in these animals we find 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 palate of Buccinum (whelk) and its allies is used by these 

 animals as a file, with which they bore holes through the shells of the 



1 " Annals of Natural History," Ser. 2, Vol. x. (1852), p. 413. 



