PALATES OF GASTKOPODA 



931 



FIG. 706. Palate of Hyalinia cellaria. 



verse rows are usually more or less curved, as shown in fig. 706, 

 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 transverse row present 

 a modification of that symmetry, 

 the prominences on the inner 

 side of each tooth being sup- 

 pressed, whilst those on the outer 

 side are increased ; this modifica- 

 tion may be observed to augment 

 in degree as we pass from the* 

 central line towards the edges. 



The palatal tube of the 

 marine Gastropods 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 a common limpet (Patella) we find the principal 

 part of the tube to lie folded up, but perfectly free, in the abdominal 

 cavity, between the greatly elongated intestine 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 palates exhibit a very marked separation between the central 

 and the lateral portions (figs. 

 707, 708), the teeth of the cen- 

 tral band being frequently small 

 and smooth at their edges, 

 whilst those of the lateral are 

 large and serrated. The palate 

 of Trochus zizyphimts, repre- 

 sented in fig. 707, is one of the 

 most beautiful examples of this 

 form, not only the large teeth 

 of the lateral bands, but the 

 delicate leaf-like teeth of the 

 central portion having 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 FIG. 707. Palate of Trochus zizypliinus. 

 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 different 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. 707 ; whilst in Doris tuberculata the central band is the 



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