TONGUES OF GASTBEOPODS. 



619 



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 tongues exhibit a very marked separation 

 between the central and the lateral portions (Figs. 267, 269) ; the 

 teeth of the central band being frequently small and smooth at 

 their edges, whilst those of the lateral are large and serrated. 

 The tongue of Trochus zizyphinus, represented in Fig. 267, 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 tongue 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 con- 

 sisting 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 bands 

 in Fig. 267 ; whilst in Boris tuhereulata, the central band is the 



Fig. 267. 



Fio. 268. 



Palate of Trochus zizyphinus. 



Palale of Doris iyhercuVxta. 



part most developed, and contains a number of rows of conical 

 teeth, standing almost pei-pendicularly, like those of a harrow 

 (Fig. 268). 



346. Many other varieties might be described, did space per- 

 mit ; but we must be content with adding, that the form and 

 arrangement of the teeth afford characters of great, value in 

 classification, as was first pointed out by Prof Loven (of Stock- 

 holm) in 1847, and has been since very strongly urged by Dr. 

 J. E. Gray, who considers that the structure of the tongue is one 



