16 THE LIFE OF THE MOLLUSCA 



ventral side of the body. The radula is of the 

 many-toothed order or wanting. 



Sub-Order 2 : Ch.£:todermatina, represented by 

 a single genus Chcetoderma (Plate IX., Fig. 6), in 

 which the body is extremely long and cylindrical; 

 the foot is wanting, its position being merely indi- 

 cated by a groove ; whilst the radula is represented 

 by a single tooth. 



Class II. : The GASTROPODA (Plate II.), typi- 

 fied by the Snail and Whelk. 



In these there is a distinct head, furnished with 

 one or two pairs of contractile sense organs, the 

 "tentacles," or "horns," the form of which varies 

 greatly in different groups. The two eyes are 

 situated on the tentacles (the hinder pair when four 

 are present), and are generally carried on tubercles 

 near the base, but they sometimes appear halfway 

 up, while in the Common Snail and his kindred they 

 are, as well known, borne aloft on the summits of 

 the "horns." 



The ventral foot forms a creeping disc, and sur- 

 mounting it the visceral organs within the covering 

 mantle are borne as a twisted hump, generally 

 covered by a shell formed in a single piece. This 

 shell is attached to the body by a powerful muscle, 

 which serves to withdraw the animal into its house. 



The torsion of the body is brought about by a 

 double process (Plate IV., Fig. i, A-D). Firstly, as 



