16 THE LIFE OF THE MOLLUSCA 
ventral side of the body. The radula is of the 
many-toothed order or wanting. 
Sub-Order 2: CHZTODERMATINA, represented by 
a single genus Chatoderma (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. 
Crass 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. 1, A-D). Firstly, as 
