68 THE GASTROPODA 
tion of the summit of the spire, in Stylifer and allied parasitic 
forms (Fig. 20). 
1. The Foot.—This is primitively and normally formed by a 
powerful mass of ventral muscles with a more or less elongated 
c » 
Fic. 44. 
A, Triton variegatwm, animal and shell, seen from the right side. a, siphonal notch of the 
shell occupied by the siphonal fold of the mantle-skirt ; b, edge of the mantle-skirt resting on 
the shell; c, cephalic eye; d, cephalic tentacle ; e, proboscis or buccal introvert in a state of 
eversion ; f, foot; g, operculum; h, penis; 7, under-surface of the mantle-skirt, forming the 
roof of the sub-pallial chamber. (From Lankester, after Poli.) B, sole of the foot of Hemifusus 
tuba, to show, a, the pore of a pedal gland ; b, median line of the foot. (From Lankester, after 
Souleyet.) 
ventral creeping surface (Fig. 44, B). But this primitive condition 
may be modified in relation to different conditions of existence. 
Thus, among sedentary Gastropods the foot is reduced to a simple 
discoidal prominence in such fixed forms as Vermetus (Fig. 45) and 
Fig. 45. 
Vermetus triqueter, with broken shell; left-side view. co, columellar muscle; f, foot; m, 
mouth ; 0, aperture of the shell ; op, operculum ; ov, eggs ; pa.s, mantle slit ; p.t, pedal tentacles ; 
sh, shell. (After Lacaze-Duthiers.) 
Magilus. In Bathysciadium (Fig. 126) the ventral surface of the 
foot assumes the form of a sucker, the central portion of which is 
covered by a thick cuticle, and the circumference is ciliated. In 
the parasitic forms Stylifer and Zhyca the foot is atrophied and is 
