GASTEROPODS 335 
In some of the species it is amazingly large and powerful, as in 
Polynices (Natica), and in most of the forms which live along 
sandy beaches. These are sometimes provided in front with a 
wedge-shaped process called the propodium, which serves admir- 
‘ ip nN, 
sap 
Sigaretus levigatus, showing excessive development of the propodium (pr.) and metapodium 
(met.) in a mollusk living in sand (the shell, which covers only the liver and adjacent parts, has been 
removed) : J, liver; s. ap., aperture of proboscis, here deflected from the median line; ¢, t, tentacles ; 
JS, foot. 
ably as a plow to push aside the heavy wet sand through which 
the animal forces its way. In Nassa, which is so common all 
along our coasts, the foot has two terminal appendages or 
points behind. 
fe 
Oliva textilina showing how the front part of the foot (jf) is devel- 
oped into a sort of fender, the propodium (pr.): e, e, eyes; m. ap., front ap- 
pendage of mantle; m. ap.’, hinder appendage of mantle, folded into the 
suture when the animal is at rest; si., siphon; ¢, ¢, tentacles. 
There is no single feature of mollusks more important than the 
foot, for upon the many modifications of this organ the various 
molluscan classes are founded. The name Gasteropoda means 
“stomach-foot,” the latter organ being merely a thickening of 
