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- 



met 



fHfiiiretiis tevigatiu, showing excessive development of the propodium (pi:) and nietapodinm 

 (met. ) in a mollusk living in sand (the shell, which covers only the liver and adjacent parts, has been 

 removed) : I, liver; s. ap., aperture of proboscis, here deflected from the median line; t, t, tentacles ; 

 /, 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. 



m.a.f' 



Oliva textilina showing how the front part of the foot (/) 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; t, t, 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 



