CHAPTER V1I.—GasrerRopops. 
The univalve or Gasteropod mollusks are much more active 
and intelligent than the bivalves. They have a head, with 
eyes and tentacles or feelers. The mouth is at the end ofa 
proboscis. The 
foot is flat and 
constructed for 
crawling. There 
is often a siphon 
to conduct water 
Fig. 32. A Gasteropod, Nassa trivittata. 
to the gills. The aperture of the shell is closed when the 
animal is retracted, by a small plate, the operculum. 
The name Gasteropod means stomach-foot, in allusion to the 
fact that the entire lower surface of the body is thickened and 
flattened to form a broad muscular disk for crawling. This 
disk or “foot” is marked f. in the figure. Upon the back 
part of the foot, the operculum, 0, is carried. The proboscis 
or rostrum is marked r in the figure. On each side of it, the 
tentacles, ¢, are situated, near the bases of which are the eyes. 
The tube marked s in the figure, conducts water to the gill 
cavity which is in the last whorl of the shell. When the ani- 
mal retreats into the shell the water is squirted from the gill 
cavity to make room for the foot. 
The Gasteropods may conveniently be divided into two 
classes, according to the shape of the aperture. In the her- 
bivorous species the base of the aperture is rounded; in the 
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