Gastropoda. 129 



lum, and grows by concentric rings to keep pace with the 

 continually widening aperture. 



The Lingual Ribbon. In the floor of the mouth is a rib- 

 bon-shaped membrane bearing on its upper surface many 

 rows of fine, sharp teeth. This ribbon passes over a pad of 

 cartilage, being pulled forth and back by muscles. It acts 

 like a rasp, wearing away the surfaces to which it is applied. 

 As it is worn away in front, it is pushed forward by a new 

 growth behind. In addition to the lingual ribbon, many 



FIG. 8 1. FIG. 82. FIG. 83. 



THREE SPECIES OF POND SNAILS. 



In Figs. 82 and 83 the aperture is closed by an operculum. 



mollusks of this group have also one or more jaw plates 

 in the mouth, against which the ribbon works. 



The Foot. The foot in the gastropods is broad and flat. 

 Resting upon this wide foot, the animal creeps or glides, 

 leaving behind a slimy trail of mucus, which is abundantly 

 secreted. The foot is symmetrical, and the anterior end is 

 more or less distinctly marked off as the head. 



The Digestive System. The mouth opens on the front 

 or under surface of the head. Watch a snail in an aqua- 

 rium to see how the mouth works. From the mouth ex- 

 tends a short gullet, sometimes dilated into a crop, to the 



