86 



Invertebrate. 



FIG. 51. 



Tooth ribbon or radula of the Whelk or 

 Hucclnum ; a, c, 

 . medial teeth. 



the front teeth are worn the ribbon grows forward. 



This band can be easily found in the common limpet 



or whelk where it 

 exceeds an inch in 

 length. 



In all but the 

 little elephant's tooth 

 shell, the mantle 

 lobes do not entirely 

 include the body, and 

 the shell consists only 



, lateral teeth of one row ; of One ValvC. It 



varies in shape, some- 

 times being conical as in the limpet, but usually it is 

 spirally coiled, the curvature being due to the mode 

 of growth, as one side of the animal grows rapidly, 

 the other slowly, or not at all ; hence the body becomes 

 coiled towards the aborted side, and the gills and 

 other organs are generally developed only on one side. 

 In most coiled shells, curvature is towards the left 

 side, throwing the mouth round to the right side ; in 

 a few rare cases, or as an anomaly of growth, the coil 

 may be reversed, winding to the right, and with the 

 mouth at the left side. The bodies of these molluscs 

 usually project, but they can be retracted into their 

 shells. Progression takes place with a gliding motion, 

 produced by the undulatory movement of the under 

 side of the foot, as may be seen by placing a snail on 

 the outside of a window pane, and watching it from 

 within. The foot sometimes bears at its hinder part 

 a little shelly lid which, when the animal is retracted 

 into the shell, acts as a door to shut up the cavity ; 



