104 THE LIFE OF THE MOLLUSCA 



their kindred. Most of the members of these groups 

 are furnished with thick opercula, which are not 

 withdrawn far within the mouth of the shell. With 

 the capacity, however, for retreating further and 

 further into the shell, and so out of the more im- 

 mediate reach of danger of violent injury, the oper- 

 culum, always an encumbrance, tends to become less 

 and less ponderous. 



Other inter-tidal forms belonging to families 

 higher in the molluscan scale have also, under the 

 necessity of facing similar conditions, developed 

 strong shells; such are Littorina, Purpura, Nassa, 

 and, among tropical genera, Pterocera, Turbinella, and 

 Strombus. The last named, indeed, is the most 

 difficult of all shells to break, resisting even the lusty 

 application of a geological hammer. 



When, however, the foreshore is quitted in favour 

 of deeper water, where no surf breaks, and where the 

 sea-bottom is composed of soft sand or silt, a pon- 

 derous shell ceases to be essential for protective 

 purposes and becomes a positive disadvantage in 

 locomotion. This drawback is further increased in 

 the case of Gastropoda that are carnivorous, as the 

 higher forms mostly are, for even the slow-moving 

 Bivalves on which they feed require greater activity 

 to seek out and capture than does a rooted plant. 

 Hence the reduction in shell and operculum shown 

 by the inhabitants of the laminarian as contrasted 

 with those of the littoral zone. 



