104 THE LIFE OF THE MOLLUSCA 
their kindred. Most of the members of these groups 
are furnished with thick opercula, which are not 
withdrawn fai within the mouth of the shell.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. 
