rocky seashore can be most unforgiving habitat,as waves pound unyielding stone.many intertidal creatures have responded by evolving hard outer shells,which also protect them from predator and the sun's drying heat.molluscs such as limpets have low,volcano-shaped shells that present little resistance to waves,while the periwinkle's shell is thick,tough,and rounded,so that if detached it is soon rolled to rest in a gulley.another aid to survival is a good grip.starfish and sea-urchains have hundreds of tiny tube feet ,while limpets and sea-snails have a single large suction foot