Back    
Sea star clinging to an undercut rock wall

This vertical and undercut rock surface is a perfect example of how abiotic factors, slope + gravity, would act to select for organisms that can securely cling to such a challenging surface. Snails and sea stars are two groups of animals capable of moving across just such a surface, each group accomplishing it uniquely, sea stars with tube feet, and snails with a large, muscular, mucous-covered foot.