It is hypothesized that difference between containers and between sediments will produce structural differences in the resulting communities by intensifying competition for resources, and by excluding certain classes of predators. It is also hypothesized that the level of the effect of container, substrate, and time on community structures will be dependent on tidal level. Paine (1969) and Dayton (1972) have demonstrated that the structure of some communities is largely determined by the effects of foundation, or keystone, species. It is suggested that the structure of classic-substrate communities may be determined similarly.