Tremendous anatomical diversity exists throughout the nonarticulated Corallinaceae, while at the same time the hardness conferred by the copious calcite in the cell walls and certain other structural features serve to unify the group. All coralline algae are filamentous and, in almost all instances, the filaments are aggregated into multiaxial thalli. Although some thin coralline crusts are made up solely of hypothallia and epithallia, most crusts also have perithallia made up of filaments initiated by dividing hypothallial cells. In fact, ribbon corallines are more or less flexible and appear to have evolved a means of projecting their conceptacles higher into the water, as have the articulated coralline algae. In the structural context marl are irregularly shaped, coralline algae usually made up of several protuberances projecting from a common center. Rhodoliths are coralline nodules that are usually larger than marl and that have grown by layers around a center, or nucleus.