Dinoflagellates have silaceous walls - that is silicon (sand) walls. Most other organisms have carbonaceous cell walls. Silicon has a harder time forming chain compounds than carbon (also a group 4 element), and cannot form the variety of structures available to carbonaceous compounds. Silicaceous structures are more stable, however, and make stronger walls. Coral reefs are formed from the silica skeletons of the coral algae that live and die there; their remains building up into towerblocks and intricate cityscapes.