Foraminifera, also known as forams, may be free-floating, planktonic species, but the vast majority are benthic (bottom-dwellers). The shells, or tests, of the planktonic and most benthic species are composed of calcite, which is the same mineral that comprises bivalve shells. Others construct shells composed of organic material, sediment particles, or mineral secretions. In order that there may be room for additional growth, the tests of most forams are chambered. However, some species exhibit alternate designs, such as open tubes or hollow spheres.