Diatoms have a cell wall composed primarily of silica, which forms a pillbox-like shell, known as a frustule, around the organism. The cell wall is unusually beautiful and contains intricate markings and designs. The color of the living tissue of diatoms is yellow-brown instead of the green frequently seen in other creatures that use light as a source for energy. There are an estimated 6 million diatoms per cubic foot of seawater, most of which are free-floating. Some, however, are bottom dwellers or grow on other algae and plants. Although not well understood, diatoms have the power of locomotion. Through a slit along the side of the cell, tiny microfibrils protrude and can be used to help the microscopic creatures move.