Diatoms are unicellular, microscopic algae easily recognized by their distinctive siliceous cell walls. These protective cell walls are diverse in form and covered with intricate patterns of pores and striations that researchers use to identify and classify the many different species. Each cell has overlapping upper and lower valves held together by rings of silica called cingula or girdle bands. These siliceous components are collectively known as the frustule and are much like a petri dish or a hat box made of glass in which the cell resides.   All diatoms are classified within two morphological groups. The centric diatoms exhibit radial symmetry, while the pennate diatoms have non-radial symmetry and usually instead exhibit symmetry of one or more of the valvar, apical, or transapical planes. Pennate diatoms may possess a slit-like structure called a raphe along the surface of one or both valves that allows the cell to move by secreting polysaccharides.   Diatoms are among the largest contributors to global primary production and form the base of aquatic food webs in both marine environments and freshwaters. Unicellular forms may be planktonic or attached to a substrate, and cells may also be joined in filaments or colonies. The diatoms are a powerful ecological tool used to investigate past conditions and monitor environmental changes over time. The siliceous valves remain intact after the cells have died and have created an extensive fossil record exhibiting the evolutionary changes of the group's morphology over 150 million years. In addition, the preserved remains known as diatomite or diatomaceous earth have many industrial and commercial applications in products such as foods, filters, paints, and cosmetics. There are at least 10,000 known species, and perhaps as many as a million. Diatom taxnomy is based on valve morphology rather than reproductive compatibility.