or What Do They Look Like?
Diatom cell walls are made up of two distinct halves, or valves. The older, larger valve is called the epivalve, and the smaller, younger valve is the hypotheca. Primitively, each valve is shaped like a cylinder, with the open end of the hypovalve facing and inside the open end of the epivalve. The two valves are connected by a girdle region, which often lacks areolae, and raised intercalary bands. These features are often visible in edge-on (girdle) views.
Diatom tests often bear one or more spines, setae, or other projections. The setae are quite dramatic as they may be many times longer than the diameter of the cell. Test projections influence positioning of diatoms vertically in the water column by increasing drag.
For the diatomologist, test projections are one easy-to-see way
of distinguishing different types!
Diatom valves, or tests, are punctuated with areolae, numerous 0.1 - 0.6 micron hexagonal holes. Areolae are thought to be necessary for the cell to obtain nutrients and dissolved gasses such as CO2 from the seawater, since a solid silica sheet would be impermeable to molecules and ions. Many tests also have sub-areolar structure visible only in electron microscopes. The porous nature of diatomaceous earth is the key to its suitability as a fluid filter and an insulator and for other industrial uses.
Areolae are arranged in very
attractive, regular patterns. The beauty of diatom tests, at least
partially due to the areolar patterns, has attracted microscopists
for over two hundred years, and continues to attract them in the era
of the electron microscope.
Some species of both central and pennate diatoms assemble into colonies. The individuals may be connected by their valve faces, edge-on, or interlaced with setae with the valves scarcely touching.