These cells are solitary and typically have an elongated, needle-like shape with long, extended acicular edges, which can sometimes curve or have a slight sigmoid shape. The central portion of the cell has a spindle-like form, and two chloroplasts are generally present. The frustules, or the cell walls, are lightly silicified. The valves, which include the keel and raphe canal, exhibit a twisted structure. The girdle bands are numerous and, like the valves, are long and narrow. This unique combination of features characterizes these organisms.