Diatoms may be divided into two main classes. The centrate diatoms, or Centrobacillariophyceae, are generally radially symmetrical, like the specimen of Melosira shown above at left. Pinnularia, at right, is a pennate diatom, member of the Pennatibacillariophyceae, showing its typical bilateral symmetry.
Members of both classes may be found in either fresh or salt water, though centrate forms tend to predominate in marine habitats, while pennate diatoms are more typical of freshwater environments.
The diatoms are only one of several photosynthetic groups of Chromista with silica skeletons. Others include the Prymnesiophyta and Silicoflagellata.