This takes into account the above problems and also the reliability of a particular taxon as a biostratigraphic marker. Variation in size is large even within the same species. The nucleus is central with cytoplasmic strands extending from it over the vacuole. Unlike many other protistans, the diatoms have no flagella or rhizopodia. Benthic varieties can glide over the substrate on layer of mucus and planktonic varieties stay afloat using internal fat globules for buoyancy.
Diatoms often appear to be brown.
They produce oils and fatty acids as an end-product of the photosynthetic process. In areas where they are abundant this can create an oily slick on the surface of the water. They usually occur singly, but may also form chains held together by protoplasmic threads or strands of mucilage or spiny projections that lock together. Some groups take stellate forms or make twisted spiral patterns. Unlike radiolarians, they are easy to grow in the laboratory and more is understood about their reproductive biology and ontogeny.