Bacillariophyta A division of microscopic algae (known as diatoms) which are mostly unicellular, but which may be colonial or filamentous. The cell wall (frustule) is composed of silica and consists of two halves, one of which overlaps the other like the lid on a box. The frustule is commonly delicately ornamented. Most diatoms are photosynthetic, but there are also species that lack chlorophyll and live heterotrophically among decaying marine algae. Some species are capable of a gliding motility when in contact with a surface. Cells with bilateral symmetry are said to be pennate, while those with radial symmetry are called centric. Pennate diatoms are found in both freshwater and marine habitats, either as plankton or attached to rocks, etc.; centric diatoms are predominantly marine and planktonic. The silica frustules are an important constituent of deep-sea deposits. There are more than 10 000 species of diatoms.

Bacillariophyta A phylum of algae comprising the diatoms. These marine or freshwater unicellular organisms have cell walls (frustules) composed of pectin impregnated with silica and consisting of two halves, one overlapping the other. Diatoms are found in huge numbers in plankton and are important in the food chains of seas and rivers. Past deposition has resulted in diatomaceous earths (kieselguhr) and the oil reserves of these species have contributed to oil deposits.