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A protist, belonging to the division Bacillariophyta, in which the cell wall (frustule) is composed of silica and consists of two halves, one of which overlaps the other like the lid of a box. Frustules are often delicately ornamented. Most diatoms are unicellular, but some are colonial or filamentous. Most are photosynthetic, but some species lack chlorophyll and live heterotrophically among decaying marine algae. Pennate (i.e. bilaterally symmetrical) diatoms occur in both freshwater and marine habitats; centric diatoms (i.e. radially symmetrical) occur predominantly as part of the marine plankton. There are more than 10 000 species. See diatomaceous earth; diatomite; diatom ooze.

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