FrustuleThe shell of the cell, made of silica, the material of glass. It is in two halves that fit together like a box and its lid.
EpithecaThe larger half of the shell. Its sides fit over the smaller half, as a lid fits over a box.
HypothecaThe smaller half of the shell. When a diatom divides, each new cell keeps one old half and builds a new smaller half.
ChloroplastsThe place where the cell makes food from light. They lie in the thin layer of cytoplasm along the walls.
NucleusThe nucleus lies near the middle of the cell. Strands of cytoplasm hold it in place.
VacuoleA large sac of fluid that fills much of the cell. The cytoplasm is a thin lining round it.
Oil dropletsDrops of oil in the cytoplasm. Oil is the food reserve of a diatom.
The shell 4
RapheA slit that runs along the face of the shell. Through it the living cell inside meets the water outside.
StriaeRows of tiny pores in the glass of the shell. Their pattern differs from one kind of diatom to another.
Girdle bandBands of silica round the middle of the shell. They lie where the two halves overlap and hold them together.
Central noduleA thickening of the wall in the middle of the shell face. The two parts of the raphe end on each side of it.
How this list was compiled: Every name was checked on 2026-10-01 and kept only where two independent sources use it for a diatom. (1) Wikipedia, text only: Diatom and Frustule, https://en.wikipedia.org/wiki/Diatom and https://en.wikipedia.org/wiki/Frustule . (2) George M. Briggs, Inanimate Life, the chapter on diatoms, an open college botany textbook, https://milnepublishing.geneseo.edu/botany/chapter/the-diatoms/ . (3) G. S. West, A Treatise on the British Freshwater Algae (1904), the chapter on the Bacillarieae, public domain, https://archive.org/details/treatiseonbritis00west . (4) Mishra and others, All New Faces of Diatoms, Frontiers in Microbiology 8 (2017), an open-access review read through Europe PMC, https://europepmc.org/article/PMC/PMC5496942 . Licence: own work
The drawing: Drawn for this site on 2026-10-01 from plain geometry; nothing traced. Licence: own work
How it works
To label the diatom cell, write a name on the line for each number. Numbers 1 to 7 are on a diatom cut open and seen from the side. Numbers 8 to 11 are on three empty shells.
Read the full guide
Word bank to label diatom cell parts
In alphabetical order, to copy onto the board: central nodule, chloroplasts, epitheca, frustule, girdle band, hypotheca, nucleus, oil droplets, raphe, striae, vacuole.
Which mark takes which name
1 is the bracket at the left: the whole shell (frustule). 2 is its outer half, on top like a lid (epitheca), and 3 the half tucked inside (hypotheca). Flat plates along the walls are chloroplasts (4), the round body with a dark spot is the nucleus (5), the large sacs are vacuoles (6) and the small drops are oil droplets (7). On the long shell, 8 is the slit (raphe) and 11 the knot that parts it (central nodule). Rows of dots are striae (9). The plain bands round the last shell are the girdle band (10).
When marking, shell or cell wall is right on line 1. An older book says chromatophores on line 4 and connecting band on line 10.
A class list may have valve, the plate at each end of the shell. It is drawn, but has no number on this sheet.
Questions and answers
What is a diatom?
A single-celled alga in a shell of silica. Diatoms live in the oceans, in fresh water and in soils, as single cells or in colonies shaped like ribbons, fans, zigzags or stars.
What color is a diatom?
Yellowish-brown. That is the color of its chloroplasts, which hold the pigment fucoxanthin as well as chlorophyll. It suits the plates at number 4 on a sheet that gets colored in.
How big is a diatom?
Single cells range from 2 to 2,000 micrometers, and most are 20 to 200. The sheet is a schematic and is not drawn to scale.
Can diatoms move?
Mostly they drift, carried by currents and by water stirred by the wind. Some pennate (long) diatoms glide over a solid surface on sticky mucilage that they give out through the raphe, the slit at number 8.
What is diatomaceous earth?
A soft rock made of the shells of dead diatoms, which crumbles easily into a fine powder. It is used to filter water, as a mild abrasive and in cat litter.