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
This diatom cell diagram has two views. Cut open shows the two halves of the shell and the living cell inside. The shell shows three empty shells from outside. Tap a name, then press Quiz me.
Read the full guide
A box, its lid and the cell inside
The shell is called the frustule. Its larger half, the epitheca, fits over the smaller half, the hypotheca, the way a lid fits over a box. Inside, the nucleus sits in the middle with a large vacuole on each side, the chloroplasts lie along the walls, and the small drops are oil.
Two shapes of shell
Seen face on, a diatom shell is round (centric) or long and boat-shaped (pennate). The rows of dots are striae, and the slit along the boat-shaped shell is the raphe. Girdle bands run round the middle of a shell, where its halves overlap.
The hypotheca is always the smaller half: its sides are tucked inside the epitheca.
Questions and answers
What is the shell of a diatom called?
The frustule. It is made of silica, the material of glass, and is pierced by rows of tiny pores. It comes in two halves that fit together like a box and its lid.
What is the difference between the epitheca and the hypotheca?
They are the two halves of the shell. The epitheca is slightly larger and fits over the hypotheca. When a diatom divides, each new cell keeps one old half and builds a new, smaller half inside it.
What is inside a diatom cell?
A nucleus near the middle, one or more large vacuoles, chloroplasts that make food from light, and drops of oil, which are the food reserve of the cell.
What are centric and pennate diatoms?
The two main shapes of diatom. A centric diatom is round, with a pattern that runs out from the center. A pennate diatom is long, with two matching sides, and many pennate diatoms have a slit called a raphe.