Transverse flagellumA ribbon-like flagellum that lies in the cingulum. Its waves turn the cell and help to push it along.
Longitudinal flagellumA flagellum that runs down the sulcus and trails behind. It beats to push the cell forward.
Cingulum (girdle)The groove that runs round the middle of the cell. It divides the armor into an upper and a lower half.
SulcusThe groove that runs from the cingulum down the lower half. The longitudinal flagellum lies in it.
EpithecaThe half of the armor above the cingulum. It is the front end as the cell swims.
HypothecaThe half of the armor below the cingulum. The sulcus runs down it.
Inside and horns 4
ThecaThe armor of the cell, built of cellulose plates. A dinoflagellate without it is called naked.
NucleusA large nucleus whose chromosomes stay tightly packed. They stay that way even when the cell is not dividing.
ChloroplastsThe place where the cell makes food from light. About half of all dinoflagellates have them.
HornsLong points drawn out from plates of the armor. This cell has one at the top and three below.
How this list was compiled: Every name was checked on 2026-10-03 and kept only where two independent sources use it for a dinoflagellate. (1) Wikipedia, text only: Dinoflagellate, Peridinium and Ceratium, https://en.wikipedia.org/wiki/Dinoflagellate , https://en.wikipedia.org/wiki/Peridinium and https://en.wikipedia.org/wiki/Ceratium . (2) University of California Museum of Paleontology, Dinoflagellata: More on Morphology, https://ucmp.berkeley.edu/protista/dinoflagmm.html . (3) G. N. Calkins, The Biology of the Protozoa (1926), public domain, https://archive.org/details/biologyofprotozo00calk . (4) H. B. Ward and G. C. Whipple, Fresh-water Biology (1918), public domain, https://archive.org/details/freshwaterbiolog00ward . Licence: own work
The drawing: Drawn for this site on 2026-10-03 from plain geometry; nothing traced. Licence: own work
How it works
This is a drawing of what a slide of dinoflagellates under a microscope shows, with every part labeled. Outside shows the armor, its two grooves and the two flagella. Inside shows the cell cut open, beside a horned cell.
Read the full guide
Two grooves, two flagella
The armor, or theca, is built of plates. The cingulum is the groove round the middle: the epitheca lies above it and the hypotheca below. The sulcus is the groove down the lower half. The transverse flagellum lies in the cingulum, and the longitudinal flagellum runs down the sulcus and trails behind.
Inside, and the horned kind
Cut open, the cell shows its large nucleus and its chloroplasts. The cell beside it has the same two grooves, but its plates are drawn out into long horns.
Questions and answers
What do dinoflagellates look like under a microscope?
Most are single cells with a groove round the middle and a second groove down one side. Armored kinds are covered in plates, and some, such as Ceratium, have long horns. The two flagella make the cell whirl as it swims.
What are the two flagella of a dinoflagellate?
The transverse flagellum is a wavy ribbon that lies in the cingulum, the groove round the cell. The longitudinal flagellum lies in the sulcus and trails behind. Together they turn the cell and push it forward.
What is the difference between the cingulum and the sulcus?
Both are grooves in the surface of the cell. The cingulum runs round the middle like a belt and holds the transverse flagellum. The sulcus runs from the cingulum down the lower half and holds the longitudinal flagellum.
What is the theca of a dinoflagellate?
The armor of the cell, made of cellulose plates. The part above the cingulum is the epitheca and the part below it is the hypotheca. Dinoflagellates without this armor are called naked.