Diagram of a Flame Cell – One Cell, Then the Flatworm

All 10 parts on the diagram

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One flame cell 7

Flame cell The cell that closes the blind end of a tubule. Its cup filters the fluid between the body's cells.
Cilia A tuft of long cilia that beats like a flame. Its beating draws fluid in and drives it down the tubule.
Nucleus The nucleus of the flame cell, in its cell body.
Fenestrations Small openings in the wall of the cup. Water and small molecules pass in; larger proteins stay outside.
Tube cell The next cell, a sleeve the cup fits into. It takes useful molecules back from the fluid.
Lumen The hollow space inside the cup and the tube. The filtered fluid collects here and flows away.
Excretory tubule The fine tube that leads away from the flame cell. It joins others and ends at a pore.

In the flatworm 3

Excretory canal Long coiled canals run the length of the body. The fine branches drain into them.
Branches Fine branches reach every part of the body. Each ends in a flame cell.
Excretory pore A small opening on the back of the worm. The fluid leaves the body here.

How this list was compiled: Every name checked on 2026-10-02 in two sources. Wikipedia, Flame cell, Nephridium and Planarian, https://en.wikipedia.org/wiki/Flame_cell. R. W. Hegner, College Zoology (1912, public domain), the excretory system of Planaria, https://archive.org/details/collegezoology00hegn. T. J. Parker and W. A. Haswell, A Text-book of Zoology (1910, public domain), the flatworms, https://archive.org/details/textbookofzoolog00park. H. T.-K. Vu and others, eLife 2015, an open-access study of the planarian excretory system, https://doi.org/10.7554/eLife.07405. R. L. Chevalier, Evolution, Medicine, and Public Health 2025, open access, https://europepmc.org/article/PMC/PMC12409785. Three open-access papers gave a second source for tube cell, fenestrations and excretory canal: P. J. Skelly and others, PLoS Pathogens 2014, https://europepmc.org/article/PMC/PMC4133383; K. Ichimura and T. Sakai, Anatomical Science International 2017, https://europepmc.org/article/PMC/PMC5315740; T. Rozario and P. A. Newmark, Experimental Parasitology 2015, https://europepmc.org/article/PMC/PMC13044859. Licence: own work

The drawing: Drawn for this site on 2026-10-02 from plain geometry; nothing traced. Licence: own work

How it works

This diagram of a flame cell shows the cell cut along its length. A flame cell is the filter at the blind end of a flatworm's excretory tubule. The second view shows flame cells in a planarian.

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One flame cell

The cell body holds the nucleus. A cup reaches from it, and a tuft of cilia beats inside the cup. Fluid from between the body's cells passes in through the fenestrations. It collects in the lumen and flows past the tube cell into the excretory tubule.

In the flatworm

Switch to In the flatworm. Coiled excretory canals run down the two sides, and a cross tube joins them behind the head. Fine branches end in flame cells. The fluid leaves at an excretory pore on the back. Press Quiz me to find each part yourself.

The cup's wall is drawn as blocks with gaps so the openings show. The real openings are far too small for a school microscope.

Questions and answers

What is a flame cell?

A flame cell is an excretory cell of flatworms and rotifers. It closes the blind end of a fine tubule and works as a filter, a little like part of a kidney. A group of flame cells with its tubules is a protonephridium.

What is the function of flame cells?

They filter the fluid between the body's cells and help keep the animal's water and ion balance steady. The beating cilia draw fluid in through small openings and drive it along the tubule to a pore.

Why is it called a flame cell?

Because of its tuft of cilia. Under a microscope the tuft beats with a steady wave that looks like a flickering candle flame.

Which animals have flame cells?

Flatworms such as planarians and tapeworms have them, and so do rotifers. In a planarian the fine branches of the excretory canals end in flame cells all through the body.