Flame cell
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A flame cell is a specialized excretory cell found in most "lower" freshwater invertebrates, including Nemotada, Platyhelminthes (except the tubellarian order Acoela), flatworms, rotifers and nemerteans; these are the simplest animals to have a dedicated excretory system. Flame cells function like a kidney, removing waste materials. Bundles of flame cells are called protonephredia.
The flame cell has a nucleated cell body, with a "cup-shaped" projection, with cilia covering the inner surface of the cup. The beating of these cilia resemble a flame, giving the cell its name. The cup is attached to a tube cell. The inner surface of the tube cell is coated in flagella. The beating of the cilia and flagella help move liquid through the tube cell. The tube opens externally through a nephropore, or, in the trematoda, into an excretory bladder. The function of these cells is to regulate the osmotic pressure of the worm, and maintain its ionic balance. Microvilli in the tube cell may be used to reabsorb some ions.