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flame cells Ciliated cells that form part of the excretory and osmoregulatory system of platyhelminths, rotifers, and nemertine worms. This system, known as a protonephridium, consists of branching tubules that open to the exterior through excretory pores; flame cells occur at the ends of the tubules, into which their cilia project. Fluid, containing water and nitrogenous waste products, diffuses into the flame cells and is directed through the tubules to the exterior by movements of the cilia, which resemble the flickering of flames. Flame cells that possess only one cilium are known as solenocytes; these are found in the protonephridia of marine worms of the phylum Priapulida.