Ciliated cells that form part of the excretory and osmoregulatory system of flatworms (see platyhelminthes), 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, consisting of water, ions, and nitrogenous waste products, filters between cells into the tubules, where it is directed to the exterior by movements of the flame cell cilia, which resemble the flickering of flames. Along its route the fluid is modified by reabsorption and secretion of ions across the tubule walls to produce the urine. Flame cells that possess only one cilium are known as solenocytes; these are found in the protonephridia of marine worms of the phylum Priapulida.