Protonephridia - Excretion
Protonephridia are more primitive and occur mainly in acoelomate and pseudocoelomates animals. An animal may have two or more extensively branched protonephridia, each internally terminating in several bulb-like structures called flame cells or flame bulbs. In the lumen of each flame cell is found a tuft of cilia.
Figure: Excretory system of planarian
Flame cells are common in flatworms and nemertines. In some of the coelomate worms that have protonephridia (e.g., polychaetes), the flame bulbs are Excretory pore replaced by a solenocytes. The solenocytes have a single flagellum projecting into the lumen instead of the ciliary tuft as found in the flame cells. It has been suggested that the undulating movements of the ciliary tuft or flagellum generate sufficient negative pressure for filtration and the force required to propel the fluid through the tubules. Flame cell system functions in animals without a circulatory system and pick up substances only from the tissue fluids.