Practice question

Flame cells appear individually as ciliated terminal cells, yet collectively they connect via branching tubules forming protonephridial system, simplest nephridial type among bilaterians. Each protonephridium comprises closed tubule ending in flame bulb with filtration slits and opening externally via pore, contrasting with metanephridia that start with open ciliated funnel draining coelomic fluid. Ultrafiltration driven by ciliary beating creates negative pressure pulling fluid through weir. Thousands of flame cells provide osmoregulation in freshwater flatworms. Nephrons refer to vertebrate glomerulus-tubule units, Malpighian system originates from insect hindgut, thus protonephridia uniquely characterize acoelomates and some pseudocoelomates.