Practice question

Excretion in flatworms relies on protonephridial system, not metanephridia, Malpighian tubules or green glands characteristic of other phyla. Numerous terminal flame cells, each bulb-like cyrtocyte with tuft of beating cilia creating flickering appearance, drive ultrafiltration of interstitial fluid via negative pressure. Filtrate passes through branching tubules opening via nephridiopores, actively regulating water in freshwater species and eliminating nitrogenous waste mainly as ammonia through diffusion. This closed system lacks internal opening to coelom, unlike metanephridia in annelids which possess ciliated funnel called nephrostome. Protonephridia exemplify primitive excretory design consistent with acoelomate organization and absence of circulatory system.