Metanephridia - Excretion
Metanephridia are found in coelomates (e.g., annelids). These animals have evolved a close circulatory system intimately associated with the excretory organs resulting in direct exchange of material between the two systems. In these worms an isosmotic fluid appears initially in the nephridium due to filtration of the coelomic fluid through the ciliary mesh of the nephrostpme. Salt is later withdrawn from the fluid as it passes through the extensively looped duct and finally dilute urine is discharged.