Protonephridia are made up of many flame cells. In the flame cells, there are cilia at the 'tips'. They beat, creating a sort of vacuum that sucks out substances on the other side of the membrane. This gets filtered, things are reabsorbed as appropriate & the waste is excreted through the nephridiopore (your one opening). This is mostly for osmoregulation.
Metanephridia is in segments, there's an internal opening (nephrostome) & an external opening (nephridiopore). Body fluid is drawn in from the nephrostome, substances are filtered, selected or reabsorbed as necessary. The wastes are excreted from the nephridiopore. This functions in both
