The excretory system of Nereis vexillosa is dependent on structures known as nephridia or “little kidneys.” In Nereis vexillosa all body segments except the first and last contain an individual pair of nephridia because members of the Nereis genus are errant Polychaetes, which means the anterior and posterior segments lack nephridia. Similar to all other organisms included in the Nereis genus, Nereis vexillosa contains longer nephridial tubes than other members of the Annelida phylum.