The protonephridium is an organ found in the excretory system of several invertebrates such as those in phylum Platyhelminthes. Thousands of protonephridium compose a network of protonephridia, which is commonly used to refer to the complete network. These networks are found on the body walls of the invertebrate. Each protonephridia is made of a blind-ended tubule with a flame bulb which caps the end of the branch.
The flame bulbs in the protonephridia generate currents which draw interstitial fluids into the protonephridia tubules. A filtrate solution made from metabolic nitrogenous waste is pulled from the fluids and processed as urine, which exits through the excretory canals and pores.
Protonephridia are comparable to the kidney organs found in vertebrate animals. Accordingly, both the protonephridium (and the overarching category nephridia) and the nephron, the basic cellular unit of the kidneys, use the prefix nephr-, which derives from the Greek word for kidney, nephros.