protonephridium

A protonephridium is a primitive excretory organ found in many invertebrates. It functions to remove metabolic wastes from the body cavity and expel them to the exterior through a pore called a nephridiopore.1

The anatomy of a protonephridium includes a hollow cell, often referred to as a flame bulb or solenocyte, which is connected to a duct. This cell contains cilia or a flagellum that beats to move filtered fluid, or urine, down the duct and out of the body. This system is crucial for tissue specialization by allowing organisms to eliminate waste without needing all cells to be in direct contact with the external environment.1