nephridiopore

A nephridiopore is the external opening of a nephridium, which is a unit of the excretory system in many invertebrates. Nephridia function to expel waste products from the body cavity to the exterior, typically in aquatic environments. This specialized system evolved to allow for tissue specialization by removing the necessity for all cells to be in direct contact with seawater for waste diffusion.1

There are two main forms of nephridia:
* Protonephridia: These are simpler and found in organisms like flatworms. They consist of a hollow cell (a flame bulb or solenocyte) and a duct leading to a nephridiopore. Cilia or flagella within the flame bulb help filter and move waste down the duct to the outside.1
* Metanephridia: These are more complex and found in segmented invertebrates such as earthworms. They open directly into the body cavity and use cilia to draw fluids into the tubule, where useful substances are reabsorbed before waste is expelled through a nephridiopore.12