nephridial gland

The term "nephridial gland" is synonymous with "nephridium". A nephridium is a fundamental unit of the excretory system in many invertebrates, serving to expel waste products from the body cavity to the exterior. This evolutionary development allowed for greater tissue specialization by removing the necessity for all cells to be in direct contact with seawater for waste diffusion.12

There are two primary forms of nephridia:
* Protonephridia: These are simpler and more primitive, found in organisms like flatworms. They consist of a hollow cell (flame bulb or solenocyte) within the body cavity connected to a duct leading to an external opening called a nephridiopore. Cilia or flagella within the cell help move filtered fluid down the duct.2
* Metanephridia: These are more complex and found in segmented invertebrates such as earthworms. They lack flame cells and open directly into the body cavity. Cilia lining the tubule draw in cavity fluids, and specialized cells actively reabsorb useful substances as the fluid moves towards the exterior.2