nephridium

A nephridium (plural nephridia) is a tubular excretory organ found in many invertebrates. It functions in removing metabolic wastes and in regulating water balance and ion concentrations, often integrating excretion with osmoregulation.

There are two main categories of nephridia. Protonephridia are closed tubules that end blindly with a flame cell or solenocyte at their distal end. Cilia or flagella drive filtrate through the tubule, which leads to an external pore. This type is common in flatworms, some nemerteans, and certain larval stages of other groups, and it often provides a primary means of waste removal and osmoregulation.

Metanephridia are typically found in segmented animals such as annelids. In these, each body segment usually bears a pair of nephridia. They begin with a nephridial funnel called a nephrostome that opens into the coelomic cavity, collects fluid, and channels it through a convoluted tubule where selective reabsorption and secretion occur. The processed filtrate exits to the exterior through a nephridiopore. Metanephridia thus effectively manage waste elimination and internal chemical balance.

In earthworms, the extensive system of metanephridia per segment illustrates how these organs support large-scale osmoregulation and nitrogenous waste excretion. Across invertebrates, nephridia can vary in form and function, with protonephridia and metanephridia representing common archetypes. Some groups rely more on other organs, such as Malpighian tubules in insects, but the term nephridium remains a general label for tubular excretory structures derived from diverse evolutionary origins. The wastes typically reflect environmental conditions, with ammonia, urea, or uric acid as the principal end products depending on habitat and physiology.