nephrostome

A nephrostome is the funnel-shaped, ciliated opening that forms the entry point of a metanephridium, connecting the coelomic cavity to the tubular part of the excretory system in many invertebrates. It serves to draw coelomic fluid into the nephron for processing as urine, aided by the beating of its ciliated lining.

In segmented worms such as annelids, each nephrostome is located at the beginning of a nephridium and opens into the body cavity. The ciliated rim creates currents that drain coelomic fluid into the internal tubule, which then conducts filtrate toward the nephridiopore, through which waste is expelled to the exterior. The nephrostome thus marks the transition from the coelom to the tubular excretory pathway.

Functionally, the nephrostome participates in filtration and osmoregulation. It helps regulate the composition of body fluids by initiating the movement of coelomic fluid into the tubule, where selective reabsorption and secretion modify the filtrate before excretion. The structure and size of nephrostomes can vary among species, reflecting adaptations to different habitats and excretory requirements.

The term is most commonly applied to annelid physiology, where paired nephrostomes are characteristic of many metanephridia. Etymologically, it derives from Greek roots meaning kidney and mouth.