nephridia

Nephridia are excretory organs found in many invertebrates that remove metabolic wastes and help regulate water and ionic balance. They vary in structure but commonly operate by filtering coelomic or interstitial fluids and selectively reabsorbing essential substances while secreting wastes for elimination. In addition to excretion, they contribute to osmoregulation and nitrogen waste management across species.

Protonephridia are the simplest nephridial type and occur in some flatworms, rotifers, and larval forms of other animals. A network of tubules bears a tuft of ciliated cells known as flame cells or solenocytes at the closed end. Cilia beat to draw interstitial fluid into the tubules, where selective reabsorption and secretion occur, and the processed fluid exits through an external pore. This design provides filtration and urine-like excretion without a carrying central duct.

Metanephridia are more complex, found in many segmented worms such as earthworms. Each body segment typically contains a pair of metanephridia open to the coelom by a ciliated funnel called a nephrostome and terminating with a nephridiopore to the exterior. Inside, tubules reabsorb water and essential ions, while wastes are concentrated and expelled. This segmented arrangement supports continuous waste removal and homeostasis as the animal grows.

Excretion can differ among phyla; some mollusks possess metanephridia, while insects rely on Malpighian tubules rather than nephridia. Overall, nephridia illustrate a modular approach to waste disposal in invertebrates, enabling osmoregulation and toxin removal across diverse habitats.