Which excretory structure is characteristic of annelids?

• A. Metanephridia (tubes of cilia move fluid)
• B. Nephridia
• C. Malpighian tubules
• D. Gland cells

Answer: (A)

Annelids process waste with metanephridia, a paired, segmentally arranged excretory tubule. In each body segment, a metanephridium begins with a ciliated funnel called the nephrostome that collects waste from the coelomic fluid. The filtrate then travels through the tubule, where useful substances are reabsorbed, and wastes are concentrated and released outside through a nephridiopore. This setup—segmental metanephridia—is characteristic of annelids and sets them apart from other groups. Malpighian tubules, for example, are typical of insects, while the general term nephridia is not the specific, repeated, segmented arrangement seen in annelids. Gland cells do not constitute the specialized excretory system in these worms.