In contrast to the closed Protonephridia, another type of excretory tubule, the metanephridium, has internal openings that collect body fluids. Metanephridia are found in most annelids, including earthworms (Figure 40.8). Each segment of the worm has its own pair of metanephridia, which are serpentine tubules immersed in the coelomic fluid of that segment. As in most animals with closed circulatory systems, blood vessels are intimately associated with the excretory tubules of the earthworm; a network of capillaries envelops each metanephridium. The tubule drains to the outside of the body through a nephridiopore.
At the opposite end of a metanephridium is the nephro-stome, a ciliated funnel that collects coelomic fluid from the body segment just anterior. (Notice again that the metanephridium, unlike the protonephri-dium of flatworms, is open at both ends.) As the fluid moves along the tubule, the transport epithelium bordering the lumen pumps essential salts out of the tubule, and the salts are reabsorbed into the blood circulating through the surrounding capillaries.
The urine that exits through the nephridiopore is hypoosmotic to the body fluids of the earthworm. By excreting this dilute urine in amounts up to 60% of the body weight of the worm per day, the metanephridia offset the continuous osmosis taking place across the skin of the animal from the damp soil.