Metanephridia
Most annelids, such as earthworms, have metanephridia
(singular, metanephridium), excretory organs that collect fluid
directly from the coelom (Figure 44.12). Each segment of a
worm has a pair of metanephridia, which are immersed in coelomic fluid and enveloped by a capillary network. A ciliated
funnel surrounds the internal opening. As the cilia beat,
fluid is drawn into a collecting tubule, which includes a storage
bladder that opens to the outside.
The metanephridia of an earthworm have both excretory
and osmoregulatory functions. As urine moves along the
tubule, the transport epithelium bordering the lumen reabsorbs
most solutes and returns them to the blood in the capillaries.
Nitrogenous wastes remain in the tubule and are
excreted to the outside. Earthworms inhabit damp soil and
usually experience a net uptake of water by osmosis through
their skin. Their metanephridia balance the water influx by
producing urine that is dilute (hypoosmotic to body fluids).