Nephridia, coiled tubular duct-like organs, filter and remove waste from an earthworm's body. In less developed worms such as the flatworm or the rotifer, the nephridia are not as specialized and are located in various cells throughout the creature's body. In more advanced segmented worms, like the earthworm, the nephridia typically are grouped in pairs throughout the segments of the worm's body. These pairs of nephridia are named for their location in the worms anatomy. Each group shares similarities, they are all waste conduits, but each also has distinctive functions.
The pharyngeal nephridia consists of large numbers of smaller nephridia grouped into three sets of paired tusks. These grouped nephridia filter nitrogen from the clusters of surrounding capillaries. The ends of all the pharyngeal nephridia meet and form a large waste duct that dumps into the pharynx and buccal chamber, a tube extending from the mouth to the pharynx.
The septal nephridia are located on both sides of the septa--thread-like structures that hold the skin to organs below it--in rows of forty or fifty individual nephridia. They appear after the 15th segment, and they empty into a pair of excretory ducts that lead to the main gut cavity. This system of of nephridia is responsible for maintaining a water balance or osmoregulation in the earthworm.
Hundreds (200 to 250 per segment) of integumentary nephridia are located throughout the earthworm's body wall (in all but the first two segments). These tiny nephridia serve to channel nitrogenous waste from inside the worm to the outside soil.
- earthworm image by ril from Fotolia.com