Excretion in earthworm is performed by long, thin and coiled tubules known as nephridia. It is found in all segment except first two. They are the small microscopic coiled tube with the thin wall which is glandular and vascular. According to location, nephridia are of three types they are:
Septal nephridia are attached to the intersegmental septa.They are attached in septa in 4 rows 2 on anterior and 2 on the posterior side.The nephridia consists of three parts:
It is ciliated funnel having elliptical opening bounded by two lips i.e. the upper and the lower lip. Nephrostome opens into the body through the short narrow ciliated neck.
The body of nephridia consists of the astraight lobe and twisted lobe having epical lobe. The twisted lobe has two limbs they are: distal limb and the proximal limb. The proximal limb is attached to the nephrostome and terminal duct whereas the distal limb is attached to the straight lobe.
It is a short narrow ciliated tube which is attached to the septal excretory canal on either side. The septal excretory canal opens into supra intestinal excretory duct. From supra intestinal excretory duct, small ductules open into the intestine which is guarded by a sphincter.
They are found in three pairs, in tuft or bunch on either side of the alimentary canal in 4th, 5th, and 6th segments. Each tuft consists of hundreds of ciliated tubules. The terminal duct of the tuft opens to the common thick walled duct. The common duct of 6th segment opens into the buccal chamber and that of 4th and 5th opens into the pharynx. They are enteronephric.
The integumentary nephridia are found scattered on the inner side of skin or body wall in all segment except first three segments. The terminal duct of this nephridia opens outside into nephridiopore so they are mesonephric. In each segment, there are 200-250 nephridia but in clitellum 10 times more is found. So, it is also called forest of nephridia.
Physiology of Excretion
The nephridia are richly supplied with blood vessels and the glandular cells which extract water and nitrogenous waste like urea, ammonia, amino acid etc. from the blood. The septal nephridia remove nitrogenous waste matters from the coelomic fluid. The enteronephric nephridia play an important role in the digestive process as it moistens the food and helps in digestion. The nitrogenous compound is used by symbiotic bacteria which lives inside the alimentary canal. The enteronephric nephridia are also called protonephridia as it helps in digestion. It also helps in osmoregulation, as the water which passes through the alimentary canal is absorbed in an intestine, and water is conserved in dry regions. The salts while passing through nephridia is selectively reabsorbed.
Earthworm is ureotelic because its nitrogenous waste matter consists of 40% urea, 20% ammonia, 40% amino acid and other nitrogenous compounds.