Excretory system of Earthworm (megascolex)

Excretory System In earth worm, the excretory system is the metanephridium having internal opening the nephrostome and external opening the nephridiopore. There are three sets of excretory organs in earth worm. They are 1. Pharyngeal nephridia 2. Integumental nephridia 3.Septal nephridia 1.Pharyngeal  Nephridia: Pharyngeal nephridia are located anterior side of the pharynx and hence the a Thcy open into the gut, so they are tad enteronephrie nephridia. They are the metanephridia

Pheretima has 3 pairs of bunches nephridia. They are located in the 4th. 6th segments

excretory duct opens into the intestine by a single duct at the level of each septum. Supra-intestinal

Each bunch is formed of numerous nephridial tubules. One end is closed. The other ends of all the tubules open into a common duct which opens into the gut. The common ducts of the pharyngeal nephridia of the 4th and 5th segments open into the pharyntx.But the common ducts of pharyngeal nephridia of 6th segment open into the buccal chamber.

 2. Integumentary Nephridla :Integumentary nephridia are present on the inner surface of the body wall (integument) and hence the name. They are metanephridia. They open to the outside. Hence they are also called exonephric  nephridia. They occur in all the segments except the first two segments. There are 200 to 250 nephridia in each segment. In the clitellar segments, there are 2000 to 2500 nephridia in these segments; so these nephridia with numerous numbers in the clitellar segment are called forests of nephridia. The integumentary nephridia are V-shaped. Each nephridium is in the form a tubule. One end ends blindly and the other end opens to the outside by a nephridiopore.

3.Septal Nephridia :Septal nephridia are attached to the septum of earthworm.They open into the gut, hence they are also called enteronephric nephridia. They are attached on either side of the septum. There are about 50 nephridia on each side of the septum. Each septal nephridium is a syntytial glandular mass. It consists of a nephrostome, a neck, a main body and a terminal duct The nephrostomes is in the form of a funnel which opens into the body cavity. The mouth of the funnel is ciliated The nephrostome is followed by a short neck. The neck leads into the main body. The main body consists of a straight lobe and a twisted lobe. The twisted lobe is formed of a proximal limb and a distal limb. They are spirally twisted on each other. The straight lobe is continued into the distal limb. The distal limb leads into the proximal limb. The proximal limb continues as the terminal nephridial duct.

The entire nephridium has ciliated tracts. The neck has one ciliated tract, the body has two ciliated tracts and the terminal duct has one. There arc 4 parallel tubules in the straight lobe, three tubules in the base of each limb and two tubules in the apex of each limb of the twisted lobe. There is a single tubule in the neck as well as in the tenninal duct. There are 4 rows of septal nephridia on each septum, two rows on the anterior face and two rows on the posterior face. The terminal duets of nephridia open into a septal excretory canal. There are two sep-tat excretory canals on each face of the sep-tum. They run towards the median line and open into a pair of supra-intestinal excre-tory ducts. They extend from the 15th seg-ment to the posterior end. Each supraintestinal excretory duct opens into the intestine by a single ducta at the level of each septum.

Physiology of Excretion Earthworms are ammonotelic animals as the urine contains ammonia. The urine also contains small amount of urea, uric acids and amino acids. The nephridium is highly vascular. The nephriclial cells extract nitrogenous wastes from the blood. The septalnephridium collects wastes from the coeloinic fluid through the neplutostome. The nitrogenous wastes are sent out through the nephridiopores. In the case of entero nephric nephridia, the wastes are poured into the gut from where they are passed out through the anus.