Table of Contents
In this excretory system organs of earthworms post we have briefly explained about septal nephridia, structures of septal nephridia, arrangement of excretory system of earthworm, integumentary nephridia ,pharyngeal nephridia and excretory system of earthworm physiology.
Excretory system of earthworm alimentary canal is a long tube that runs from the beginning to the end of the body. Earthworms eat leaves and decaying organic materials that have been mixed up with the soil.
The parts of the alimentary canal and their secretion differ from other creatures depending on their food. The alimentary canal starts in the mouth (buccal or oral cavity) and runs through the pharynx, oesophagus (5-7 segments), muscular gizzards (8-9 segments), stomach (9-14 segments), intestines, and lastly the anus. As the food particles pass through the various compartments of the alimentary canal, they are gradually digested.
The soil particles and other debris are ground up by the muscular gizzards, and the humic acid in the hummus is neutralised by the calciferous glands in the stomach. In the gut, typhlosole (26-35 segments) increases the surface area available for absorption.
Excretory System of Earthworm
It is present from 15th segment on either side of the septa.40 to 50 nephridia are present on either side of septa. so,80 to 100 nephridia are present in each segment. Each nephridium has a nephrostome, neck, straight lobe, twisted loop and terminal duct. The twisted loop has two parts- the proximal and distal parts.
There are four, three, two, and one intracellular canal in straight lobe, proximal part, distal part and terminal duct respectively. Septal nephridia collects excreta from coelomic fluid through nephrostome and from blood through intracellular excretory canals.
They collected excreta and pass it to septal excretory canal through terminal duct. Septal excretory canal opens into supra intestinal excretory duct which in turn pours the excreta into intestine. In this way septal nephridia helps in conservation of water which is absorbed in the intestine. So, septal nephridia are called enteronephric nephridia.
A typical septal nephridium consists of a) nephrostome, b) neck c) body of nephridium, and d) terminal duct.
It’s also known as nephridiostome or ciliated funnel. It is the nephridium’s proximal flattened funnel-shaped structure in the coelom.
It has an elliptical mouth-like opening that leads into a big central cell’s intracellular canal; the opening’s borders are encircled by a wide top lip and a smaller lower lip. Several rows of tiny ciliated marginal cells line the lips, and the central canal is similarly ciliated.
The nephrostome leads into a short and narrow ciliated canal forming the neck. It joins the nephrostome to the body of nephridium.
The body of nephridium has two parts a short straight lobe and a long twisted loop. The loop is formed by two limbs the proximal limb and the distal limb. Both these limbs are twisted spirally around each other; the number of twists varies from nine to thirteen.
The neck of nephridium and the terminal duct join together and remain connected with the proximal limb of the twisted loop, while the distal limb becomes the straight lobe. Internally the nephridium is made of a connective tissue matrix having long coiled nephridial duct forming loops.
There are four such canals in the straight lobe, three in the lower part and two in the upper part of the limbs of twisted loop. Two canals of the straight lobe out of the four are ciliated like the ciliated canal of the neck.
d. Terminal Duct
It is short and narrow with a terminal excretory duct. It joins the nephridium with septal excretory canal.
The nephridia hang freely in the coelom and are attached only by their terminal ducts. They open by their terminal ducts into two septal excretory canals lying on the posterior surface of the septum, one on each side of the intestine, each begins ventrally but dorsally it opens in the supra-intestinal excretory duct of its own side.
The supra-intestinal excretory ducts are two parallel longitudinal canals lying above the gut and below the dorsal vessel. These excretory ducts begin from the 15th segment and run to the last segment, they communicate- with each other for a short space behind each septum, then either the right or the left duct opens by a ductule into the lumen of the intestine near the septum.
Thus, each segment has one such opening into the intestine of either the left or the right supra-intestinal excretory duct. The waste collected by the nephridia is discharged through the excretory canals and ducts into the lumen of the intestine. Such nephridia opening into the intestine are called enteronephric nephridia.
In each segment of the body from 7th to the last segment, numerous nephridia are found attached inside the lining of the body wall. These are called integumentary nephridia which are about 200-250 in each segment except the segment of the clitellar region where they number 2,000-2,500 in each segment. These nephridia are small-sized, without nephrostome and without any opening into the coelom.
Hence, they are called closed type of nephridia. Each integumentary nephridium is V-shaped with a short straight lobe and a twisted loop, its lumen has two ciliated canals. Each nephridium opens by a nephridiopore on the outer surface of the body wall directly. Since the integumentary nephridia discharge the excretory wastes directly outside, hence, they are called exonephric nephridia.
These nephridia lie in three paired tufts, one on either side of the anterior region of the alimentary canal in the segments 4th, 5th and 6th. The tufts of pharyngeal nephridia also contain blood glands.
Each pharyngeal nephridium is about the size of a septal nephridium but it is of the closed type having no funnel or nephrostome. It has a short straight lobe and a spirally twisted loop, its lumen has ciliated canals. Ductules arise from each nephridium and unite to form a single thick- walled duct on each side in each segment.
The two ducts of nephridia of segment 6th open into the buccal cavity in segment 2nd and the paired ducts of nephridia of segments 4th and 5th open into the pharynx in segment 4th.
These nephridia also discharge their wastes into the alimentary canal and are, therefore, enteronephric but such enteronephric nephridia which open into the anterior region of the alimentary canal (buccal cavity and pharynx) are called peptonephridia because they may have taken the function of digestive glands.
Recently it has been reported that the pharyngeal nephridia of P. posthuma produce a variety of enzymes like amylase, chimosin, prolinase, prolidase, dipeptidases, aminopeptidase, lipase, etc., which hydrolyse various foodstuffs. Thus, such nephridia work like the salivary glands.
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 live 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.