Excretory System of Nephridia (Earthworm)!
These are of three types according to their location in the body:
1. Septal nephridia;
2. Integumentary nephridia
3. Pharyngeal nephridia.
1. Septal Nephridia:
These are found situated on the inter-segmental septum between 15th and 16th segments to the posterior side of the body.
Each septum bears nephridia on both the surfaces arranged in semicircles around the intestine, two rows in front of the septum and two behind it. Each septum has about 40 to 50 nephridia in front and the same number behind, so that each segment possesses 80 to 100 septal nephridia except the 15th segment which has only 40 to 50 nephridia. These are not found in the segments up to 14th.
The septal nephridia may be considered typical of all the nephridia of Pheretima. Each septal nephridium (Fig. 66.22) consists of nephrostome, neck, body of nephridium and the terminal duct.
It is also known as ciliated funnel or nephridiostome. It is the proximal flattened funnel-shaped structure of the nephridium lying in the coelom.
It has an elliptical mouth-like opening leading into an intracellular canal of the large central cell, the margins of the opening are surrounded by a large upper lip and a smaller lower lip. The lips are provided with several rows of small ciliated marginal cells and the central canal is also ciliated.
The nephrostome leads into a short and narrow ciliated canal forming the neck. It joins the nephrostome to the body of nephridium.
(iii) 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.
(iv) Terminal Duct:
It is short and narrow with a terminal excretory duct. It joins the nephridium with septal excretory canal.
Relation of septal nephridia with intestine:
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 (Fig. 66.24). 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.
2. Integumentary 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.
3. Pharyngeal 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.
Physiology of Excretion:
Like other animals, in earthworms also, the protein catabolism results in the formation of nitrogenous waste substances like certain amino acids, ammonia and urea.
Uric acid is not found in the earthworms. However, the amino acids are degraded to form free ammonia and the urea is synthesised in the chloragogen cells which are released into the coelomic fluid and also in the blood for its removal. Free amino acids are not excreted but traces of creatinine occur in the urine.
Moreover, the nitrogen excreted in different forms in a well fed worm is about 72% NH3, 5% urea and remaining other compounds, while in a starved worm NH3 8.6%, urea 84.5% and remaining being other compounds. But generally, the excretion is 42% NH3, 50% urea, 0.6% amino acids and remaining being other compounds.
So, we can say that in a well fed earthworm, NH3 predominates the nitrogenous excretory wastes, hence, it is ammonotelic, while a starved one is ureotelic.
An earthworm excretes the nitrogenous wastes in the form of urine which generally contains urea, water, traces of ammonia and creatinine. Nephridia excrete these substances from the body of earthworm. The various excretory wastes from the coelomic fluid are drawn into the nephrostomes of septal nephridia or into the excretory canals of other nephridia along with some other useful substances.
These products are either discharged into the intestine (by enteronephric nephridia) or outside by the nephridiopores (by exonephric nephridia). The body of nephridia also absorbs some wastes. However, the useful substances are reabsorbed and the passing out waste remains concentrated for various nitrogenous compounds.
The excreted waste substances are removed out from the body with faeces. The nephridia, in addition to excretory, are also osmoregulatory in function.
The nephridia help in conserving water by reabsorption from the excreted products during summers and winters, so they pass hypertonic urine in relation to blood. During rainy season, the urine is dilute due to lesser reabsorption of water. The enteronephric nature of nephridia provides another device for conserving water.