Question

Which is the excretory structure present in earthworm? Describe

their types along with their name & number of segment where they

are located?​

Answer 1

Answer:

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.

Pheretima. Different types of nephridia and general plan of their distribution

(i) Nephrostome: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.

(ii) Neck:

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.

Nephrostome of Earthworm

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.

Pheretima. The arrangement of septal nephridial system in relation to the intestine

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.

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.

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