Excretory System of Earthworm

In earthworms ( Pheretima posthuma ),  excretion occurs by segmentally arranged coiled tubules called nephridia. 

These unbranched tubes open up into the coelom by a ciliated funnel called a nephrostome. 

Such nephridium with nephrostome are called metanephridium.

Also, in Pheretima, nephridia are small-sized called micro nephridia or metanephridia ( Nereis and leeches have comparatively large-sized metanephridia).

Nephridia occur in all segments of the body except in the first two.

Excretory System of Earthworm Diagram 

According to their location in the body, nephridia are divided into 3 types :

Types of Nephridia :

I. Septal nephridia 

II. Integumentary nephridia 

III. Pharyngeal nephridia 

Septal nephridia: 

Behind the 15th segment on the anterior and posterior faces of the septa occurs the septal nephridia arranged in two rows on each face of the septa. A row on either side of the intestine. There may be 80-100 of them per segment.

Septal nephridia ( entire) and L.S. of Nephrostome 

The terminal duct of all nephridia in each row opens into a septal excretory canal. Two such canals run upwards along the septum and open into a pair of supra-intestinal excretory ducts, situated side by side in the mid-dorsal line above the intestine and below the dorsal vessel extending from the 15th to the last body segment. These longitudinal excretory ducts open in the intestine in each segment through narrow ductules near the septum. Since the septal nephridia open into the gut, they are said to be enteronephric.

Integumentary nephridia:

Integumentary nephridia are small in size without a funnel or nephrostome and are closed type with no opening into the coelom. 

They are V-shaped with a short straight lobe and twisted loop. Its lumen has two ciliated canals. 

They are scattered on the entire inner or parietal surface of the body wall from the 3rd to last segment. There are about 200- 250 nephridia in each segment which increases to more than 2000 in the clitellar segment. 

Each nephridium opens by nephridiopore on the outer surface of the body wall. 

They are mesonephric micro nephridia as they discharge waste externally.

Pharyngeal nephridia :

Pharyngeal nephridia are similar in size and structure to the septal nephridia but they lack coelomic funnels. In the segment 4th 5th and 6th, they occur in paired masses lying beside the pharynx and oesophagus. Their terminal ducts join together and give rise ultimately to one pair of ducts in each of segments 4th 5th and 6th. 

The ducts of the pharyngeal nephridia of the 6th segment open into the buccal cavity in the second segment whereas the ducts of the nephridia of segments 4th and 5th open into the pharynx. 

The pharyngeal nephridia are intertwined with the blood glands.

* Note: For more detailed explanation check out the link.

Nephridia: Types, Function and Physiology of Excretion in Annelids

Physiology of excretion : 

Nephridia are chiefly organs of osmoregulation. They are richly supplied with blood vessels and lined by cilia.  The epithelial cells extract waste material,  mainly urea from the blood. This material travels from the body of the nephridium to its terminal duct which discharges it either directly to the exterior or into the alimentary canal. 

Function of nephridia : 

The nephridia covers three functions

• filtration 
• reabsorption and 
• chemical transportation.

In open septal nephridia, filtration takes place at the funnel whereas in integumentary and pharyngeal nephridia,  it takes place through the walls of nephridia. This also occurs in septal nephridia where its parts are quite narrow.

Integumentary nephridia being exonephric, discharges excretory material to the outer body surface through nephridiopore. Pharyngeal and septal nephridia being enteronephric, discharge them into gut lumen from where they are eliminated with faeces.

 The filtrate consists of blood plasma minus the colloids together with coelomic plasma. 

While passing from the fine vessels in the nephridia, large amounts of useful substances like glucose, amino acids, phosphates, chlorides etc, are reabsorbed. This process is called selective reabsorption. 

Finally, protein-free urine containing ammonia, urea and allantoin is put out.

Earthwormsm are less ureotelic than other terrestrial animals. 

 The urine of earthworm is hypotonic,  as it contains a far lower concentration of salt than the blood and coelomic fluid. It contains 40% urea, 20% ammonia, 40% amino acids and other nitrogenous compounds but no uric acid or urates.

Some excretory matter gets rid of when the coelomic fluid passes out through the dorsal pores.

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Also :

Nervous system of Earthworm : Pheretima posthuma 

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