Excretory System of Earthworm | Zoology QnA MCQs For Students

Excretory System of Earthworm- Zoology Notes for Students

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.

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 

Diagram of Excretory system of Earthworm showing location of different types of 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.

Diagram of Septal nephridia of Earthworm 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.

Earthworms 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.

Question: 

Write short notes on structure and function of nephridia in earthworm. 

Answer:

Nephridia are the unit of excretion in earthworm. They are present in each segment of earthworm except in the first two. They are coiled unbranched tubules which open up into coelom by a ciliated funnel called nephrostome. 

Such a nephridia with nephrostome are called metanephridium.

In earthworm, nephridia are small sized called micronephridia or metanephridia (Neries and leeches have comparatively large-size metanephridia).

 Depending upon location they are of three types 

1. Integumentary nephridia 

• Small size without funnel. 
• Closed type (with no opening into the coelom) 
• V-shaped with short straight lobe and twisted loop with two sea with no two civiliated canals 
• Present scattered in entire inner or parietal surface of body wall from 3rd to last segment. 
• Numbers around 200 to 250 in each segment and increased to more than 2000 in clitellar segment. 
• Opens by nephridiopore on outer surface of bodywall. 
• They are mesonephric micronephridia, as they discharge waste externally.

2.Septal nephridia 

• Present from 15th to last segment on anterior and posterior faces of septa. 
• Arranged in two raws on each face, on either side of intestine.

• There maybe 80-200 per segment.

Structure: 

The terminal duct of nephridia in each row opens into septal excretory canal which run upwards and open into pair of Supra-intestinal excretory duct which are situated side by side in mid-dorsal line above the intestine below the dorsal vessel.

• They open in intestine in each segment through narrow ducts near the septum.
• Septal nephridia are called entero nephric since they open into the gut. 

3. Pharyngeal nephridia 

• Situated in 4th, 5th and 6th segment, in pair, beside the pharynx and oesophagus.
• Their terminal ducts join to form one pair of duct in 4th, 5th and 6th segment.
• Ducts of 6th segment open into buccal cavity in second segment. 
• Ducts of 4th and 5th segment open into pharynx.
• The pharyngeal nephridia are intertwined with the blood gland. 

Function of nephridia 

Nephridia are primarily excretory in function but secondarily they may serve as conveyors of genital products to the exterior.

 Nephridia are the units of excretion in earthworm ( annelids). They remove metabolic waste from the animal body. 

They function in following steps, 

Filtration

In septal nephridia, it takes place at the funnel where as in integumentary and pharyngeal nephridia it takes place through the  walls of nephridia.

 Reabsorption 

The filtrate consists of blood plasma minus the colloids together with coelomic plasma,  which while passing through fine vessels in nephridia are reabsorbed. 

Substances like glucose, amino acids, phosphates, chlorides etc, are reabsorbed by the process of selective reabsorption. 

Chemical reabsorption 

• Finally protein free urine, containing ammonia, urea and allantoin is put off. 
• The urine of earthworm is hypotonic containing 40% urea, 20% ammonia, 40% amino acid and other nitrogenous compounds but no uric acid or urates.
• The excretory matter gets rid of when the coelomic fluid passes out through the dorsal pores. 

Question: Write short note on, exonephric and enteronephric nephridia with examples 

Answer:

Exonephric nephridia 

Nephridia are termed exonephric or metanephric when they open directly to the exterior through nephridiopores. 

For example Meganephridia of Neries, Hirudinaria and Lumbricus and integumentary micronephridia of Pheretima. 

Enteronephric nephridia 

Nephridia are termed enteronephric when they lack nephridiopores and open into the excretory canal or alimentary canal.

 Thus, nephridia without nephridiopore is called enteronephridia. 

For example, septal and pharyngeal nephridia of earthworm.

The enteronephric system helps in the conservation of water in body because water present in the excretory product is again reabsorbed in the intestine.

Question:

Write short note on 'Forests of nephridia'.

Answer:

On the body wall (integument) of earthworm, in each segment accept the first two segments, are found scattered nephridia called integumentary nephridia. There are around 200-250 in segments other than 14th,15 th and 16th segment i.e. clitellar segments.

In clitellar region,their number is about ten times more than that in other segments. Thus clitellar segments are often called as the 'Forest of Nephridia'.

 The integumentary nephridia are smallest among three types of nephridia.

They discharge their contents directly to the outside hence they are called ectonephric (or exonephric) nephridia.

They help to keep the skin moist for cutaneous respiration. 

Question:

Write short note on 'Chloragogen cells'.

Answer:

They are also called yellow cells. These cells are excretory in function. The Chloragogen cells take up excretory matter from the blood capillaries of the gut and from the coelomic fluid of the coelom. When the cells are filled with excretory matter, they are either taken by septal nephridia or pass directly to the outside on the surface of the skin through the dorsal pores.

They also store glycogen and fat. Thus, they are supposed to be analogous to the liver of vertebrates.

Some of the nitrogenous excretory substances like guanin are extracted from the bloodstream by chloragogen cells. 

These cells collect and store excretory products and on becoming heavily laden with excretory materials, they pinch of into the coelomic fluid from where they are eliminated through dorsal pores or by nephridiopores.

These small but numerous cells have bulging. when stained with iodine solution, they become yellow.

Question:

Write short note on ' Adaptations of excretory system to environment in earthworm '.

Answer:

The adaptations shown by excretory system to environment in earthworm are,

• Different types of nephridia ( septal, integumentary and pharyngeal ) contribute to excretion and osmoregulation based on location and environmental demand. 
• Selective reabsorption is an essential adaptation for terrestrial life, were moisture conservation is critical. 
• Earthworms can alter there excretion, based on soil condition.In moist condition they excrete mainly urea (ureotelic), but in very wet condition it excrete ammonia (ammonotelic), thus demonstrating adaptation to environment. 
• The water and salt balance is continuously regulated by nephridial system via osmoregulation, ensuring that the earthworm does not dehydrate in dry soil nor accumulate access fluid in wet condition. 
• Aestivation during dry or hot climate is also an adaptation where earthworm burrow deeper, coil up and secrete protective mucus, to reduce metabolic rate and water loss.

Multiple Choice Questions (MCQs)

1. The excretory organs of earthworm are:

(a) Coelum

(b) Flame cells

(c) Nephridia

(d) Gizzard

Answer: (c)

2. Earthworm excretes mainly:

(a) Uric acid

(b) Ammonia and urea

(c) Creatine

(d) Water only

Answer: (b)

3. Which of the following segments contains forests of nephridia?

(a) Pharyngeal

(b) Clitellar

(c) Typhlosolar

(d) Intestinal

Answer: (b)

4. Which nephridia type discharges wastes directly outside the earthworm’s body?

(a) Pharyngeal

(b) Septal

(c) Integumentary

(d) Enteronephric

Answer: (c)

5. In well-fed earthworm, the major form of excreted nitrogenous waste is:

(a) Ammonia

(b) Urea

(c) Uric acid

(d) Amino acid

Answer: (a)

6. Enteronephric nephridia expel excretory products into:

(a) The environment

(b) The coelomic fluid

(c) The alimentary canal

(d) The skin

Answer: (c)

7. In earthworms, which cell is responsible for production of urea?

(a) Chloragogen cell

(b) Flame cell

(c) Epidermal cell

(d) Parietal cell

Answer: (a)

Next: Most important question asked most often.

Flight Adaptation in Birds

Also :

Nervous system of Earthworm : Pheretima posthuma 

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Rekha Debnath.

About the Author: This educational content on Zoology is written by Rekha Debnath, M.Sc. & M.Phil. in Zoology, with a focus on university-level academic topics. Read the full Author Credentials and Background here.