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Earthworm: Morphology, Clitellum, Structures Common to Most Segments, Practice Problems and FAQs

Earthworm: Morphology, Clitellum, Structures Common to Most Segments, Practice Problems and FAQs

You all might have seen earthworms in the rainy season. Do you remember those creepy things that come out normally during the rainy season? Once the rain hits the ground, it creates lots of vibrations on the soil. These vibrations cause the earthworms to come out of their burrows in the soil to the surface. This is due to the fact that the earthworms can easily move greater distances across the soil surface when it is wet. They also need a moist surface for survival as they respire through the moist skin.

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                                                                 Fig: Earthworm

You know earthworms move through the soil and loosens it. This increases aeration in the soil and also aids in the movement of water. Hence they are called nature's ploughmen. Farmers use earthworms also for making manure by a process called vermicomposting. Now you are interested in knowing more about earthworms. Let’s understand more about the structure of earthworms in this article.

Table of contents

  • Morphology
  • Clitellum
  • Structure common to most segments
  • Practice problems
  • FAQs

Morphology

Earthworms belong to the phylum Annelida and class Oligochaeta under the Animal Kingdom. It lives in burrows. For type studies Pheretima posthuma is commonly used. It is commonly known as nightcrawlers or farmer’s friends. Earthworms have long and cylindrical bodies. They are reddish brown in colour and are terrestrial invertebrates. They are the inhabitants of the uppermost layer of moist soil. The body is metamerically segmented with 100 to 120 segments. These segments are known as metameres.


                           Fig: Segmented body

The dorsal surface of earthworm bears a median line. It is the dorsal blood vessel.


                                              Fig: Dorsal blood vessel

The ventral side of earthworm is characterised by genital openings. It includes male genital apertures and a female genital aperture.


                                                                Fig: Genital openings

Clitellum

It is a bulged and prominent dark band which is present in mature worms. It possesses glandular tissues and is present between 14th to 16th segments. Clitellum secretes a fluid which on exposure to air hardens to form the egg case or cocoon.


                                                                  Fig: Clitellum

Body divisions based on clitellum

The body of earthworm is divided into three regions on the basis of clitellum as follows:

  • Pre-clitellar segments
  • Clitellar segments
  • Post-clitellar segments


                                        Fig: Body divisions based on clitellum

Pre-clitellar segments

Pre-citellar segments consist of three structures. They are as follows:

  • Peristomium
  • Spermathecal apertures
  • Female gemital pore
Peristomium

The first segment of earthworm is known as the peristomium. It contains mouth and prostomium. Prostomium is a fleshy lobe overhanging the mouth. Prostomium is sensory in function.


                                      Fig: Peristomium

Spermathecal apertures

There are four pairs of spermathecal apertures in the ventro-lateral side. They are located on the intersegmental groove between 5th to 9th segments. They are responsible for storage of sperms that are released during copulation.


                           Fig: Spermathecal apertures

Female genital pore

Female genital pore is present in the mid-ventral line of the 14th segment. It is a single median aperture.


                                                      Fig: Female genital pore

Clitellar segments

Clitellum is a bulgy part of the earthworm. It secretes mucus and helps during reproduction. It includes the 14th to 16th segment.


                                                   Fig: Clitellum

Post-clitellar segments

The post-clitellar segments consist of male genital pores. These are present in pairs on the ventro-lateral side. It is present on the 18th segment. Anus is present on the last segment at the posterior end. Genital papillae are also present.


                                                       Fig: Male genital pore

Anus

It is a slit like aperture present in the last segment called anal segment or pygidium at the posterior end.


                                                                          Fig: Anus

Genital papillae

These are one pair of projections present each on the 17th and 19th segments. They lie in line with the male genital apertures. They possess cup-like depressions on them.


                Fig: Genital papillae

Structures common to most segments

There are two structures that are common to most segments. They are as follows:

  • Nephridiopores
  • Setae
  • Dorsal pores

Nephridiopores

They are considered as a part of the excretory system. They expell nitrogenous wastes from the body of the earthworm and are present in all segments except the first two segments.


                                                                  Fig: Nephridiopores

Setae

Setae are minute and S-shaped hair-like bristles. These are present in all segments except 1st segment, last segment, and clitellum. Setae lies in setal sacs. They can be extended or retracted. They help in locomotion.


                                                                          Fig: Setae

Dorsal pores

On the dorsal surface of the earthworm there are present minute pores called dorsal pores. These are present in the intersegmental grooves between all the segments except the first twelve segments and the last segment. Coelomic fluid comes out through these pores to keep the body moist.


                                                                 Fig: Dorsal pores

Practice problems

1. The earthworm's _______ segments have a noticeable black ring of glandular tissue called clitellum.

  1. 14th to 16th
  2. 14th to 18th
  3. 15th to 19th
  4. 15th to 17th

Solution: Metameric segmentation can be seen in the earthworm. An earthworm has 100 and 150 segments. Clitellum is a thick glandular tissue found between the earthworm's 14th and 16th segments. During the breeding season, clitellum secretion is very active. It secretes mucus and helps during reproduction. Hence the correct option is ‘a’.


                                                        Fig: Clitellum

2. Determine which of the following statements concerning earthworm setae is correct.

  1. Setae are the L- shaped partition in earthworm
  2. Setae are present in all the segments of the earthworm
  3. Setae are absent in the first, last and clitellar segments of the earthworm
  4. The primary role of setae is sensory

Solution: Setae are S-shaped structures seen in the earthworm's epidermal pits. They can be found throughout the earthworm's body, with the exception of the first, last, and clitellar segments. Setae lies in setal sacs. They can be extended or retracted. They help in locomotion. Hence, the correct option is c.


                                                                               Fig: Setae

3. Identify the structure in the given image of earthworm:

Answer: The given image shows clitellum which is a dark band that is bulged and noticeable. It is found in fully grown earthworms. It is made up of glandular tissues. It can be found between the 14th and 16th segments. It secretes mucus and helps during reproduction.

4. Determine the correct statement regarding nephridiopores.

  1. They pump blood
  2. They excrete out waste products
  3. They secrete enzymes
  4. They secrete mucus

Solution: Nephridiopores are a component of the excretory system. The earthworm's body expels nitrogenous wastes through these nephridiopores. These are present in all segments except the first two segments. Hence, the correct option is b.


                                                                          Fig: Nephridiopores

FAQs

1. Define peristomium?
Answer:
Peristomium is the first segment of an earthworm. It has a mouth and prostomium in it.


                                            Fig: Peristomium

2. Can an earthworm bite a person?
Answer:
Earthworms cannot bite or sting. They are nature’s ploughmen and help farmers in increasing the fertility of the soil.


                               GIF: Earthworms

3. What is the habitat of an earthworm?
Answer:
Earthworms are terrestrial organisms. They normally live in the top layers of soil. But they burrow deeper layers of the soil to find moisture when the Earth dries out or freezes. They move through the soil and loosens it. This increases aeration in the soil and also aids in the movement of water. They also improve drainage of water and bring nutrients to the soil surface.


                    Fig: Habitat of earthworm

4. What is the lifespan of an earthworm?
Answer:
Earthworms may live up to 8 years. The size for an earthworm depends on the species. It ranges from less than half an inch to nearly 10 feet. For example, the longest known earthworm in the world, stretching to more than 2m, is the Australian Giant Gippsland earthworm Megascolides australis.


          Fig: Megascolides australis

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