Phylum Annelida, Practice Problems and FAQs

After a rainfall, you must have observed some small crawling creatures in your garden, commonly known as earthworms. Ever wondered where they come from all of a sudden? Earthworms require constant moisture. If they are above ground, they need moisture, otherwise they usually get dehydrated. When it rains, the surface is sufficiently damp for worms to survive and stay hydrated. They can roam around, locate mates more easily or can move to another territory. So they come outside after rainfall.

Fig: Earthworm

Since earthworms are residents of soil, they bond really well with farmers. They are called farmer's friends. But why? Earthworms make the soil fertile and never let down farmers when it comes to helping in growing crops. Hence, farmers consider earthworms as their best buddies. This favourite worm of the farmers then belongs to which phylum? Any ideas?

If we closely observe the earthworm we can find only a segmented body. Apart from that it is simply a worm. But earthworm represents one of the large phylum of invertebrates, which is Annelida. So now we are going to meet some more interesting animals of phylum Annelida and will find out what are the specific features of the worms that belong to this particular phylum.

Phylum Annelida

The word ‘Annelida’ has been derived from two words ‘annulus’ meaning little ring and ‘lidos’ meaning form. Annelids differ from other worms by their segmentation in the body. Now let’s discuss the general characteristics of phylum annelida.

General characteristics of phylum Annelida

Habitat

Annelids thrive in both aquatic as well as terrestrial environments.

Fig: Habitat of annelids

Habit

Annelids are either free-living or parasitic.

Symmetry

Annelids are bilaterally symmetrical (the body can be divided into two equal halves through a single plane passing through the centre of the body).

Fig: Bilateral symmetry in annelids

Level of organisation

They have an organ system level of body organisation (division of labour can be seen between different organ systems).

Germ layer organisation

Annelids are triploblastic (have three germ layers which include external ectoderm, middle mesoderm and internal endoderm).

Fig: Germ layers

Presence of coelom

Annelids are coelomates, that means a body cavity is present between the body wall and the gut wall which is lined by mesoderm or coelomic epithelium. They are the first coelomate animals. They are called schizocoelomates, as the coelom or body cavity is divided into compartments by internal partitions.

Fig: Triploblastic germ layer organisation in annelids

Segmentation

Body of annelids is externally and internally divided into segments with serial repetition of organs in the segments and a coordination between them to attain a specific function. These body segments are called metameres. The external division of the worm can be observed due to the presence of ring-like grooves along the body called annuli. Internally the segments are separated by transverse septa. Structures like the gut, blood vessels and nerve extend through the entire length of the body and almost all the segments have a serial repetition of their constituent units.

Fig: Metameric segmentation in annelids

Body wall of annelids

The body wall consists of thin, non-cellular cuticles and is usually moist. A single layer of epidermis is present below which lies above the longitudinal and circular muscle layers.

Locomotion in annelids

Annelids can move by using their muscles and also by some unjointed appendages like parapodia or setae.

By muscular aid

There are two types of muscles that help in locomotion in annelids:

Longitudinal muscles

The muscles run lengthwise along the body which helps the worm to decrease in length and increase in width.

Circular muscles

Muscles that encircle the body of the annelid helping the worm to increase its length and decrease its width.

Fig: Muscles involved in locomotion in annelids

Locomotion is facilitated by the alternate contraction of the longitudinal and circular muscles. The contraction of inner longitudinal muscles of a section of the worm causes an increase in width in the preceding metameres. Subsequent contraction of outer circular muscles causes reduction in width accompanied by an increase in length which helps the worm to proceed forward.

GIF: Locomotion in earthworm

By parapodia

In aquatic annelids, like Nereis, special unjointed appendages are present called parapodia. These are present on the lateral sides of the worm. It helps in swimming.

Fig: Parapodia in Nereis

GIF: Locomotion in Nereis

By setae

In earthworms, bristles like structures called setae are present. They are located on each segment of the body of the earthworm. They prevent them from slipping backwards.

Organ systems in Annelids

Now we are going to discuss different organ systems like the digestive system, circulatory system, nervous system, and excretory system of phylum Annelida.

Digestive system in annelids

Annelids have a complete digestive system. This is because they have two different openings, one for ingestion (mouth) and the other for egestion (anus).

Fig: Digestive system in annelids

Circulatory system in annelids

Annelids have a closed circulatory system, which means the transport of blood occurs through enclosed spaces such as blood vessels.

Fig: Circulatory system in annelids

Nervous system in annelids

Nervous system in annelids is made up of ganglia present in pairs. The paired ganglia are connected to the double nerve cord present on the ventral side of the body by means of lateral nerves.

Fig: Nervous system in annelids

Excretory system in annelids

Specialised organs composed of coiled tubular structures known as nephridia are present in annelids. Nephridia consists of a nephrostome. It is a ciliated funnel which hangs normally into the coelom which from there leads to the nephridial duct. These aid in the osmoregulation and excretion of waste products from the body.

Fig: Excretory organ in annelids

Mode of reproduction and development of annelids

Sexual nature

They are monoecious or dioecious.

Monoecious

It is a condition where there is presence of both sexes in the same organism. For example, leeches and earthworms.

Dioecious

It is a condition where there is presence of sexes in separate organisms. For example, Nereis.

Mode of reproduction

They exhibit sexual reproduction mediated by fusion of gametes. Even though earthworms are hermaphrodites or have both sex organs, they rarely self-fertilise. This is due to non-synchronous maturation of male and female sex organs. For example, earthworms are protandrous (male reproductive organs mature earlier than female reproductive organs). Fertilisation of gametes takes place inside the parental body (female body) and is termed as internal fertilisation.

Fig: Sexual reproduction in earthworm

Mode of development

Development is mostly direct (the embryo resembles the adult) except in a few cases such as Nereis.

Classification of phylum Annelida

The animals of this phylum are classified into 5 classes on the basis of presence or absence of setae.

Class Polychaeta

There are numerous setae present in the parapodia of these animals. Parapodia are the structures used for locomotion and respiration. Clitellum is absent. Examples include Nereis (sand worm), Aphrodite (sea mouse), Chaetopterus (paddle worm ) etc.

Fig: Nereis

Class Oligochaeta

They have setae for locomotion, but these setae are few in number. Clitellum is permanently present. Examples include Pheretima, Lumbricus, Megascolex, Tubifex etc.

GIF: Pheretima (earthworm)

Class Hirudinea

They do not have parapodia and setae for locomotion. They are ectoparasites, so have suckers present at both ends. A specialised connective tissue is present below the muscular layer called botryoidal tissue. It is made up of adipose tissue and used for storage of fat. Examples include Hirudinaria, Hirudo, Pontobdella etc.

GIF: Hirudinaria (Leech)

Hirudin

It is a peptide naturally occurring in the salivary glands of blood-sucking leeches. Examples include Hirudo medicinalis. This compound possesses anticoagulant properties. It is important for the feeding habit of leeches. They feed on blood. This compound allows the blood to flow from the body of the host after the worm makes the initial puncture on the skin.

Class Archiannelida

Parapodia and setae are absent. Clitellum is also absent. External segmentation is not very distinct. Examples include Polygordius, Dinophilus etc.

Class Echiurida

They have no external and internal segmentation. Rarely, there are setae for locomotion. Examples include Bonellia, Echiurus etc.

Practice Problems

Q1. I am a blood sucking annelid. I need blood to grow and reproduce. I help in sucking out bad blood from humans and I am also used to treat diseases in humans. Identify who I am?

A. Nereis
B. Leech
C. Earthworm
D. None of the above

Solution: Among the options given, leech is an annelid that sucks blood for growth and reproduction and is commonly used in treating diseases in humans. Hence, the correct option is b.

Q2. While digging soil, a farmer found a lot of crawling creatures. Farmer’s friend told him that the worms are beneficial in increasing the fertility of the soil. Farmer also observed that the worms had a segmented body and fed on dead and decaying matter. Identify the organism and its respective phylum from the options given below.

A. Earthworm, Annelida
B. Tapeworm, Platyhelminthes
C. Nereis, Annelida
D. Ascaris, Aschelminthes

Solution: Earthworm, a member of the phylum Annelida, helps in increasing the fertility of soil and hence is also called farmer’s best friend. They feed on dead and decaying matter and help in decomposition. Earthworms have a segmented body and the segments are known as metameres. Tapeworm, Nereis, and Ascaris do not play any role in increasing soil fertility. Hence, the correct option is a.

Q3. Identify the odd one out with respect to Annelids among the following.

A. Presence of metameres
B. Presence of parapodia
C. Presence of coelom
D. None of the above

Solution: Annelids are segmented worms. Their body is divided into segments internally as well as externally called metameres. Annelids like Nereis use parapodia which are special unjointed appendages used for locomotion and swimming. They are coelomates, which means a body cavity between the body wall and the gut wall lined by mesoderm or coelomic epithelium is present. So the presence of metameres, presence of parapodia and presence of coelom are the characteristic features of annelids. Hence, the correct option is d.

Q4. Which of the following is common to Nereis, Aphrodite, or Chaetopterus?

A. They belong to class Polychaeta of phylum Annelida
B. They have numerous setae present in parapodia
C. They do not have clitellum
D. All of the above are correct

Solution: Nereis, Aphrodite, Chaetopterus are the members of class Polychaeta of phylum Annelida. Characteristic features of class Polychaeta are the presence of numerous setae in parapodia and absence of clitellum. Hence, the correct option is d.

FAQs

Q1. Do earthworms regenerate when they are cut into two?
Answer: Earthworms show regeneration. If we cut them into pieces each of which grows a new head and tail. Rarely, a fragment may grow heads at both ends too.

Q2. Why is hirudin considered important?
Answer: Hirudin is a polypeptide discovered in the salivary glands of the leech Hirudo medicinalis, and it is one of the most powerful thrombin inhibitors found in nature. The specific nature of the hirudin-thrombin interaction has been studied using a variety of biochemical and molecular biologic approaches. The anticoagulant hirudin is derived from the body tissues of the European medicinal leech (Hirudo medicinalis), while another chemical produced from Amazonian leeches is used to dissolve existing blood clots.

Q3. What is a sand worm?
Answer: Nereis is also known as a sand worm. Alitta virens (also known as sandworm or king ragworm) is an annelid worm that burrows in moist sand and mud. Michael Sars, a biologist, was the first to describe it in 1835. It belongs to the Nereididae family as a polychaete.

Q4. Do earthworms have heart?
Answer: The earthworm, which is possibly the most well-known of all the annelids, possesses five aortic arches, which are heart-like structures. The aortic arches, along with the dorsal and ventral veins, allow blood flow through the closed circulatory system and to both ends of the body.

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