Have you ever seen an earthworm in your garden or on your farm? The Phylum Annelida includes earthworms. The Phylum Annelida has a wide range of body forms. Annelids can be found in terrestrial, marine, and freshwater settings, however in terrestrial ecosystems, the presence of water or humidity is critical for their existence. Earthworms, polychaete worms, and leeches are all members of the Phylum Annelida. Because of their metameric segmentation, they are also known as segmented worms. This phylum’s animals form parasitic and commensal symbioses with other species in their environment. The phylum Annelida contains approximately 16,500 species. Earthworms, polychaete worms, and leeches are all members of this phylum. Because of its important property of metamerism, or real segmentation, annelids are commonly referred to as “segmented worms” throughout embryonic stages.

In this article, we will discuss in detail Phylum Annelida: Characteristics, Classification & Examples. Continue reading to know more.

Phylum Annelida Definition

Annelids are defined as triploblastic, bilaterally symmetrical, coelomate invertebrates possessing a metamerically segmented body formed of small rings like segments.

Fig: Examples of Phylum Annelida

What is the History of Phylum Annelida?

The Annelids have an ancient history; the early Zoologists included annelids as a part of the Phylum Vermis and other unsegmented worms. But in \(1798\), George Cuvier, a French Zoologist, separated them from the Phylum Vermis. In \(1809\) another French Zoologist, Lamarck, used the name Annelida for the higher segmented worms. The term Annelida has been derived from the Latin word “Annellus”, which means small rings. There are around \(16500\) species included in the phylum Annelida.

What are Characteristics of Phylum Annelida?

The characteristics features of Phylum Annelida are as follows.

1. Habitat: They are mostly marine, freshwater, and terrestrial.
2. Habit: They are living or parasitic.
3. Symmetry: They are bilaterally symmetrical. 
4. Germ layers: They are triploblastic animals.
5. Body Cavity: They have true coelom.
6. Metamerism: The body is divided into several segments or metameres. The successive segments of the body are separated externally by ring-like constrictions called annuli. 
7. Cephalization: They show true cephalization.
8. Locomotion: In earthworms the contraction and relaxation of the longitudinal and circular muscles helps in locomotion.
9. Circulatory system: They have a closed circulatory system. 
10. Nervous system: They have a nerve ring around the pharynx and a double ventral nerve cord with ganglion. The sensory receptors like taste buds, photoreceptors cells like eyes and statocyst bodies are also present.
11. Respiration: The respiration takes place through the body surface, gills or parapodia.
12. Digestive System: They have a developed and complete digestive system which starts from the mouth and ends at anus.
13. Excretory system:Nephridia is the main excretory organ that helps in excretion and osmoregulation.

Fig: Nephridia

14. Reproduction:
a. They reproduce asexually by fragmentation in few polychaetes.
b. They show sexual reproduction. They are bisexual. In Hirudinaria the fertilization is internal or in earthworms the fertilization is external.

What is the Classification of Phylum Annelida?

Barnes \((1987)\) divided Annelida into \(3\) classes. The details of these \(3\) classes are given below:

Fig: Classification of Phylum Annelida

1. Polychaeta –
(a) They are the largest class of annelids.
(b) These are mostly found in marine environments.
(c) They have distinct head bearing tentacles and eyes.
(d) They are monoecious or dioecious.
(e) Parapodia is used in locomotion which contains many chitinous bristles called setae.
(f) They lack clitellum.
(g) They are hermaphrodites.
(h) The trochophore larva is formed during their development.

2. Oligochaeta –
(a) They are mainly terrestrial and freshwater.
(b) They have no distinct head. Tentacles and palps are absent.
(c) They possess a closed blood system.
(d) They are monoecious.
(e) Locomotion takes place by peristalsis. Parapodia and setae are absent.
(f) They are hermaphrodites.
(g) Nephridia is the main excretory organ.
(h) Larval stage is absent during development.

3. Hirudinea – 
(a) They are mainly freshwater, and few are marine, parasitic, and terrestrial.
(b) Dorsoventrally flattened body.
(c) Locomotion takes place by anterior and posterior suckers.
(d) They are monoecious.
(e) Parapodia and setae are absent.
(f) They are mostly hermaphrodites.
(g) They have a segmented body. Each segment has additional circular rings known as annuli.
(h) Larval stage is absent during development.

Phylum Annelida Examples

The examples of Phylum Annelida are mentioned below:

Nereis:

1. The systematic position of Nereis is –
Kingdom: Animalia
Phylum: Annelida
Class: Polychaeta
Order: Phyllodocida
Family: Nereididae
Genus: Nereis

2. They are commonly known as clam worms or sandworms.
3. They are marine nocturnal and carnivorous animals.
4. They are found in the burrows on tidal areas of the seashore.
5. The body of Nereis is divided into head, trunk, and pygidium.
6. They contain a close type of circulatory system.
7. Parapodia are the locomotory organs.
8. Fertilization takes place in sea water.

Fig: Nereis

Pheretima (Earthworm)

1. The systematic position of Pheretima is –
Kingdom: Animalia
Phylum: Annelida
Class: Oligachaeta
Order: Opisthopora
Family: Megascolecidae
Genus: Pheretima
2. They are cylindrical, soft-bodied, long segmented.
3. They live in burrows in soil.
4. Nephridia is the main excretory organ.
5. They possess a closed circulatory system.
6. They feed and reproduce only at night.

Fig: Pheretima

Hirudinaria (Leech)

1. The systematic position of Hirudinaria is –
Kingdom: Animalia
Phylum: Annelida
Class: Hirudinea
Order: Gnathobdellida
Family: Hirudinidae
Genus: Hirudinaria
2. They are also known as Indian cattle leech.
3. They have a triradiate mouth which is guarded by velum.
4. They are found in freshwater, ponds, lakes, swamps.
5. They are elongated dorsoventrally flattened segmented.
6. In leeches the alimentary canal is complete.
7. They have anterior and posterior suckers for adhesion and locomotion, respectively.

Fig: Hirudinaria

Do you know?

1. The longest worm Giant Gippsland Earthworm of Australia reaches \(3\,{\text{m}}\) in length.

2. Baby earthworms are not born, they hatch from cocoons which are smaller than a grain of rice.

Economic Importance of Phylum Annelida 

The economic importance of Phylum Annelida is:
1. Earthworms are considered farmers’ best friend as they improve the fertility of soil.
2. Leeches are used in the production of medicines.
3. Earthworms are commonly used baits for fishing.
4. They are used as food by many animals like frogs, snakes, lizards, and many birds.
5. They are used in zoological laboratories for dissection as they are easily obtained.

Summary

Earthworms and worms like creatures having segmented bodies are included in Phylum Annelida. They are present in places like freshwater bodies, damp soil and marine water. They have \(3\) distinct classes. These organisms have a simple body consisting of brains, a nervous system, circulatory system, and a digestive system. They play an important role in increasing the fertility of the soil.

Frequently Asked Questions

Here are some frequently asked questions about Phylum Annelida here:

Q.1. What are five characteristics of Phylum Annelida?
Ans: 1. They are mostly marine, freshwater, and terrestrial and are living or parasitic.
2. They are bilaterally symmetrical, coelomate and triploblastic animals.
3. The body is divided into several segments or metameres. The successive segments of the body are separated externally by ring-like constrictions called annuli.
4. They show true Cephalization.
5. Nephridia is the main excretory organ that helps in excretion and osmoregulation.

Q.2. What are the examples of Phylum Annelida?
Ans: The examples of Phylum Annelida are Nereis, Pheretima, Hirudinaria, Tubifex, Aphrodite etc.

Q.3. What are the unique features of phylum Annelida?
Ans: Annelids are triploblastic, bilaterally symmetrical, coelomate invertebrates possessing a metamerically segmented body formed of small rings like segments. 

Q.4. What is the importance of Phylum Annelida?
Ans: The Phylum Annelida are important in many ways: the earthworm makes the soil healthy for agriculture, leeches are used in the production of medicines.

Q.5. Which class do earthworms belong to?
Ans: Earthworm belongs to class Oligochaeta.

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