Phylum Annelida: Characteristics, Classification, Examples

Phylum Annelida (Annelids) are triploblastic, animals with fragmented bodies also known as segmented worms. With an expected 22,000 species, the phylum is morphologically fluctuated and isolated into four significant gatherings. The Phylum Annelida has a wide range of body shapes. Annelids live in terrestrial, marine, and freshwater settings, although the presence of water or humidity in terrestrial areas is critical to their existence. Earthworms, polychaete worms, and leeches are members of the Phylum Annelida.

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Meaning of Annelida

Annelida is a phylum formed from the Latin word annellus, which means "little ring." Because they grow a protostome in embryonic stages, annelids are sometimes known as "segmented worms." They may be found in a variety of habitats, including marine, terrestrial, and freshwater environments. Earthworms, leeches, and Polychaete worms are examples of annelids.

Annelids have highly-developed neural and digestive systems, as well as sexual dimorphism in their reproductive methods. Annelids are invertebrate creatures with bilateral symmetry. They are triploblastic and coelomate. The greatest distinctive aspect of annelids is their segmented bodies.

Characteristics of Annelida

The length of Annelida species ranges from less than 1 millimetre to more than 3 metres. Annelids are all bilaterally symmetrical, with elongated cylindrical bodies separated both outwardly and internally by a regular, linear succession of segments.

Annelida's distinguishing characteristics include:

1. Metamerism

Annelids have three distinct body sections. The majority of the body is made up of repetitive units known as segments. Each segment is separated from the adjoining segments by septa and has a fluid-filled chamber within it known as a coelom.

In annelids, segments form progressively and are created throughout development from growth zones placed at the rear end of the body, hence the youngest segment in the annelid's body is always the most posterior. The only non-segmental components of the annelid body are the head and the terminal post-segmental section known as the pygidium. The head is made up of two parts: the prostomium and the peristomium. Annelids have the incredible ability to regenerate segments that have been torn down into fragments, a process known as regeneration.

2. Chaetae

Chaetae, also known as setae, are bundles of chitinous, thin-walled cylinders held together by sclerotized protein and are a distinguishing characteristic of annelids. Chaetae are cuticular structures that form within epidermal follicles and are created by a microvillar border of some invaginated epidermal cells. Chaetae vary greatly in appearance, from long thin filaments to robust multi-pronged hooks.

3. Clitellum

The clitellum is a feature found on the front section of the worm that aids in the transmission of sperm from one worm to another. It also produces a cocoon inside which fertilisation occurs.

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Classification of Phylum Annelida

The following Phylum Annelida examples are provided:

Polychaeta

1. They are often referred to as clam worms or sandworms.
2. They are carnivorous marine nocturnal creatures.
3. Polychaeta can be discovered in burrows along the seashore's tidal zones.
4. The body of Nereis is separated into three parts: the head, the trunk, and the pygidium.
5. They have a similar sort of circulatory system.
6. The locomotory organs are the parapodia.
7. Fertilization occurs in marine water.

Polychaeta 

Oligochaeta 

1. They have a cylindrical shape, a soft body, and lengthy segmented bodies.
2. They dwell in burrows in the dirt.
3. Nephridia is the primary excretory organ.
4. They have a closed circulatory system.
5. They eat and reproduce exclusively at night.

Oligochaeta 

Hirudinaria (Leech)

1. They are sometimes referred to as Indian cattle leeches.
2. They have a triradiate mouth protected by velum.
3. They live in freshwater, ponds, lakes, and marshes.
4. Hirudinaria are elongated dorsoventrally flattened segmented elongated dorsoventrally flattened elongated dorsoventrally flattened.
5. The alimentary canal in leeches is complete.
6. They have anterior and posterior suckers to aid in adhesion and movement.

Hirudinaria 

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Things to Remember

• The phylum Annelida appeared after the roundworms. 
• These are segmented worms with far more complex traits like coelomates, which allow them to coordinate digesting. 
• They were the first to exhibit segmentation and possessed a closed circulatory system. 
• They develop a new excretory system known as nephridia. 
• They are much improved over Platyhelminthes and lack several molluscan characteristics.

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