Phylum Annelida – The Segmented Worms

The word Annelida is derived from Latin which means “little ring“.

The majority of the worms which we recognize are considered in this phylum. They are segmented and commonly called annelids.

General Characteristics of Phylum Annelida
Grade of organization

The animals are triploblastic and coelomate.

Shape and Symmetry

The body is metamerically segmented. These animals reveal bilateral symmetry.


The annelids consist of worms, which may be marine (Nereis), freshwater (stylaria), or found in moist or damp soil (earthworms). Some are parasites, for example, Hirudo (leech).

Body Cavity

The body ends up being divided transversely into a variety of similar parts or sections called segments. The subdivisions might be indicated externally by constriction of the body surface. Internally, the segments are separated from each other by septa extending across the coelom.

Annelids have true coelom i.e. the mesoderm splits into the parietal layer which lines the body wall, and the visceral layer which covers the alimentary canal, the area in between the two layers of mesoderm, is the coelom and is filled in by coelomic fluid, which serves- as hydrostatic skeleton likewise.

Digestive system

The gastrointestinal system remains in the kind of alimentary canal which is divided into distinct parts, each carrying out a specific function. It has two openings, the mouth at the anterior end, and the anus at the posterior end. The mouth is overhung by a lobed structure, the prostomium. In parasitic types, the digestion system is improperly developed.

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Organs and Organ systems

The annelids show specialization of body structures. The organ systems are well established.

Various systems of the body such as the gut, blood vessels, and nerve cord are constant throughout the length of the body penetrating each individual segment.

Excretory System

Excretion happens by specialized structures called nephridia. These are ciliated organs present in each segment in the body cavity.

Nervous system

A well established central nervous system exists in annelids. It comprises of an easy brain and a solid double, longitudinal, ventral nerve cord. Nerves emerge in each section from the nerve cord.

Circulatory System

Annelids are the first group of invertebrates which have developed a closed circulatory system – a system in which a circulatory fluid called blood flows in a network of vessels referred to as blood vessels. It carries gases and nutrients.

Respiratory system

The breathing system is not present. The exchange of gases is by diffusion through the skin into blood capillaries. The skin is kept moist by, mucus, and coelomic fluid.

Body Wall

The body wall contains muscles which help in mobility. The muscles are of 2 types:

  1. Circular Muscles: These are arranged along the circumference of the body.
  2. Longitudinal Muscles: These are organized along the length of the body.
Movements in Phylum Annelida

The mobility is brought about by the interaction of muscles and the hydrostatic skeleton. The contraction of circular muscle produces pressure in the coelomic fluid that requires the body to lengthen. Likewise, the contraction of longitudinal muscles produces pressure in the coelomic fluid that would trigger the body to widen.

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The organs of locomotion in annelids are chitinous chaetae or setae embedded in sacs (earthworm) or on parapodia present in the body wall (e.g., Nereis). Chaetae are absent in leech.

Reproduction in Phylum Annelida

Sexual reproduction

The common mode of reproduction is sexual. The majority of annelids (Earthworm, leech) are hermaphrodite (having both male and female). In some annelids (e.g., Nereis) the sexes are different, the fertilization is external.

Life cycle

A free-swimming trochophore larva is produced during the life cycle.


Phylum Annelida comprises:
  1. Class Polychaeta
  2. Class Oligochaeta
  3. Class Hirudinea
Class Polychaeta

These have a unique head region with eyes and structure known as palps and tentacles. Sexes are normally separate. The organs of locomotion are parapodia. They are mainly marine (marine). During advancement, these give rise to a trochophore larva.

Common examples are Nereis, Chaetopterus.


Class Oligochaeta

These animals have internal and external division. Organs of locomotion are setae. Head region not prominent or distinct. They are hermaphrodite (bisexual). No larva formed throughout development in Pheretima Posthuma and other earthworms. They might be terrestrial or aquatic.

e.g. Lumbricus terrestris


Class Hirudinea

They have a body with a fixed variety of segments. Each segment has extra circular rings or markings called annuli. They do not have organs of locomotion and relocation due to the contraction of their body and with the help of suckers. Mainly hermaphrodite and trochophore larva are formed during development.

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They are aquatic. No unique head exists but leeches have chitinous jaws for making a hole in the skin of the host. They also have an anticoagulant secretion which is entered the wound to permit the smooth flow of blood into its digestion system where it can be saved for a very long time.

e.g. Hirudo medicinalis (medicinal leech).


Economic Importance

Burrowing activity of earthworms permits greater penetration of air into the soil and improves the drain capacity of the soil.

It also makes it possible for roots to grow downwards through the soil more quickly. Blending and churning of the soil are caused when earth which contains inorganic particles is brought up to the surface from lower areas. The earthworm is possibly the most active segmented worm in churning the soil, therefore it is frequently called as a natural plough.



Annelids are the triploblastic coelomates with bilateral symmetry and segmented bodies. The annelids may be freshwater, marine water, or terrestrial organisms. The coelom is filled with coelomic fluid and serves as a hydrostatic skeleton.

They have a digestive system, a kind of alimentary canal. The mouth is overhung by a lobed structure, the prostomium. They have well-developed nephridia for the excretion of wastes from the body. The exchange of gases is by diffusion through the skin into blood capillaries.

The body muscles are of two types longitudinal and circular. The organs of locomotion in annelids are chitinous chaetae or setae. They reproduce sexually and a free-swimming trochophore larva is produced during the life cycle.