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Phylum Annelida

Introduction:

  • Phylum Annelida includes over 9000 species.
  • Also known as ringed worms or segmented worms.
  • These are animals with bodies made up of repetitive annuli/rings.

Topics covered

  • General characteristics of phylum Annelida
  • Classification of phylum Annelida
  • Examples

General characteristics of phylum
 

Detailed explanation:
 

Habit and habitat

  • Terrestrial (free living) or aquatic(freshwater or marine)organisms.
  • Usually free-living.
  • Some species are parasitic in nature.

Level of organisation

  • Organ-system level of organisation.

Symmetry

  • Bilateral symmetry

Germ layers

  • These are triploblastic animals i.e., the body consists of three germ layers.

Segmentation

  • These are Metamerically segmented i.e their body is divided (externally as well as internally)into segments or metamers.
  • Externally it is divided by ring-like grooves called annuli while internally it is divided by transverse septa.
  • The first segment or metamer is known as peristomium and an outgrowth prostomium arises from the peristomium.

Body

  • Elongated, soft , segmented body.

Body plan

  • Tube-within-tube body plan.

Body wall

  • Body wall is made up of layers-
    - Cuticle - Thin, moist,albuminous and non-cellular outermost layer.
    - Epidermis - single layered.
    - Muscular layer - Circular and longitudinal muscles that help in locomotion.

Coelom (body cavity)

  • Eucoelomates i.e true cavity present.
  • From an evolutionary point of view, they were the first organisms that have a schizocoelic cavity.
  • Body cavity or coelom is filled with the coelomic fluid.

Digestion

  • Digestive system is complete and extends through the entire body.
  • Digestive glands were developed for the first time in Annelids.

Respiration

  • Respiration takes place by the moist skin (cutaneous respiration).
  • Some have gills (branchial respiration)

Excretion

  • Specialised coiled tubular structures called nephridia are present, which help them in osmoregulation and excretion.
  • Ammonia is the nitrogenous waste in aquatic form while urea is the nitrogenous waste in land or terrestrial form.

Blood vascular system or circulatory system

  • Circulatory system is of closed type.
  • Pulsatile heart is present.
  • Blood is red due to the presence of respiratory pigments like haemoglobin or erythrocruorin which is dissolved in the blood plasma.
  • Blood corpuscles are absent.
  • The true blood vascular system is absent in leeches.
  • From an evolutionary point of view, the heart and closed circulatory system first appeared in the annelids.

Skeleton

  • Usually absent; soft bodied animals.
  • Hydroskeleton is formed by the coelomic fluid that helps in maintaining the shape of the body

Locomotion

  • Aquatic Annelids like Nereis have lateral muscular appendages, parapodia which helps them in swimming.
  • Terrestrial Annelids like Earthworm have S-shaped chitinous setae which are embedded in their body and move with contraction and relaxation of circular muscles.

Nervous system

  • Nervous system consisting of the -
    - Nerve ring (Brain)
    - Paired ganglia connected by lateral nerves to a solid, double and ventral nerve cord.

Reproductive System

  • Reproduction is sexual.
  • Both unisexual and bisexual organisms are found.
  • Nereis (an aquatic form) is dioecious or unisexual but Earthworm and leeches are monoecious or bisexual.

Development

  • Development is direct or indirect.
  • In indirect development, free swimming ciliated trochophore larvae are formed.

Special characteristics of phylum Platyhelminthes

  • Presence of nephridia for excretion and osmoregulation.
  • Presence of metameric segmentation.
  • Presence of heart and closed circulatory system.
  • Presence of haemoglobin in plasma.
  • Presence of parapodia and setae for locomotion.

Classification of phylum Annelida
 

Detailed explanation:
 

Phylum Annelida is divided into following classes, on the basis of presence and absence of setae -

1. Polychaeta

  • They have well developed parapodia with numerous setae.
  • The development is indirect, undergo metamorphosis with a free swimming trochophore larva.
  • Examples : Nereis etc.

2. Oligochaeta

  • They have a few setae on their body.
  • Development is direct in them i.e the larval stages are not formed.
  • Example - Pheretima, (Indian Earthworm), Tubifex (European Earthworm) etc.

3. Hirudinea

  • These do not have parapodia or setae.
  • These are ectoparasitic leeches.
  • Development is direct in them.
  • Beneath the muscular layer of the body wall and surrounding the alimentary canal is found botryoidal tissue (peculiar connective tissue).
  • Leeches do not have a blood vascular system. They have a haemocoelomic system.
  • Examples : Hirudinaria (Blood sucking leech)

Examples
 

Detailed explanation:
 

1. Nereis

  • It is commonly known as clam worm or sand worm or rag worm.
  • Found at the sea shore in tubular burrows.
  • Unisexual ordioecious.
  • Fertilization is external, that occurs in sea-water.
  • Development is indirect in them i.e free-swimming trochophore larvae are formed.
  • They bear parapodia for swimming.

    nereis
     

2. Pheretima (Earthworm)

  • Terrestrial organisms found in the wet soil containing rich organic matter.
  • Monoecious or hermaphrodite i.e., both male and female reproductive parts are present in the same organism.
  • Setae are present in them for locomotion.

    earthworm
     

3. Hirudinaria (Blood sucking leech)

  • These are ectoparasites.
  • Sanguivorous animals as they feed upon blood.
  • The saliva of the leech contains an anticoagulant, called hirudin which prevents clotting of blood during blood meal.
  • Parapodia and setae are absent in them.
  • They are hermaphrodite but cross fertilisation occurs in them.
  • The five pairs of eyes are present on their dorsal surface.

    leech
     

4. Aphrodite

  • Commonly known as “Sea mouse”

Frequently Asked Questions: FAQs
 

Q1. Which is the locomotory organ of organisms belonging to the phylum Annelida?
Ans :

  • Parapodia and S-shaped chitinous setae are the locomotory organ of organisms belonging to the phylum Annelida.
  • Parapodia is found in aquatic Annelids for swimming.
  • While S-shaped chitinous setae are found in the terrestrial Annelids.

Q2. Which is the respiratory organ of Annelids?
Ans :

  • Respiration in Annelids generally takes place by the moist skin (cutaneous respiration).

Q3. Which is the excretory organ of Annelids?
Ans :

  • Excretion in Annelids takes place by specialised coiled tubular structures called nephridia which also help them in osmoregulation.

Q4. What is the excretory product of phylum Annelida?
Ans :

  • Ammonia is the excretory product in aquatic forms while urea is the excretory product in land or terrestrial form.

Q5. Which larva is common to phylum Annelida and Mollusca?
Ans :

  • Trochophore larvae (free swimming and ciliated).

Q6. Why is the colour of the blood of Annelids red in colour ?
Ans :

  • The colour of the blood of Annelids red in colour due to the presence of haemoglobin pigment.

Q7. Differentiate between monoecious and dioecious animals.
Ans :

  • Monoecious animals are those in which both the male and female reproductive parts are present in the same animal (eg: Earthworm)
  • While dioecious animals are those animals in which male and female reproductive parts are present in separate animals (eg: Nereis).

Q8. Define a sanguivorous animal with an example.
Ans :

  • Sanguivorous animals are those animals which feed on blood.
  • Example - Hirudinaria (blood-sucking leech).

Q9. Which anticoagulant is present in the saliva of the blood sucking leech ?
Ans :

  • The saliva of the leech contains an anticoagulant, called hirudin which prevents clotting of blood during blood meal.

Q10. Which annelid is called a farmer's friend?
Ans :

  • Earthworms are called the farmer’s friend because they help in the fertility of the soil.
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