Phylum : Annelida

The body surface of Annelids are segmented and distinctly marked out into segments or metameres and, hence, the phylum name Annelida (Latin, annulus : little ring)

Habitat: Annelids can be free-living or parasitic. They occupy both aquatic (marine or freshwater) and terrestrial habitats.


Nereis Pheretima (Earthworm) Hirudinaria (Blood sucking leech).

Body plan: The members have a tube-within-tube body plan and organ-system level of organization.

Symmetry: They are bilaterally symmetrical.

Germ layer: Annelids are eucoelomate and triploblastic organisms.

Body characteristics:

  • Body shows characteristic metameric segmentation.
  • The body walls possess circular and longitudinal muscles. The alternate contraction and relaxation of these muscles help in their movement.
  • Locomotory organs are paired lateral appendages known as chetae or setae.
  • In aquatic annelids like Nereis, lateral appendages called parapodia help in swimming.

Worm.gif Annelid.jpg


  • Body plasma contains haemoglobin. The circulatory system is of open type.
  • Respiration is cutaneous, and the skin is kept moist and vascularised.
  • Digestion is extracellular. Excretory organ is nephridia. It also helps in osmoregulation.
  • Nervous system consists of a dorsal brain and a paired ganglion which is connected by nerves to a double ventral nerve cord.

Annelid.jpg 23-15.gif

Reproduction: Most annelids are hermaphrodites, except Polychaeta members (eg. Nereis). Hence, reproduction is by sexual means.

Embryo Development: The development is indirect.



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