Characteristic features of phylum Annelida

Salient features of Annelids

  • They are mostly aquatic and few are terrestrial forms.
  • Most of them are free living and some are ectoparasites.
  • The body is elongated, cylindrical or flattened. It is divided into segments by ring-like grooves called annuli.
  • True metameric segmentation is present. The number of segments is fixed with no gain or loss of segments in their life.
  • Body is triploblastic and bilaterally symmetrical.
  • They have tube within a tube body plan.
  • The body cavity is a true coelom (schizocoelomate or eucoelomate), as it is lined by mesodermal epithelium.
  • Alimentary canal is complete and segmentally arranged. It is divided by vertical septa into compartments.
  • Circulatory system is of closed type, and appears for the first time in animal kingdom.
  • Blood is red in color due to haemoglobin (Hb) which dissolves in plasma.
  • The heart is neurogenic.
  • Respiratory organs are absent, so gaseous exchange occurs by general body surface.
  • Excretory system consists of coiled tubes, the nephridia which are ectodermal in origin.
  • They may be ammonotelic or ureotelic.
  • They are hermaphrodite and sexual reproduction occurs with the formation of gametes.
  • Fertilization may be internal or external.
  • Development may be direct (with no larval stage) or indirect (with a larval stage).


Phylum Annelida is divided into following four classes.

Class: Polychaeta

  • They are marine forms.
  • Body bears a distinct head that consists of two pairs of eyes and tentacles.
  • Body is segmented with unjointed locomotory appendages called parapodia.
  • Parapodia bears setae which are long and numerous. Setae help in fixation.
  • Clitellum is absent.
  • Sexes are separate and fertilization is internal in water.
  • Development includes a larval stage called Trochophore
  • e.g. Nereis (the sandworm), Aphrodite (the sea mouse), Chaetopterus (the paddle worm) etc.

Class: Oligochaeta

  • They may be either aquatic or terrestrial.
  • Body lacks a head and the locomotory organs are setae.
  • Setae are small and many in number which arise from setigerous
  • Clitellum is present throughout the life.
  • Sexes are united (hermaphrodite) and fertilization is external.
  • Development is direct.
  • e.g. Pheretima ( the earthworm), Tubifex (the blood worm) etc.

Class: Hirudinea

  • They are found in damp and moist places and few are ectoparasites.
  • Parasitic forms are sanguivorous and consist of an anticoagulant (Hirudin) in their saliva.
  • Body is segmented and the number of segments is fixed, i.e 33 segments.
  • Body lacks a head and setae.
  • They are haemocoelomate and circulatory system is of open type.
  • Clitellum is temporary and develops during the breeding season, in 19th to 11th
  • Five pairs of dorsal eyes are present.
  • Alimentary canal is complete with many crop and caeca.
  • There is an anterior sucker and often a posterior sucker for locomotion and feeding blood.
  • In leech, mesoderm splits and forms Botryoidal tissue which is excretory in nature.
  • Sexes are separate and development is direct.
  • e.g. Hirudinaria (the cattle leech), Pontobdella (the fish leech) etc.

Class: Archinnelida

  • They are exclusively marine.
  • Head is with tentacles and sense organ.
  • Parapodia and setae are usually absent.
  • External segmentation is faint, internal segmentation is formed by septa.
  • Sexes may be separate or united.
  • Development includes a larval stage called trochophore larva.
  • e.g. Polygordius (living fossil), Protodrillus etc.

Characteristic features of phylum Annelida