(L. Annelus – a little ring)

The term Annelida was coined by Lamarck.

VVI Examples of Phylum Annelida for Entrance Exams are:
  • Nereis (Sandworm or Clam worm)
  • Aphrodite (Sea mouse)
  • Chaetopterus (Paddle worm)
  • Arenicola (Lungworm) 
  • Pheretima (The common earthworm)
  • Lumbricus (English or European earthworm)
  • Tubifex (Blood earthworm)
  • Megascolex
  • Hirudinaria (Cattle leech)
  • Hirudo (Medicinal leech)
  • Pontobdella (Skate sucker)
  • Polygordius (Living fossil)

Characteristics of Phylum Annelida

· Triploblastic, bilaterally symmetrical, first coelomate (Schizocoel derived from splitting of mesoderm), segmented animals with the organ-system level of body organization, tube within a tube body plan and complete digestive system.
· Mostly aquatic, both fresh water and marine.
· Generally burrowing, some sedentary or free-living while others are ectoparasites.
· They are metamerically segmented i.e. external segmentation (grooves) corresponds with internal segmentation (septa).
· Locomotary organs are paired setae or chaetae or in each segment which is accompanied by the musculature of skin.
· Digestive system is well-developed which consists of a straight alimentary canal and digestion is entirely extracellular.
· Respiration takes place through moist general body surface by diffusion.
· Excretion occurs by nephridia present on each segment.
· Circulatory system is closed type appears first time in the animal kingdom or first heart bearer.
Blood is red in colour due to haemoglobin. 
Haemoglobin is dissolved in plasm.
· Presence of Neurogenic heart. Some polychactes contain chlorocruorin (green pigment).
· Body is covered by a transparent moist non-chitinous cuticle secreted by ectoderm.
· Nervous system is well-developed.
· The nervous system consists of a pair of dorsal cerebral ganglia, a pair of peri-pharyngeal connectives (nerve ring), and a double solid ventral nerve cord having a ganglion and lateral nerves in each segment.
· Hermaphrodite or exhibit sexual dimorphism.
· Larva is Trochophore type e.g. Nereis, Pheretima posthuma, Hirudinaria, etc.
· Various species of polynoe (scale worm) and chaetopterus (paddle-worm) are luminous in dark.
· Respiratory pigments are chlorocrunin and hemocyanin.

Classification of Phylum Annelida

On the basis of locomotory organs, they are divided into 4 classes.

(i) Polychaeta (Poly – many, chaite – hair)
· They are marine and carnivorous.
· Body is elongated and segmented.
· Body bears a distinct head. The Head bears two pairs of eyes and tentacles.
· Clitellum is absent.
· Cirri or branches may be present for respiration.
· Sexes are separate, fertilization is external and development is indirect.
· Locomotary organs are parapodia.
· No clitellum and cocoon-formation and a free-swimming trochophore larval stage are present.

Examples: Nereis (Sandworm or Clam worm)
Aphrodite (Sea mouse)
Chaetopterus (Paddle worm)
Arenicola (Lungworm)

(ii) Oligochaeta (Oligo–few, chaite–hair)
· Locomotary organs are setae found in each segment.
· Mostly terrestrial, some are freshwater.
· Body lacks a distinct head.
· Body is segmented externally as well as internally.
· Clitellum is present.
· Hermaphrodite, development is direct (absence of larval stage), fertilization is external by cocoon formation.

Examples: Pheretima (The common earthworm)
Lumbricus (English or European earthworm)
Tubifex (Blood earthworm)

(iii) Hirudinea
· They are ectoparasites or sanguivorous with an anterior and posterior sucker.
· Locomotary organs are suckers.
· Hirudin (an anticoagulant) is present in saliva.
· Distinct head absent but the presence of suckers.
· Clitellum absent or temporarily developed in 9th-11th segments during the breeding season.
· Presence of Botryoidal tissue.
· Hermaphrodite
· Fertilization: external in the cocoon.
· Development: direct.
· Body consists of a definite number of segments (i.e. 33).

Examples: Hirudinaria (Cattle leech)
Hirudo (Medicinal leech)
Pontobdella (Skate sucker)

(iv) Archiannelida
· Exclusively marine forms.
· No external segmentation but internal segmentation is complete.
· Distinct head is absent.
· Clitellum is absent.
· No parapodia and no setae.
· Sexes are usually separate and they rarely unite.
· Trochophore larva is present.

Examples: Polygordius (Living fossil)

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