Phylum Annelida: General Characteristics and Classification

Phylum Annelida: General Characteristics and Classification


General characteristics of Phylum Annelida

  1. (Annelus: little ring)
  2. Kingdom: Animalia
  3. Habitat: mostly aquatic, some are terrestrial
  4. Habit: free living
  5. Symmetry: bilateral symmetry
  6. Coelom: coelomate (Body cavity is a true coelom, often divided by internal septa)
  7. Body is metamerically segmented
  8. Grade of organization: organ system grade
  9. Germ layer: triploblastic
  10. Body possesses 3 separate sections, a prosomium, a trunk and a pygidium.
  11. Digestive system: complete and developed
  12. Respiration: by general body surface
  13. Nervous system: nervous system with an anterior nerve ring, ganglia and a ventral nerve chord.
  14. Circulatory system: Has a true closed circulatory system.
  15. Excretion: by nephridia
  16. Reproduction: Sexual and gonochoristic or hermaphoditic.
  17. Fertilization: Internal or external
  18. Development: direct with no larval stages


Classification of Phylum Annelida

Phylum Annelida is divided into four main classes, primarly on the basis of setae, parapodia, metameres and other morphological features.

Class 1 Polychaeta

(Polys: many; chaite:hair)

  • Habitat: They are marine, terrestrial, and freshwater.
  • Archetypical protosome development (schizocoely).
  • True coelomates (schizocoelomates).
  • Bilaterally symmetry, segmented worms.
  • Complete digestive system.
  • Closed circulatory system.
  • Well-developed nervous system.
  • Excretory system: Both metanephridia and protonephridia.
  • Lateral epidermal setae with each segment.
  • Dioecious or hermaphroditic.
  • Fertilization: external
  • Examples: Nereis (sandworm), Syllis, Sabella


Class 2 Oligochaeta

(Oligos: few, chaite: hair)

  • Habitat: mostly terrestrial and few are freshwater
  • Body metamerically segmented
  • Clitellum present
  • Hermaphrodite but cross fertilization occur
  • Fertilization: external
  • Cocoon formation occur
  • Examples: Pheretima posthuma (Earthworm), Lumbricus, Stlaria, Tubifex


Class 3 Hiradinea

(Hirudo: leech)

  • Habitat: primarily freshwater annelids but some are marine, terrestrial and parasitic
  • The body has definite number or segments.
  • The tentacles, parapodia and setae are totally absent.
  • They are hermaphrodite.
  • Fertilization: internal and a larval stage is absent.
  • Examples: .Hirudinaria (Leech)


Class 4 Archiannelida

(Arch; first)

  • Habitat: They are strictly marine.
  • The body is long and worm like.
  • The setae and parapodia normally absent.
  • They may be unisexual or hermaphrodite.
  • The development: indirect forming trochophore larva.
  • Examples: .Protodrillus. Dinophilus, Protodrilus

Phylum Annelida: General Characteristics and Classification