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

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