Phylum Annelida (Segmented worms)

• Habitat - They may be aquatic either marine or fresh water; or terrestrial; free-living and sometimes parasitic.
• Level of organization - They exhibit organ-system level of body organization and bilateral symmetry.
• Body wall - They are triploblastic.
• Coelom - They are coelomate animals.
• Body is metamerically segmented. The body surface is distinctly marked out into segments of metameres and hence, the phylum name Annelida.
• Locomotion - They possess longitudinal and circular muscles which help in locomotion.
• Aquatic annelids posses lateral appendages, parapodia which help in swimming.
• Circulatory system is closed.
• Osmoregulation and excretion is by Nephridia.
• Neural system - It consists of paired ganglia connected by lateral nerves to a double ventral nerve cord.
• Nereis is dioecious, but earthworm and leeches are monoecious.
• Reproduction is sexual.
• Examples: Nereis, Pheretima (earthworm), and Hirudinaria (blood sucking leech)
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