ANIMAL KINGDOM Part-12, NCERT Class-11

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Phylum – Annelida

• Latin, annulus : little ring
• May be aquatic (marine and fresh water) or
terrestrial; free-living, and sometimes parasitic.
•They exhibit organ-system level of body
organisation and bilateral symmetry. 
• Triploblastic, metamerically segmented 
•coelomate.
• Their body surface is
distinctly marked out into segments or
metameres , which is why the phylum name is Annelida 
•They possess longitudinal and circular muscles
which help in locomotion. 
• Aquatic annelids like Nereis possess lateral appendages, parapodia, which help in swimming. 
• Circulatory system Closed. 
•Nephridia (sing. nephridium)
help in osmoregulation and excretion.
• Neural system consists of paired ganglia (sing. ganglion)
connected by lateral nerves to a double ventral
nerve cord.
• Nereis, an aquatic form, is dioecious, but earthworms and leeches are monoecious.
• Sexual Reproduction.

Example:
• Nereis
•Pheretima (Earthworm)
•Hirudinaria (Blood sucking leech).

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