β Description: Segmented worms,the most important character of this phylum is metamerism.The segmentation in some of the platyhelmenthis is not true segmentation.
β Symmetry: Bilateral
β Germ Layers: Triploblastic
β Coelom: Eucoelomate (true coelom)
β Coelom Evolution: First appearance of coelom
β Body Type: Worm-like
β Segmentation:
β Type: Metameric (homonomous)
β New Segment Formation: Occurs at the posterior just in front of pygidium
β Nerve Cord: Large, longitudinal, ventral
β Body Plan: Tube within a tube (Protostomic)
β Organization: Organ-system level
β’ Alimentary Canal: Straight
β’ Excretory System:
β Organs: Nephridia
β Origin: Ectodermal (from nephroblastic cell)
β’ Anus:
β Location: Last body segment
β Post-anal Tail: Absent
β’ Cuticle: Non-chitinous, albuminoid
β’ Circulatory System:
β Type: Closed
β Heart: First appearance
β RBC: Absent
β Respiratory Pigment: Haemoglobin
β’ Embryonic Similarity: Resembles Mollusca in segmentation of muscles and nervous system
β’ Digestive Gland: First appearance
β’ Notes:
β Clamworm: Possesses 2 pairs of eyes
β Nereis: Gregarious, nocturnal, and carnivorous; mature form is Heteronereis
Fig.Nereis
β Chaetopterus: Exhibits great power of regeneration
β Eunice: Example of Errantia
β’ Classification:
β Criteria: Based on setae position/arrangement and presence of sense organs
