Mostly aquatic, some terrestrial. Burrowing and tubicolous, Some are commensal and parasitic.
Body elongated and bilaterally symmetrical. Triploblastic. True coelom.
Metamerically segmented into similar metameres.
Epidermis made from a single layer of columnar epithelial cells, covered externally by a thin cuticle not made of chitin.
Body wall demo-muscular. Outer muscle fibres are circular, inner longitudinal.
Locomotory organs are segmentally repeated chitinous bristles, called Setae.
Coelom is mostly well developed except in Leeches.
Digestive system straight and complete.
Blood vascular system closed.
Respiration by moist skin or gills of Parapodia.
Excretory consists of metamerically disposed coiled tubes, called Nephridia.
Sensory organs: Tactile organs, taste buds, statocysts, photoreceptor cells, eyes in some.
Classes: Polychaeta, Oligochaeta, Hirudinea, Archiannelida