Class 11 Biology chapter 4 notes describe the characteristic features of Phylum Annelida as follows.

• They can be aquatic (both marine and freshwater) or terrestrial; free-living or parasitic.
• They have organ-system level of body organization as well as bilateral symmetry. They are coelomate, triploblastic and metamerically segmented animals. Their body surface is clearly divided into segments or metameres, giving rise to the phylum name Annelida (Latin, annulus: little ring).
• They have both longitudinal and circular muscles that aid in locomotion.
• Aquatic annelids, such as Nereis, have lateral appendages called parapodia that aid in swimming. There is a closed circulatory system set up. Nephridia (also known as nephridium) aids in osmoregulation and excretion.
• The neural system is made up of paired ganglia (sing. ganglion) that are linked by lateral nerves to a double ventral nerve cord. Reproduction is a sexual process.
• Nereis, Pheretima (Earthworm) and Hirudinaria (Blood sucking leech) are some examples.