Salient features of phylum Annelida :
(a) Annelida occur in fresh water, sea water or moist soil. Some are free living, some are burrowing and a few are parasitic.
(b) The annelids are characterised by metameric segmentation, viz, the body is divided externally by ring like grooves, the annuli and internally by transverse septa. The segments are called metameres.
(c) They show bilateral symmetry.
(d) Annelids are triploblastic animals.
(e) They have organ system level of organisation.
(f) Except leeches, unjointed chitinous setae are often present. Some annelids have unjointed, locomotory structures, the parapodia (para-parallel, podia-feet).
(g) Circulatory system is usually closed type. Blood is red due to the presence of respiratory pigment haemoglobin, found dissolved in the plasma. Free amoeboid blood corpuscles are present, but there are no red blood corpuscles.
(h) Exeretory system consists of coiled tubular structures, called nephridia. Ammonia is chief excretory waste.
(i) The nervous system consists of a nerve ring and a solid, double, midventral nerve cord with ganglia. A ganglion is an aggregation of nerve cells.
(j) Tactile (touch) receptors, gustatoreceptors (receptors of taste) and photoreceptors (sensitive to light) are usually found. Some forms have statocysts (balancing organs).
(k)Both unisexual (e.g., Nereis) and bisexual (e.g., Earthworm, Leech) forms are found.