(i) Annelids are the first segmented animals to evolve.
(ii) They are aquatic or terrestrial, free living but some are parasitic.
(iii) They are triploblastic, bilaterally symmetrical, schizocoelomates
(iv) Organization : organ system level organisation.
(v) The coelom with coelomic fluid creates a hydrostatic skeleton and aids in locomotion.
(vi) Body : metamerically segmented and the body surface is divided into segment or metameres.
(vii) Internally the segments are divided from one another by partitions called septa. This phenomenon is known as metamerism.
(viii) Nereis have lateral appendages called parapodia, which help in swimming.
(ix) Circulatory system : closed type and the respiratory pigments are hemoglobin and chlorocruorin.
(x) Nervous system : paired ganglion connected by the lateral nerves to the double ventral nerve cord.
(xi) They reproduce sexually.
(xii) Development is direct or indirect (Larva : trochophore ) eg. Lampito mauritii (Earth worm).