Write the general characters of the phylum annelida.

Explanation

(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 and exhibit organ system level of body organisation.
(iv) The coelom with coelomic fluid creates a hydrostatic skeleton and aids in locomotion.
(v) Their elongated body is metamerically segmented and the body surface is divided into segment or metameres.
(vi) Internally the segments are divided from one another by partitions called septa. This phenomenon is known as metamerism.
(vii) Aquatic annelids like Nereis have lateral appendages called parapodia, which help in swimming.
(viii) The circulatory system is of closed type and the respiratory pigments are hemoglobin and chlorocruorin.
(ix) Nervous system consists of paired ganglion connected by the lateral nerves to the double ventral nerve cord.
(x) They reproduce sexually. Development is direct or indirect and includes a trochophore larva, eg. Lampito mauritii (Earth worm).