Describe the characteristics of Phylum $Annelida$.

(N/A) The key characteristics of Phylum $Annelida$ are as follows:
$1$. Body Organization: They exhibit organ-system level of body organization and are bilaterally symmetrical.
$2$. Germ Layers: They are triploblastic,coelomate,and protostome animals.
$3$. Segmentation: Their body surface is distinctly marked out into segments or metameres,hence the name $Annelida$ (Latin,$annulus$: little ring).
$4$. Locomotion: They possess longitudinal and circular muscles which help in locomotion. Aquatic annelids like $Nereis$ possess lateral appendages,called parapodia,which help in swimming.
$5$. Circulatory System: $A$ closed circulatory system is present.
$6$. Excretion: Nephridia (singular: nephridium) help in osmoregulation and excretion.
$7$. Nervous System: Neural system consists of paired ganglia connected by lateral nerves to a double ventral nerve cord.
$8$. Reproduction: $Nereis$ (an aquatic form) is dioecious,but earthworms and leeches are monoecious. Reproduction is sexual.