GENERAL CHARACTERISTICS OF ANNELIDA

L,.annelus=little ring; eidos=form

Most of them are aquatic, but some are terrestrial too.
They may be burrowing or tubicolous and parasitic or commensal.
Triploblastic animals i.e., body derived from 3 germ layer (ectoderm, mesoderm and endoderm).
Body is bilaterally symmetrical.
Body is soft, elongated and is externally covered by a thin layer of cuticle. No exoskeleton is present.
Body is metamerically segmented into many similar metameres.
Organ-system grade of body organization.
A true coelom is found, which is totally lined by mesodermal epithellium.
Locomotory organs are segmentally arranged many chitinous bristles called setae, embedded inside the skin or borne in sac like muscular muscular outgrowths of skin called parapodia.
No distinct head is present. Appendages unpaired.
Body wall is dermomuscular and is highly contractile. It is composed of outer circular and inner longitudinal muscles.
Respiration takes place through normal body surface or by gills of parapodia and head.
Digestive system is complete type. Digestion is extracellular.
Excretion takes place through metamerically arranged paired nephridia.
Blood vascular system is closed type with definite blood vessels. Blood is red due to presence of haemoglobin or erythrocruorin.
Nervous system consists of a pair of cerebral ganglia, a double ventral nerve cord and lateral nerves in each segment.
Sense organ includes tactile organs, taste buds, statocyst and eye with lens in some.
Reproduction may be sexual or asexual. If asexual, then by fragmentation.
Regeneration is common.
Development direct or indirect (through Trochophore larval form).
They may be monoecious or dioecious.
In dioecious forms, external fertilization occurs, but in monoecious animals, internal fertilization takes place.