Kingdom Animalia Classification:The name Annelida was first used by Lamarck in 1809 for the higher segmented worms. The word annelida has been derived either from the Latin word "annullus" meaning little ring or French word "anneler" meaning to arrange in rings.Annelida are triploblastic, bilaterally symmetrical, coelomate and segmented metazoan.It includes well known earthworms and leeches.GENERAL CHARACTERS OF ANNELIDA:Cephalization is more pronounced with a distinct head bearing tentacles, eyes etc.They exhibit homonomous metamerism. Body is divided into segments which are separated externally by intersegmental grooves and internally by intersegmental septa.Body is divided in to three regions - Prostomium, Trunk and Pygidium.Prostomium:Anterior to the true segments lies prostomium which carries the mouth.Trunk: It consist of longitudinal series of similar segments.Pygidium: Post segmented region posteriorly known as pygidium.There is a single pre-oral segment called prestomium.Body wall consists of cuticle, epidermis, dermis, musculature and parietal peritoneum.Body cavity is a true coelom. Coelom filled with coelomic fluid is incompressible acts as hydrostatic skeleton.Locomotory structures are setae or parapodia.Alimentary canal is straight muscular tube. Digestive glands are present in the wall of alimentary canal.Respiration takes place through the body wall and in the richlyvascularised parapodia which acts as gillsin some polychaetes.Blood vascular system develops first time in Annelids. Blood vascular system is of closed type. Respiratory pigment is hemoglobin or chlorocruorin green and haemoerythrin pink or voilet) dissolved in the plasma.Excretory organs are nephridia metanephridia. Nephridia are ectodermal in origin.Nervous system consists of nerve ring around pharynx and two ventral nerve cords. Brain and segmental ganglia are present.Sense organs like eyes and photoreceptors, chemoreceptors and mechanical receptors are present.Sexes are separate or united.Fertilization is external, internal in hirudineans.Clevage is holoblastic and spiral.Development is direct in bisexual annelids and indirect in unisexual annelids. Larva form is called Trochophore larva.CLASSIFICATION OF PHYLUM

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