Annelida

A characteristic feature of annelids is metameric segmentation i.e body shows external division by circular ring like annuli and corresponding internal division by septa. segments thus formed are called metamers. they are found in marine habitat, freshwater habitat and in moist soil also . symmetry is bilateral. body is triploblastic that is made up of 3 germ layers. they have organ level of body organisation. another characteristic feature is presence of setae in most of species. Setae are absent in leeches. some species have parapodia. which act as locomotor organ.annelids are coelomates . they have true coelom, which is schizocoel. in animal evolution annelids are the first to have coelom.The coelom contains coelomic fluid with certain types of cells. Coelomic fluid inside body cavity forms hydrostatic skeleton. in most of species respiration occurs by diffusion of gases throw skin. in some species respiration occurs through gills. digestive tract is complete, that is, have both mouth and anus. excretory system contains nephridia. blood circulatory system is closed type. blood is red due to plasma dissolved pigment erythrocruorin . true Blood vascular system is lacking. nervous system consists of mid ventral nerve cord with segmental ganglia and a nerve ring at interior end. sense organs are tactile receptors,photosreceptors, gustatoreceptors and also statocysts in some species. some species like earthworm, leech are hermaphrodite, whereas others like Neries are unisexual. development is direct in most of species, that is, no larval stage in life history. in some false like Neries larval stage (trochophore larva) is present.