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This class includes sea urchins ,heart urchines ,etc. The body is ovoid , arms are absent .madreporite and anus are aboral in position . ambulacral grooves are closed. pedicellariae are three jawed.developement includes echinopluteus larva .
 eg :echinus ,echinodiscus etc.
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These are marine,solitary and sedentary.Radially symmetrical with pentamerous symmetry.body is exoskeleton with spiny covering and is made up of calcareous plates or ossicles. They show high power of regeneration.Sexes are separate.Fertilization is external.Circulation system is of open type i.e heart is absent.Respiration is by peristomial gills.Nervous system is simple with ring around the mouth and radial nerves around the arms. Development is indirect.
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