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Nilanjan Bhowmick AIR 3, CSIR NET (Earth Science)
Eduncle Best Answer
Echinoids, or sea urchins, have a robust, internal skeleton (the test) composed of numerous, fixed, calcite plates. Instead of arms the test has five narrow zones formed from perforated plates (the ambulacra) through which the tube feet emerge. These porous segments alternate with broader areas (the inter-ambulacra) that lack pores.