"Astropyga radiata " is a large urchin with a interambulacral plates.
2.
They are arranged into interambulacral and ambulacral regions and the arms have an ambulacral groove on the underside from which the tube feet project.
3.
The upper surface, or " tegmen ", is divided into five " ambulacral areas ", including a deep groove from which the tube feet project, and five " interambulacral areas " between them.
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