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"Apical disc of sea-urchin. In the centre is the periproct bearing the anus; around it are five genital or inter-radial plates (g.), one of which is modified as the madreporite (m.); beyond these five ocular or radial plates (o.); i.r.a., an inter-radial or inter-ambulacral area, with spines only; r.a., a radial or ambulacral area, with spines and openings for tube-feet." -Thomson, 1916

Sea Urchin

"Apical disc of sea-urchin. In the centre is the periproct bearing the anus; around it are five genital…