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Morphological features of a Monorhaphis spicule along its length in reflected light (a–c) and with SEM (d). a Lower part of the spicule displaying the annular surface (AL). b Intermediate region showing annular (AL) and tuberculate layers (TL). c Upper portion of the spicule showing the plain glassy surface (PG). d Broken lower portion of a spicule showing all three structural units superimposed
