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(A) Images of the scleractinian coral Acropora digitifera. (B) SEM images of ultrastructural features of the coral Balanophyllia europaea. (C) Sectional view of calcium carbonate spines derived from the sea urchin Heterocentrotus mammillatus. (D) SEM images of the inner structures of sea urchin spines (A,B are reproduced from Ref. [227] with permission from the publisher; C,D are reproduced [226] from with permission from the publisher).
