Fossil and Recent Sponges

pp 322-340

Non-Spicular Biomineralization in Calcified Demosponges

  • R. Wood

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The sponges occupy an unusual evolutionary position as the most primitive of metazoans. However, despite their relatively small number of cell types they have evolved a considerable array of morphological forms and have evolved into a variety of ecological habitats. Although they possess no tissue sensu stricto, their cellular activities attain a significant degree of integration and they are able to secrete both calcium carbonate (as aragonite or high- or low-Mg calcite) and amorphous or hydra ted silica. These minerals may be precipitated in the form of spicules, scales, plaques, cements, desmas, massive skeletons and granules. In the absence of autogenic precipitations, foreign mineral grains may even be incorporated and so it would appear that some mineralized component is necessary to the functioning of the sponge, presumably to maintain structural rigidity or for protection. Sponges therefore offer considerable opportunities to study mechanisms of biomineralization.