Coral nucleation

The extracellular formation of crystals of CaC03 by epithelia is the most common type of skeleton-forming system and occurs in corals (Vandermeu-len, 1975), molluscs (Wilbur, 1964), some annelids (see p. 84) brachiopods (Williams, 1971), and arthropods (Travis, 1970). In the arthropods, elaborate cellular extensions penetrate the mineralized carapace and are important in the calcification process. Formation of a mineralized skeleton in these taxa involves the movement of Ca and HCOj across a layer of cells from the body fluid (absent in corals). Nucleation and crystal growth take place in an organic milieu secreted by the epithelium. 

Before turning to these organisms we note dominant features of totally-confroUed not just nucleated minerals (as illustrated by coccoUths) as compared to inorganic precipitation from the same solution, seawater (as illustrated by corals) but controlled in form by surfaces. The points to note in the case of the coccoUths are ... 

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