Ultrastructural Characteristics of the Nacre in Some Gastropods

The nacreous layer in Gibbula, Calliostoma, Trochus and Haliotis is described on the basis of scanning electron microscopic studies. The central part of each nacreous tablet contains a significant amount of calcified organic matrix which is insoluble in a chromium sulphate and a 25% glutaraldehyde solution. In most cases, the tablet is subdivided by radial vertical organic membranes into a varying number (2 to 50) of crystalline sectors. These sectors represent polysynthetically twinned crystal individuals which form cyclic or interpenetrant twins. The nacreous tablets in gastropods are compared with those in bivalves, and with the non-biogenic aragonite. The mechanical properties of the nacre, and the effects of the interlamellar conchiolin membranes upon the nucleation of the tablets, are discussed.