An examination of the ultrastructure of the shell of the cephalopod Argonauta Nodosa was carried out using scanning electron microscopy, transmission electron microscopy and polarised light microscopy. The structure of the Argonaut shell was found to consist of an inner and outer prismatic layer separated by a thin central zone which was sparsely occupied by spherulitic crystals. Fluctuations in the width and porosity of the central zone resulted in changes in the shell\u27s opacity and gave rise to the fibrous lines visible in the structure. The central zone was the region of initial growth and was the nucleating point for the crystals which formed the prismatic layers. It was concluded that deposition of material in the Argonaut shell occurred on both the inner and outer surfaces of the shell, in contrast to the single growth surface of other molluscs. The deposition process can be explained by the periodic movement of the Argonaut\u27s tentacles, which are responsible for the material secretion, from one surface to the other. In general it was found that the Argonaut exercises Jess control over the structure of its shell than is common amongst the molluscs and in particular the organic matrix of the shell does not appear to play as large a role in determining the crystal structures
