ULTRASTRUCTURAL OBSERVATIONS OF THE ARGONAUT SHELL

An examination of the ultrastructure of the shelf of the cephalopod Ar gonauta Nodosa was carried out using scanning electron microscopy, tra nsmission electron microscopy and polarised light microscopy. The stru cture of the Argonaut shell was found to consist of an inner and outer prismatic layer separated by a thin central zone which was sparsely o ccupied by spherulitic crystals. Fluctuations in the width and porosit y of the central zone resulted in changes in the shell's opacity and g ave rise to the fibrous lines visible in the structure. The central zo ne was the region of initial growth and was the nucleating point for t he crystals which formed the prismatic layers. It was concluded that d eposition of material in the Argonaut shell occurred on both the inner and outer surfaces of the shell, in contrast to the single growth sur face of other molluscs. The deposition process can be explained by the periodic movement of the Argonaut's tentacles, which are responsible for the material secretion, from one surface to the other. In general it was found that the Argonaut exercises less control over the structu re 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 rol e in determining the crystal structures.