Shell ultrastructure and ontogenetic growth in Nautilus pompilius L. (Mollusca: Cephalopoda)
Ontogenetic growth of the Nautilus shell is here reconstructed on the basis of structural studies of the initial, apical parts of adult shells. The results of these studies are compared with those obtained previously by studies on embryonic shells. The following four early growth stages can be distinguished: (1) formation of initial plate of organic matrix (periostracum) with cicatrix, (2) formation of initial calcified shell, composed of the initial organic plate and outer prismatic layer, (3) formation of three-layered shell-wall around the initial calcified shell, and (4) formation of three-layered protoseptum on the inner surface of the initial calcified shell-wall. The protoseptum differs from ordinary septa in that it lacks the siphonal tube and in that it is firmly attached to the inner surface of the initial shell-wall in order to make it less fragile, without forming a shell-chamber. During subsequent growth, two thin but distinct prismatic layers, separated by an organic sheet, are secreted on the inner surface of the ventral and lateral sides of the shell-wall. The outer of the two prismatic layers is the mantle-attachment layer. In adult shells, this layer is considerably thickened on the ventral and lateral sides of the shell aperture, and perforated by vertical canals, which in all probability housed finger-shaped epithelial extensions from the mantle. Thus, the mantle seems to be firmly attached to the apertural region of the shell during the entire ontogenetic growth of the animal. The inner of the two prismatic layers is the myostracal layer, secreted by the myo-adhesive mantle epithelium in front of the last septum. The caecum is the sac-shaped apical end of the siphonal tube. Its structure is extremely variable. Occasionally, the caecum is formed by a regularly developed connecting ring of the first septum and consists of the outer porous spherulitic layer and the inner conchiolin layer. In this case, it has the same structure as the connecting rings in the subsequent septa. Usually, however, the connecting ring of the first septum is structurally modified and heavily calcified. In the latter case, the wall of the caecum is impermeable to cameral fluid, which means that empting of the cameral fluid form the first chamber cannot take place in the majority of animals.