Aesthete channel morphology in three species of Australian chitons (Mollusca: Polyplacophora)

Abstract

Light and scanning electron microscopy are used to investigate the internal structure of the shell valves of Ischnochiton (Ischnoradsia) australis, Acanthopleura gaimardi and Cryptoplax mystica. The shell in all species is comprised of four layers (periostracum, tegmentum, articulamentum, and hypostracum), and is perforated in specific areas by one of three classes of megalaesthete (jugal, slit-ray, and multiple branch). All classes consists of a number of micraesthete channels emanating from the distal region of a single aesthete channel, but differ according to the extent of canal branching. Both aesthete and micraesthete structures terminate at the shell surface in conical caps, while ventrally, a narrow canal extends towards the mantle interface. The gross structural arrangement is probably typical of all chiton species. A possible differentiation of aesthete function between the three types of megalaesthete is discounted in respect of ultrastructural similarities.