ABSTRACT The formation of the sclerites (spicules and scales) in the girdles of five species of polyplacophorans of the order Chitonida (Acanthochitona fascicularis, Acanthochitona crinita, Lepidochitona cinerea, Ischnochiton rissoi and Rhyssoplax olivacea) has been studied at the ultrastructural level and by optical and transmission electron microscopy to analyse the relationships of the scleritome within the Aculifera. The sclerites of the studied chitons were secreted inside an invagination of the microvillous cellular membrane of a single epithelial cell located in the papillae. This invagination was initially secluded from the organic cuticle, and it represented a minute extrapallial space, favouring the saturation of ions inside. Active secretion of vesicles and organic network from the microvilli of the crystallization chamber wall was observed. These vesicles penetrated the forming sclerite. When the sclerites grew, additional secreting cells appeared at the basal zone of the crystallization chambers and the extrapallial space between the forming sclerites and the microvillous walls became nanometric. At the end of the formation of the sclerites, the crystallization chambers opened, releasing the sclerites into the cuticle. The sclerites were temporarily attached to the papillae by elongated microvilli but eventually were released and reached the cuticle surface. The pattern of secretion of the sclerites in the studied Polyplacophora was similar to that observed in most of the studied Solenogastres. This pattern, with a microvillous secreting epithelium, is similar to that of the shell-secreting mantle epithelium of conchiferan molluscs, such as the bivalves, some gastropods and the cephalopod Nautilus. It should, therefore, be considered the plesiomorphic state in Aculifera.
Salas et al. (Tue,) studied this question.