Periostracum Formation in Sepia officinalis and Loligo vulgaris and Homology with Other Molluscs Ruiz Villaespesa, Ernesto Checa González, Antonio G. Lucena Serrano, Cristina Salas, Carmen Periostracum Mollusca Cephalopods The periostracum is the outermost shell layer and the first produced during shell formation in molluscs. This organic layer isolates the extrapallial space from the external environment and provides a scaffold for subsequent calcification. In cephalopods with an internal shell, some organic shell structures are putatively homologous to the periostracum of other molluscan groups. However, neither their detailed structure nor their mode of formation has been described, leaving the extent of this homology unresolved. To address this issue, we investigated the morphology and formation of the organic layer of the dorsal shield and the gladius in embryos of the cuttlefish Sepia officinalis Linnaeus, 1758, and the squid Loligo vulgaris Lamarck, 1798, respectively, using light microscopy and transmission electron microscopy. In both species, the periostracum forms within a periostracal groove located along the lateral and anterior margins of the shell sac. As in other molluscs, secretions from columnar cells at the bottom of the groove produce a dense layer, while a translucent layer is subsequently added beneath it through secretions from cuboidal cells. The main difference is the absence of both a pellicle and of the specialized glandular cells that typically secrete it at the bottom of the periostracal groove. 2026-03-09T12:17:17Z 2026-03-09T12:17:17Z 2026-03-07 journal article Ruiz-Villaespesa, E., Checa, A. G., Lucena-Serrano, C., & Salas, C. (2026). Periostracum Formation in Sepia officinalis and Loligo vulgaris and Homology with Other Molluscs. Animals, 16(5), 841. https://doi.org/10.3390/ani16050841 https://hdl.handle.net/10481/111981 10.3390/ani16050841 eng http://creativecommons.org/licenses/by/4.0/ open access Atribución 4.0 Internacional MDPI