Molluscs demonstrate an astonishing degree of morphological diversity, and the relationships among molluscan clades have been debated for more than a century. Molluscan nervous systems range from simple 'ladder-like' arrangements of nerve cords to the complex brains of cephalopods. Chitons (Polyplacophora) are assumed to retain many molluscan plesiomorphies, lacking neural condensation and ganglionic structure, and therefore a brain. We reconstructed three-dimensional anatomical models of the nervous system in eight species of chitons in an attempt to clarify chiton neuroarchitecture and its variability. The specimen material incorporated both new data and digitised historic slide material originally used in the work of malacologist Johannes Thiele (1860-1935). Reconstructions of whole nervous systems in Acanthochitona fascicularis, Callochiton septemvalvis, Chiton olivaceus, Hemiarthrum setulosum, Lepidochitona cinerea, Lepidopleurus cajetanus, and Leptochiton asellus, and the anterior nervous system of Schizoplax brandtii, demonstrated a consistent and substantial anterior concentration of nervous tissue in the circumoesophageal nerve ring. This neural mass is further organised into three concentric tracts, corresponding to the paired lateral, ventral, and (putatively) cerebral nerve cords. These represent homologues to the three main pairs of ganglia found in other molluscs. The relative size, shape and organisation of these components is highly variable among the examined taxa, but consistent with previous studies of select species, and we formulated a set of neuroanatomical characters for chitons. These characters are parsimony-informative for reconstructing chiton phylogeny at the ordinal and subordinal levels; the identification of robust detailed homologies in neural architecture will be central to future comparisons among all molluscs, and more broadly in Lophotrochozoa. Modern evolutionary thinking, and modern tomographic technology, bring new light to an old problem. Contrary to almost all previous descriptions, the size and structure of the chiton anterior nerve ring unambiguously qualify it as a true brain with cordal substructure.
