Inside out Monoplacophora: revisiting <i>Neopilina galatheae</i> Lemche, 1957 using µCT scanning

Abstract Neopilina galatheae Lemche, 1957, first collected in the 1950s, was presented as a critical species for understanding animal evolution, particularly in the context of segmentation. Decades later, we have now revisited this iconic species using a newly collected specimen and modern micro-computed tomography (µCT). A comprehensive 3D digital model of the internal anatomy of N. galatheae revealed new complexities within the musculature, digestive, excretory, and reproductive structures, as well as insights to its feeding ecology. Although developmental data for monoplacophorans are still lacking, it has long been clear that the repeated anatomical elements of Neopilina are not true metamerism. Our new results show that the seventh muscle pair is an asymmetrical double muscle bundle, and there are independent pairs of kidneys without interconnections. Monoplacophorans remain a critical and contentious group for the phylogeny of conchiferan molluscs, though recent phylogenomic studies have shown consistency with traditional morphological hypotheses. Understanding body plan evolution in molluscs requires robust anatomical descriptions that can be compared among classes and between living and fossil species. Neopilina galatheae remains the iconic, textbook monoplacophoran; revisiting classic species with modern technology can challenge established views and contribute new perspectives on evolutionary processes.