A hallmark in snails’ anatomy is the conspicuous crossing of the pleurovisceral nerve cords present in most basal gastropod clades. This feature is called streptoneury and hitherto near-universally believed to derive from the process of torsion which is, ontogenetically, visible by a 180° horizontal rotation of the cephalopodium relative to the visceral sac, being also responsible for the formation of a cranially bent gut and the location of gills in a mantle cavity that opens to the anterior. However, a mechanical link between the ontogenetic rotation of the visceropallium and streptoneury which is implied in many textbook presentations has never been demonstrated directly. After suppressing ontogenetic torsion and replacing it with a 90° vertical tilt of the visceropallium in an individual of the freshwater apple snail Marisa cornuarietis, we could show in a 3D reconstruction based on serial sectioning that the nervous system of the non-torted snail mirrored the classical organization of normal, torted individuals and showed all features of streptoneury in this species. Furthermore, immunolabelling provided no indication that the pleurovisceral cords were fully shaped after completion of ontogenetic torsion. This singular observation provides an experimental proof for the doubts that have recently arisen about the association between streptoneury and 180° horizontal ontogenetic torsion.
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