Abstract
Bivalves have particular life orientations for each species. Species of Tellinidae and Semelidae burrow in sediment and are orientated with their commissure plane nearly horizontal and almost always rest on their left side. However, three species of the tellinid genus Cadella, which have the commissure plane nearly horizontal, lie on their right side. It is suggested that this reversed orientation in Cadella is an inversion of the normal left side orientation and appears to be the first example of behavioural inversion in bivalves.