<P> The evolution of an asymmetrical conispiral shell allowed gastropods to grow larger but resulted in an unbalanced shell . Torsion allows repositioning of the shell, bringing the centre of gravity back to the middle of the gastropod's body, and thus helps prevent the animal or the shell from falling over . </P> <P> Whatever original advantage resulted in the initial evolutionary success of torsion, subsequent adaptations linked to torsion have provided modern gastropods with further advantages . </P>

A characteristic that is differential between gastropoda and cephalopoda is that