Why is torsion considered unique to gastropods?

Torsion is considered unique to gastropods because it is a distinct developmental process that occurs during the veliger larval stage, involving a 180° twisting of the body. This anatomical shift brings the posterior portion of the body to an anterior position, placing it directly behind the head.1

While other mollusks exhibit various body plans—such as the bilateral symmetry of bivalves or the dorsoventral elongation of cephalopods—this specific, dramatic reorientation of the visceral mass is a defining characteristic exclusive to the gastropod class. In some gastropods, this process may be followed by a secondary "detorsion," but the initial occurrence of torsion remains a unique evolutionary hallmark of the group.21