Assertion : Torison can be seen in ctenidium. Reason : Ctenidium acts as the respiratory organ.

Ctenidium is a monopectinate gill situated on the right side of the branchial chamber. Ctenidium helps in respiration by beating of cilia, thus creating water current. Ctenidium is innervated by nerves from the left pleural and supra-intestinal ganglia. This nerve supply indicates that it is actually an organ of the left side but has shifted to the right side during development (torsion). Torsion or twisting is a development of gastropods, which rotates the visceropallium anticlockwise through out \(180^{\circ}\) from its initial position, so that mantle cavity with its pallial complex, is brought in front of the body in adult.