Practice question

Torsion fundamentally reorganizes gastropod body architecture during larval development. Rotation of visceropallium through 90 to 180 degrees brings originally posterior mantle cavity containing ctenidia, osphradia, anus and nephridiopores to anterior position dorsal to head and anterior to heart. Alimentary canal becomes sharply U-shaped, anus now discharging above mouth near head, creating sanitation problem. Advantage lies in ability to withdraw head-foot into shell with mantle cavity opening anteriorly facing incoming water current, enhancing respiratory flushing and predator protection, but causing fouling risk solved later by detorsion or shell slits.