Torsion is an anatomical event, which takes place during the very early part of the life of snails and slugs of all kinds. In other words, torsion is a gastropod synapomorphy, which occurs in all gastropods during larval development. Torsion is the rotation of the visceral mass, mantle and shell 180∘ with respect to the head and foot of the gastropod. This brings the mantle cavity and anus to an anterior position above the head.