Torsion (gastropod)

Torsion is the rotation of the visceral mass, mantle, and shell 180˚ with respect to the head and foot of the gastropod.

At a certain point in larval development this muscle contracts, causing an anticlockwise rotation of the visceral mass and mantle of roughly 90˚.

For example, having the place where wastes are excreted positioned above the head could result in fouling of the mouth and sense organs.

Some gastropods can also close the entrance to their shell with a tough operculum, a door-like structure which is attached to the dorsal surface of their foot.

The English zoologist Walter Garstang wrote a famous poem in 1928, The Ballad of the Veliger, in which he argued with gentle humour in favour of the defence theory, including the lines[1] Predaceous foes, still drifting by in numbers unabated,Were baffled now by tactics which their dining plans frustrated.Their prey upon alarm collapsed, but promptly turned about,With the tender morsel safe within and the horny foot without!

Another possible advantage for aquatic species is that moving the osphradium (olfactory sense organs) to an anterior position means they are sampling water the gastropod is entering rather than leaving.

This is due to the back and forth motion of the shell during movement, which would tend to block the mantle opening against the foot if it was in a posterior position.