Torsion in Gastropods
What is Molluscan Torsion?
Torsion is the rotation of the visceral mass, mantle, and shell 180˚ with respect to the head and foot of the gastropod.
This rotation brings the mantle cavity and the anus to an anterior position above the head and mouth.
A gastropod shell has only one opening that servers as both entrance and exit for all visceral organs.
Mouth and anus open on the anterior side.
The mantle cavity opens on the anterior side for easy respiration.
Benefits of torsion:
The snail’s head is drawn into the shell first, and as the foot retracts, an operculum covers the aperture.
Without torsion, withdrawal into the shell would proceed with the foot entering first and the more vulnerable head entering last.
The twist in the mantle’s sensory organ around to the head region.
This makes the snail more sensitive to stimuli coming from the direction in which it is moving.
Benefits of torsion:
Protection – head enters first.
Allows clean water to enter the mantle cavity.
Makes snail more sensitive to stimuli coming from the front.
Operculum seals the opening to prevent drying out.
Development
There are two different developmental stages which cause torsion
sca
Torsion is a process in which the viscero-pallium* rotates counterclockwise through 180° from its initial position during larval development (veliger stage).
