All molluscs, except the Gastropoda, retain ancestral bilateral symmetry of the body. Mantle cavity lies posteriorly or laterally.
Gastropods have an asymmetrical body with mantle cavity lying anteriorly, and the shell and visceral mass coiled spirally and directed posteriorly.
Anterior situation of the mantle cavity in gastropods is due to torsion or twisting of the visceral mass during development.
What is Torsion?
Process during larval development of gastropods, which rotates the viscera-pallium anti-clockwise through 180º from its initial position (w.r.t head and foot).
This results in mantle cavity and anus being brought in front of the body in adult.
Only visceral mass undergoes rotation by 180º, head and foot remain fixed.
Torsion starts before coiling of the shell (Coiling is achieved by more rapid growth of one side of the visceral mass than the other. Torsion and coiling are two separate evolutionary events).
How does Torsion occur?
Before torsion, larva is symmetrical. Mantle cavity faces downwards and backwards, alimentary canal is straight, anus open posteriorly in the middle line.
Ventral flexure of the body results in looping of alimentary canal and approximation of mouth and anus. The shell and visceral mass, originally saucer shaped, become cone shaped and then spirally coiled.
Shell lies dorsally and and forms a coil on the anterior side: Exogastric shell.
Ventral flexure is followed by a lateral torsion, dorsal / exogastric shell becomes ventral / endogastric.
Ways in which torsion can be brought about:
Growth arrest Hypothesis
Lateral torsion is probably due to arrest of growth on one side and active extension on the other.
Growth of the right side becomes retarded so that mantle cavity gradually pass around to the right side, and to the anterior side on account o greater growth of the visceral sac toward the left.
e.g. Vivipara
Muscular Contraction Hypothesis
Asymmetrical position and pull of the larval retractor muscles running from velar lobes to the shell.
They are present only on the right side, with no related muscles on the left.
Contraction brings about torsion.
Only narrow neck of the larva is twisted, and consequently, everything between head and anus undergoes an anticlockwise rotation by 180º.
e.g. Acmaea
Two stage
First 90º movement by contraction of larval retractor muscles.
Then a slow 90º torsion by differential growth.
Commonest mechanism known today. e.g. Patella.
Effects of Torsion:
Displacement of Mantle cavity
-Mantle cavity was primitively posterior in position.
-Increase in length of the ventral foot, tends to remove the mantle cavity and pallial complex away from the head.
-After Torsion, the mantle cavity opens just behind the head and its associated parts are shifted forwards.
Changes in relative positions
-Anus, Ctenidia and renal orifices project forward.
-Auricles lie infant of the ventricles.
-Original posterior face of visceral sac becomes the actual anterior face.
-Right hand side organs are now placed on the left hand side.
Digestive tract, which was originally in a straight path from mouth to anus, is now thrown in a loop
Pleuro-Visceral nerve connections become twisted into a figure of 8.
Coil of visceral sac and the shell changes from dorsal and exogastric to ventral and endogastric.
Loss of symmetry.
Significance of Torsion
Garstang (1928)
Torsion first occurred as a larval mutation of advantage to the larva, but of little direct use to adult.
Torsion helped to increase predator evasion. After torsion, mantle cavity became anterior, so head and velum could withdraw first before the foot. Operculum sealed the aperture and Cilia stop beating, so that the Gastropod can fall to the sea bottom.
Morton (1958)
Torsion promotes stability in the adult by placing bulky mass of animal near the substrate.
When mantle cavity containing gill is placed posteriorly, when the animal moved upstream, the water flow and current due to movement of animal opposed the respiratory current entering the mantle cavity. After torsion, the currents flow in same direction increasing the respiration.
Anteriorly placed chemoreceptive organs can sample the sediment and incurrent water stream and the animal orients itself properly with the help of sense organs on the head.
Detorsion:
Reversion of changes that occurred during Torsion.
Characteristic of the group Euthyneura.
Pallial complex travels back towards the posterior end along the right side.
Ctenidia point backwards, auricles move behind the ventricle, visceral loop becomes untwisted and symmetrical.
Thus, a secondary external symmetry is established.
Various degrees of detorsion are met within Euthyneura.
In least specialised Opisthobranchia and Pulmonata, detorsion is not complete, so the visceral loop remains partly twisted. Anus and Ctenidium are directed laterally.