Torsion and detorsion in Gastropoda
Torsion or twisting is a process during the larval development of the gastropods, which rotates the viscera-pallium anticlockwise through 180 0 from its initial position. Therefore, the mantle cavity with its visceral complex is brought to the front of the body in adults. Torsion is significant from the evolution perspective. Sie of Torsion : In the larval gastropods, only the visceral mass undergoes rotation through 180 0 whereas the head and the foot remain fixed. The actual site of the torsion is the neck, behind the head-foot through which the oesophagus, rectum, aorta, visceral nerve loop and shell muscle pass. Thus, actual twisting involves the neck tissue and the structure within it. Torsion is often confused with the spiral coiling of the visceral mass and shell, but the two are entirely distinct and quite independent. Mechanism of torsion : Torsion can be seen in the embryology of living gastropoda, i.e., the veliger larva. Before torsion, the larva is quite symmetrical (