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Torsion is a process in which the viscero-pallium rotates anti-clockwise through 180° from its initial position during larval development. So that the mantle cavity, with its pallial complex, is brought to the front of the body in the adult.
How Torsion Occurs
Torsion is not an evolutionary hypothesis. Its occurrence can be seen during embryogeny of living gastropods. Before torsion the larva is quite symmetrical, the mantle cavity is backward and downward, the alimentary canal is straight and anus opens posteriorly in the middle line. The shell and visceral mass originally saucer-shaped, which is first cone-shaped and then spirally coiled. When the shell lies dorsally and forms a coil anterior side then the shell is called exogastric.
The ventral flexure is followed by a lateral torsion, so that the dorsal or exogastric shell becomes a ventral or endogastric shell. The entire process takes 2 to 3 minutes as in Acmaea.
Torsion is actually effected due to the contraction of the larval retractor muscles, in which only the narrow neck of the larva is actually twisted. So the everything between the head and the anus undergoes an anti-clockwise rotation at the angle of 180°.
Thomson (1958) described five ways by which the torsion can be brought about.
In Archaeogastropods, the Acmaea, the torsion takes place by muscles contraction alone.
The rotation of 180° is completed in two stages, the first movement takes place by the contraction of larval retractor muscle and second rotation is slower by different growth. It is very common as in Patella, Haliotis etc.
The rotation of 180° takes place by only differential growth processes like Vivapara.
Rotation by differential growth processes, with anus coming to a position appropriate to the adult state like Aplysia.
Torsion is no longer recognizable as a movement of viscera-pallium, the organs in the post-torsional position from their first appearance as in Adalaria.
Effects of Torsion
The effect of torsion first time indicated by Spengel in 1881. The effects are not equally examined in all the gastropods by the general effects are described as follows:
Displacement of mantle cavity: The mantle cavity was primarily present on the posterior side. The elongation of the ventral foot which was primarily very small. After torsion, the mantle cavity opens just behind the head and its associated parts are shifted forwards.
Changes in relative position: Before torsion, the anus, ctenidia and excretory opening were placed on the posterior side and the auricles were placed behind the ventricle but after torsion the anus, ctenidia and excretory opening become anterior and the auricles lie in front of the ventricle. The original posterior face of the visceral sac becomes the anterior face so that the visceral organs morphologically of the original right side change into the left side.
Looping of alimentary canal: The alimentary canal which was originally straight from mouth to anus, after torsion, it changes into a loop.
Chiastoneury: The long, uncoiled pleuro-visceral nerve connectives become a figure of "8" after twisting. The right connective with its ganglion passes over the intestine to form the supra-intestinal connective, while the next connective pass under the intestine to form the infra-intestinal connective.
Endogastric coil: The coil of visceral sac and shell, which was primarily dorsal or exogastric become ventral and endogastric after torsion.
Loss of Symmetry of Atrophy: The anus changes their original position towards the right side of the pallial cavity so that the original symmetrical condition disturbed.
The torsion is reversible in Eu-thy-neura and its reversal is known as detorsion by which animal become untwisted and symmetrical.