Torsion and Detorsion in Gastropoda
Molluses are typically bilaterally symmetrical animals but this symmetry is lost in
Gastropoda due to two processes called torsion
The visceral hump behind the head includes the visceral mass, mantle, mantle cavity, and
foot, it rotates in a counter-clockwise direction through an angle of 180 on the rest of the
body by contraction of an asymmetric retractor muscle which arises from the right side of
the larval shell, passes over the body and gets inserted to the left side of the head. This
rotation is known as torsion.
Site of torsion:
In gastropods the head and foot remains fixed but the visceral mass undergoes rotation.
Actual site of torsion is neck, behind the head-foot, through which oesophagus, rectum,
aorta, visceral nerve loop and shell muscle pass. Thus the twisting involves the neck tissue
and structures within it.
Course of Torsion:
The pre-torsional larva is bilaterally symmetrical; the mantle cavity is posterior and ventral;
gut straight, opening posteriorly. A ventral flexure of the body changes the straight
alimentary canal to a loop and the mouth and anus come close to each other. Initially
saucer shaped visceral mass and snell first become a cone shape and later become spirally
coiled.
Process of torsion:
In torsion only a narrow part of the body and the organs which pass through it are twisted,
it is that small part which lies between the visceral hump and the rest of the body. Torsion
changes the orientation of the mantle cavity and its organs, and the organs of the left side
tend to be reduced or even lost.
Before torsion the mantle cavity opens posteriorly, ctenidia point backwards, the auricles
are behind the ventricle, the nervous system is bilaterally symmetrical, and the mouth and
the anus are at opposite ends.
After torsion the mantle cavity opens in front just behind the head, ctenidia come to lie in
front and point anteriorly, the ctenidium of the right side comes to lie on the left and that
of the left side on the right, the auricles become anterior to the ventricle, the auricle of the
right side comes to lie on the left and vice versa, the nervous system is twisted into a figure
of 8 by the crossing of the two long pleuro-visceral connectives nerve connectives running
to the viscera, and the digestive system becomes U-shaped so that the anus comes to lie in
front near the
Before torsion the visceral mass points forward and the mantle cavity is posterior in
position. This position is called exogastric. After torsion the position becomes endogastric
Molluses are typically bilaterally symmetrical animals but this symmetry is lost in
Gastropoda due to two processes called torsion
The visceral hump behind the head includes the visceral mass, mantle, mantle cavity, and
foot, it rotates in a counter-clockwise direction through an angle of 180 on the rest of the
body by contraction of an asymmetric retractor muscle which arises from the right side of
the larval shell, passes over the body and gets inserted to the left side of the head. This
rotation is known as torsion.
Site of torsion:
In gastropods the head and foot remains fixed but the visceral mass undergoes rotation.
Actual site of torsion is neck, behind the head-foot, through which oesophagus, rectum,
aorta, visceral nerve loop and shell muscle pass. Thus the twisting involves the neck tissue
and structures within it.
Course of Torsion:
The pre-torsional larva is bilaterally symmetrical; the mantle cavity is posterior and ventral;
gut straight, opening posteriorly. A ventral flexure of the body changes the straight
alimentary canal to a loop and the mouth and anus come close to each other. Initially
saucer shaped visceral mass and snell first become a cone shape and later become spirally
coiled.
Process of torsion:
In torsion only a narrow part of the body and the organs which pass through it are twisted,
it is that small part which lies between the visceral hump and the rest of the body. Torsion
changes the orientation of the mantle cavity and its organs, and the organs of the left side
tend to be reduced or even lost.
Before torsion the mantle cavity opens posteriorly, ctenidia point backwards, the auricles
are behind the ventricle, the nervous system is bilaterally symmetrical, and the mouth and
the anus are at opposite ends.
After torsion the mantle cavity opens in front just behind the head, ctenidia come to lie in
front and point anteriorly, the ctenidium of the right side comes to lie on the left and that
of the left side on the right, the auricles become anterior to the ventricle, the auricle of the
right side comes to lie on the left and vice versa, the nervous system is twisted into a figure
of 8 by the crossing of the two long pleuro-visceral connectives nerve connectives running
to the viscera, and the digestive system becomes U-shaped so that the anus comes to lie in
front near the
Before torsion the visceral mass points forward and the mantle cavity is posterior in
position. This position is called exogastric. After torsion the position becomes endogastric
