Correct Answer - B
Torsion or twisting is a prcess during larval development of gastropods, which rotates the viscero-pallium anticlockwise brought `180^(@)` from its initial position, so that mantle cavity, with its pallial complex, is through in front of the body in adult. Changes occuringin torsion are to certain extent reversible. this reversion is known as detorsion and it is very characteristic of the whole group of the euthyneura. Formerly, this condition was looked upon as an arrested stage in the torsion, but these is the same reduction of the paired parts of the pallial complex as in the specialized streptoneura. Total detorsiop, as shown by the typical opisthobranchia is accompained by the reduction of disappearancce of the shell.
