question_answer

Assertion : Detorsion is the characteristic of mollusca. Reason : Detorison is an arrested stage of torsion.

A) If both the assertion and the reason are true and the reason is a correct explanation of the assertion

B) If both the assertion and reason are true but the reason is not a correct explanation of the assertion 

C) If the assertion is true but the reason is false

D) If both the assertion and reason are false

E) If the assertion is false but reason is true

Correct Answer: B

Solution :

Torsion or twisting is a process during larval development of gastropods, which rotates the viscero- pallium anti clockwise brought  from its initial position, so that mantle cavity, with its pallial complex, is through in front of the body in adult. Changes occuring in torsion are to certain extent reversible. This reversion is known as detorsion and it is a very characteristic of the whole group of the euthyneura. Formerly, this condition was looked upon as an arrested stage in the torsion, but there is the same reduction of the paired parts of the pallial complex as in the specialized streptoneura. Total detorsion, as shown by the typical opisthobranchia is accompained by the reduction of disappearance of the shell.