Question

Which of the following molluscs is formed by a larva which has torsion?
A. Lamelledens
B. Pila
C. Sepia
D. Octopus

Answer 1

Torsion is a process in which the viscero-pallium rotates anti-clockwise through 180 degrees from its initial position during larval development which brings the mantle cavity, with the pallial complex to the front of the body in an adult.

Complete answer:
Gastropoda is the larger invertebrate class of the Phylum Mollusca and the animals within this class are called Gastropods. Torsion is the characteristic feature of gastropods and occurs in almost all of them. It takes place in their free-swimming larva stage.

Let us analyze the options to find the correct answer.
Option A: Lamelledens is the genus of Bivalve which is the class of marine and freshwater molluscs. The species of this genus do not exhibit torsion.
Option B: Pila belongs to the class Gastropoda which exhibits the characteristic feature of torsion. The species of this genus exhibit torsion.
Option C: Sepia belongs to the class Cephalopoda which does not exhibit torsion. Their development is direct and thus they lack larval stages.
Option D: Octopus also belongs to the class Cephalopoda which exhibits spiral cleavage. Their development is direct and thus they have no larval stages.

Hence, the correct answer is option (B)

Note: In some gastropods, Torsion is a reversible process. Detorsion means the reversion to the changes that have occurred during torsion. And thus, because of this the pallial complex can travel towards the posterior end along the right side.