Question

Torsion occurs in the visceral mass of
A. Gastropoda
B. Pelecypoda
C. Cephelopoda
D. Monoplacophora

Answer 1

Hint: There are two different developmental phases which cause torsion. The first phase occurs by the development of the asymmetrical velar/foot muscle which has one end attached to the left side of the shell and the other end has fibres attached to the left side of the foot and head.

Complete Answer:
Torsion is characteristic of Gastropoda. It is seen in most gastropods during the period of larval development. Torsion develops in the visceral mass. It is the phenomenon of rotator movement of the visceral mass, mantle, and shell at 1800. This results in the anterior position of the mantle cavity and the anus above the head.

Now, let us find the solution from the option.
- Torsion is characteristic of Gastropoda. It is seen in all gastropods during larval development. Torsion occurs in the visceral mass. It is the phenomenon of movement of the visceral mass, mantle, and shell at 1800. This results in the anterior position of the mantle cavity and the anus top of the head.
- Pelecypoda is recognized by a bivalve shell surrounding the body without head and lamellae gills, consisting of the mussels, scallops, oysters and clams.
- Cephalopods are characterized by a completely merged head and foot, with a ring of arms and/or tentacles surrounding the head.
- Monoplacophorans have a single symmetrical shell, which varies in shape from a flattened shield like plate to a short cone.

Thus, the correct option is A.i.e. Gastropoda.

Note: As a gastropod shell has rarely one opening, it has to perform as entry point as well as exit point for most of visceral organs. Both mouth and anus need to unblock on the anterior side. Mantle cavity also needs to unblock on the anterior side for respiration. Respiratory flow opposes locomotion after torsion which will increase availability of water interior the branchial chamber.