Question

Torsion of visceral mass is seen in animals belonging to class
a. Cephalopoda
b. Scaphopoda
c. Amphineura
d. Gastropoda

Answer 1

Hint: Animals of class-Gastropoda of phylum-Mollusca show torsion or twisting of the visceral mass during their embryonic development, leading to conversion of symmetrical embryo into asymmetrical adult.

Complete answer:
> A gastropod’s visceral mass is its collection of various internal organs that include heart, kidney, reproductive organs and digestive system. A gastropod’s digestive tract is twisted back onto itself as a result of a process called “torsion.” Torsion is actually the rotation of the visceral mass, mantle, and shell at 180˚ with respect to the head and foot of the gastropod. Torsion leads to an asymmetrical body mass where the anus actually exists over a gastropod’s head and mouth.

> Cephalopoda is most morphologically and behaviourally complex class in the phylum mollusca. Cephalopoda means “head foot” and this group has the most complex brain of any invertebrate. They are characterised by a completely merged head and foot, with a ring of arms and/or tentacles surrounding the head.

> The Scaphopoda are a distinct class of molluscs commonly known as the "tusk shells" as their shells are conical and slightly curved to the dorsal side, making the shells look like tiny tusks. The most characteristic feature of scaphopods is that the tubular shell is open at both ends, not just one end as in most molluscs.

> Amphineura is known as a class of elongate, bilaterally symmetrical, marine molluscs. They have a shell that, if present, comprises seven or eight overlapping, calcareous plates on the dorsal surface. The head if present is poorly differentiated.

Hence, the correct answer is option (D).

Note: Gastropods are the largest group of molluscs. Most of the gastropods have a single usually spirally coiled shell into which a soft body can be withdrawn, but the shell is lost or reduced in some groups. Gastropods are characterised by “torsion”, a process that results in rotation of visceral mass and mantle on the foot.