Question

The body pattern of _______ is asymmetric.
A. Snails
B. Pila
C. Octopus
D. Mussel

Answer 1

Hint: Symmetry is referred to as the arrangement of body parts in a balanced geometrical design, so that the body is divisible into equal parts by planes of division. An animal is considered to be symmetrical only when a plane passing through its centre will divide it into right and left halves. When an animal can’t be divided into similar halves it is referred to as asymmetrical.

Complete solution:
Option A. Snails: They belong to the class Gastropod, phylum Mollusca and possess a symmetrical body. They have a spiral shaped shell made up of calcium carbonate.
Option A is not correct.
Option B. Pila: Pila, also known as apple snail, belongs to the class Gastropod, phylum Mollusca. During growth, a phenomenon occurs known as torsion or twisting, because of which the adults become asymmetrical. Although, the early embryo is symmetrical. During larval development, the rotation of visceral and pallial organs through an angle of $${180}^{\circ }$$ in an anticlockwise direction on the rest of the body is known as torsion or twisting.
Option B is correct.
Option C. Octopus: It belongs to the class Cephalopoda, phylum Mollusca. In Cephalopods, the region of the head and foot is combined and modified into a structure which contains eyes and eight tentacles, hence the name cephalopod or ‘head foot’. It also possesses a symmetrical body.
Option C is not correct.
Option D. Mussel: It belongs to the class Bivalvia, phylum Mollusca. Their shell is made up of $$2$$ halves, known as the valves. It also possesses a symmetrical body as the shells are mirror opposites of one another.
Option D is not correct.

Hence, Option C is the correct answer

Note:
During torsion, rotation takes place in the visceral mass only behind the neck, so that the visceral organs of the right side occupy the left side and vice versa, but the head and foot remain fixed. Before torsion the mantle cavity is posterior in position and the visceral mass points forward. This position is referred to as exogastric. After torsion the position changes to endogastric, in which the visceral mass points backwards and the intestine lies in the whorls of the shell and the opening of the anus is on the anterior side.