Question

What is bilateral symmetry?
A) Type of arrangement of the parts and organs of an animal in which the body can be divided into two halves.
B) Type of arrangement of the parts and organs of an animal in which the body can be divided into three halves.
C) Type of arrangement of the parts and organs of an animal in which the body can be divided into four equal halves.
D) None of these.

Answer 1

Hint: In the animal kingdom, animals can be classified based on body symmetry. All animals are differentiated based on various body patterns. Body symmetry is of three types; asymmetric, radially symmetric and bilaterally symmetric. This type of organization of the body can be seen in Platyhelminthes.

Complete answer: The symmetrical axis as well as the plane is important for a body. An axis is an imaginary straight line passing through the body of an organism. For example, the axis passing through the lengthwise direction of the body of an organism is called the longitudinal axis, while the axis passing through the body which divides the body of an organism into the head and the tail portion is known as the transverse axis. The body of an organism is divided into corresponding halves through the flat surface, this surface is known as the plane. The plane divides the body into left and right halves or front and back halves is known as the longitudinal plane. The transverse plane divides the body into upper and lower halves. When a longitudinal axis passing through the body divides the body into identical left and right halves in only one plane, then it is called bilateral symmetry. The animals possessing bilateral symmetry are called as bilaterally symmetric. The phylum Platyhelminthes is the first phylum of the animal kingdom which possess bilateral symmetry. It is characteristic of higher animals which include chordates.
Hence, the correct answer is option A.

Note: The longitudinal plane is always perpendicular to the transverse plane. Bilateral symmetry arose when animals become motile on the ocean floor. It is also shown by animals of phylum Aschelminthes, Annelids, Arthropods, Molluscs and Hemichordates.