Question

Radially symmetric body plan includes __________
A. Nematodes
B. Nemertea
C. Cnidaria
D. All the above

Answer 1

Hint: Symmetry refers to any line or axis (imaginary) that can divide an object or an organism into similar parts. Most organisms have features that show symmetry. Symmetry can be – bilateral, biradial, radial and spherical. Symmetry can be external, that is determined easily by observing the object or the organism or internal, say symmetry of the lungs in humans.

Complete answer:
Symmetry is said to be radical when an imaginary axis passes through a central point that can divide the body into segments.
When an imaginary line can divide the organism into equal halves, such that both halves appear as mirror images of each other, it is referred to as bilateral symmetry.
Organisms that show spherical symmetry can be divided into two identical halves, regardless of the plane of symmetry. Here, an infinite number of symmetry axes can be drawn. This symmetry can be observed in Volvox.
Certain organisms can have both bilateral and radial symmetry. These organisms have a biradial body plan. This symmetry is seen in Ctenophores.
Nematodes are small worms with a prominent head and slender body. Their body is bilaterally symmetrical. The head however, is radially symmetrical. They are considered to have bilateral symmetry
Nemertea are also worms that show bilateral symmetry.
Cnidaria are animals that show radial symmetry.
Hence, the correct option is (C).Cnidaria has a radially symmetrical body plan.

Note:
Certain organisms do not show any symmetry i.e., some are asymmetrical like Sponges. Viruses exhibit a unique symmetry known as icosahedral symmetry. The shape of the viral capsid attributes to this symmetry. Organisms that show symmetry often have advantage over the ones that lack it. For example, certain organisms prefer to procreate with ones that show certain symmetrical features in their body.