Question

Radial symmetry of echinoderm is
(A) Primary
(B) Secondary
(C) Biradial
(D) Triradial

Answer 1

Radial symmetry is a kind of symmetry in which the organism is divided into equal halves when any of the plane passes through the central axis. The symmetry in which different symmetry is found in larval and adult form is known as secondary symmetry.

Complete answer:
Symmetry is the basis of classifying the animals into different groups. There are three kinds of symmetry:

Asymmetrical: Non identical halves produced when any plane is passed through the centre of the axis. Example: Poriferans.
Radial symmetry: Identical halves are produced on passing any plane through the centre of the axis. Example: Coelenterata, Ctenophora and echinoderms.
Bilateral symmetry: The body is divided into two equal halves on passing the single plane through the centre of the axis. Examples: Annelids, arthropods.
Echinoderms don’t have a single symmetry in the whole period of life. That’s why option A is incorrect.
Echinoderms have pentaradial symmetry because they have five rays around the centre of the axis. They don’t have Biradial and tri radial symmetry. That’s why Option C and D are incorrect.
Echinoderms have bilateral symmetry in larvae and radial symmetry in adults. Such type of symmetry is known as secondary symmetry.

Hence, option B is correct.

Additional information: On the basis of secondary symmetry, we can say that echinoderm’s secondary symmetry is evolved from the bilateral symmetry bearing individuals.
Radial symmetry allows the organism to experience an environment from all directions.

Note: Secondary symmetry is found in those organisms which have different symmetry in different periods of life. For example: Echinoderms have bilateral symmetry in their larval form whereas radial symmetry is found in adults. That’s why they have secondary symmetry.