Question

Animals radially symmetrical in adults and bilaterally symmetrical in larvae are
A. Coelenterates
B. Polychaetes
C. Echinoderms
D. Hemichordata

Answer 1

Hint: On the basis of symmetry of their body plan, animals are classified into three types, i.e. asymmetric, radially symmetric, and bilaterally symmetric. Asymmetry is observed in the organisms of the phylum Porifera. Most of the organisms are bilaterally symmetric and radial symmetry is seen as rare.

Complete answer:
All kinds of symmetries are assigned to the animals depending upon their lifestyle and basic needs. In bilateral symmetry, the body of the organisms is divided through a sagittal plane into two equal halves (left and right side), such as in human beings, butterflies, etc. These organisms possess a head and a tail, as well as a back and a front side along with the right and left sides. Bilateral symmetry is helpful for ‘cephalization’, i.e. collecting and organizing the nervous system of the body at the anterior end. It promotes streamlined or directional motion only.
In radial symmetry, the body of the organism is arranged around a central axis (such as pieces of a pie). These organisms possess a top and a bottom surface but lack left and right or front and back sides. The side of the body where the mouth is present is called the oral side and the side without mouth is called the aboral side. This type of body symmetry helps organisms to move freely in all directions. This body plan is seen mostly in the organisms of the phylum Ctenophora (such as comb jellies) and Cnidaria, also known as Coelenterata (such as sea anemones, corals, etc.).
Apart from these two phyla, radial symmetry is also observed in the organisms of the phylum Echinodermata. But in these organisms, radial symmetry is observed only in the adult stages of life, whereas in the larval stage they possess bilateral symmetry.
So, the correct answer is “Option C”.

Additional Information:
Asymmetric organisms cannot be divided into two equal halves through any plane passing from the centre of their body. Only the members of the phylum Porifera are asymmetric, but there are some fishes also that do not show any body symmetry as adults.

Note:
Although the echinoderms show both radial and bilateral symmetry in their life cycle, they are classified as bilaterally symmetric and not radially symmetric because they are evolved from bilaterally symmetrical organisms.