Animal Characterization Based on Body Symmetry | Zoology Optional Notes for UPSC PDF Download

At a fundamental level of categorization, genuine animals can be primarily categorized into three groups based on their body plan's symmetry: radial symmetry, bilateral symmetry, and asymmetry. Radial symmetry is only found in a limited number of animal groups, whereas asymmetry is a distinctive characteristic of the Porifera phylum, which includes sponges. Each type of symmetry is adapted to meet the specific requirements of a particular animal's way of life.

Radial Symmetry

Radial symmetry is the organization of body parts around a central axis, similar to the rays of the sun or slices of a pie. Animals with radial symmetry possess upper and lower surfaces but lack distinct left and right sides, as well as front and back. The two halves of a radially symmetrical animal are often referred to as the side with a mouth ("oral side") and the side without a mouth ("aboral side").
This symmetry pattern characterizes the body structures of organisms within the Ctenophora phylum, such as comb jellies, and the Cnidaria phylum, including corals, sea anemones, and other jellyfish. Radial symmetry equips these marine creatures, which may be sessile or have limited mobility or floating capabilities, to interact with their environment uniformly from all directions.

Bilateral Symmetry

Bilateral symmetry is characterized by a division of the animal along a sagittal plane, resulting in two mirror-image halves – one on the right and the other on the left. This symmetry pattern is observed in creatures like butterflies, crabs, and the human body. Animals with bilateral symmetry possess distinct "head" and "tail" regions (anterior and posterior), front and back sides (dorsal and ventral), and right and left halves. With the exception of animals displaying radial symmetry, the majority of true animals exhibit bilateral symmetry.
The evolution of bilateral symmetry, which led to the development of anterior and posterior (head and tail) ends, facilitated a phenomenon known as cephalization, involving the concentration of a well-organized nervous system at the animal's front end.
In contrast to radial symmetry, which is best suited for stationary or minimally mobile lifestyles, bilateral symmetry allows for streamlined and directed movement. From an evolutionary perspective, this simpler form of symmetry promoted active mobility and enhanced the sophistication of resource-seeking and predator-prey relationships.

Animals in the phylum Echinodermata (such as sea stars, sand dollars, and sea urchins) display radial symmetry as adults, but their larval stages exhibit bilateral symmetry. This is termed secondary radial symmetry. They are believed to have evolved from bilaterally symmetrical animals; thus, they are classified as bilaterally symmetrical.

Asymmetry

Among living organisms, only sponges belonging to the Porifera phylum exhibit a complete lack of body plan symmetry. While there are certain fish species, like flounders, that display asymmetry in their adult stage, it's worth noting that their larval forms maintain bilateral symmetry.