Radial vs Bilateral Symmetry
Symmetry, the balanced distribution of duplicate body parts, is a prominent feature in biological organisms, especially animals; but plants too exhibit interesting symmetrical features. The symmetry of animals has a long history with its existence prevails in many taxonomic phyla. Radial symmetry and bilateral symmetry are the two main types of symmetric levels found in animals, and there are some important differences between those. However, symmetry in biology is a rough idea, which is mainly due to the fact that the symmetric portions of the body are not perfectly identical but almost similar with each other.
What is Radial Symmetry?
In radial symmetry, there are identical body parts that are distributed in a circular arrangement around a central axis. Coelenterates (aka Cnidarians) and Echinoderms are the two best examples with the presence of this kind of body symmetry. Usually, animals with radial symmetry have two dorsal and ventral sides rather than left and right sides. The central axis is usually formed between oral and aboral ends of the radially symmetric organisms. Among cnidarians, the radial symmetry is prominent in both of their body forms, medusa form with tentacles arranged on the central disc-like body and polyp form with a cylindrical central body surrounded by the radially arranged tentacles.
The echinoderms exhibit a special type with five identical body parts distributed around the central axis, and this type of symmetry is known as the Pentamerism or penta-radial symmetry. Pentamerism can be observed among plants as well; flowers with five equal petals or fruits with the fivefold symmetry could be considered as examples. In addition, the radial symmetry can occur in many forms such as octamerism (eight) and hexamerism (six). All in all, coral organisms, jellyfish, starfish, sea urchin, sea cucumber, and many other examples can be considered to discuss the radial symmetry in animals.
What is Bilateral Symmetry?
In bilateral symmetry, the body can be divided into two equal halves through the central plane. When this idea is adopted into animals, the central plane, aka sagittal plane, the two halves are known as right and left. Bilateral symmetry is most prevalent in the plant leaves with midrib being the central plane that divides the two halves. The closest example for bilateral symmetry would be the human body, which can be divided into right and left halves through the sagittal plane. Indeed, all the phyla in the Animal Kingdom except Unicellular animals, Cnidarians, and Echinoderms exhibit the bilateral symmetry.
Forward and backward movements have been made convenient for animals with the bilaterally arranged bodies, especially for the terrestrial animals. It would be important to state that the animals with a central nervous system control the left and the right halves of the body by the opposite sides of the brain. In other words, left side of the vertebrates is controlled through the nervous signals originating from the right side of the brain. The colloquial statement “left-handed has the right brain” has the origins from bilateral symmetry.
What is the difference between Radial and Bilateral Symmetry?
• Bilateral symmetry has a symmetric plane while radial symmetry has a symmetric axis.
• Only two similar portions can be identified from bilateral symmetry whereas, from radial symmetry few similar portions of the body can be identified.
• All the radially symmetric animals are found in water, but bilaterally symmetric animals are found in both aquatic and terrestrial habitats.
• Bilateral symmetry is more common than radial symmetry among animals. In fact, there are more animal phyla with bilateral symmetry compared to radial symmetry.
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