Radial vs. Bilateral Symmetry Definition
We’ll start by taking a look at the differences between Radial vs. Bilateral Symmetry. Symmetry in biology refers to an organism’s bodily components being arranged around a central point or axis. The two fundamental types of symmetry are radial symmetry (wherein body parts are structured around a central axis) as well as bilateral symmetry (wherein organisms may be split along a single plane into two almost identical halves). A small number of organisms exhibit asymmetry, or loss of symmetry, throughout the body line.
| Radial Symmetry | Bilateral Symmetry |
| Similar body parts are arranged around a central axis | Similar body parts are arranged on either side of the sagittal axis |
| Organisms have a top and bottom but no left or right sides | Organisms have front and back, head and tail, and left and right sides |
| Organisms are typically sedentary or slow-moving | Organisms typically have a wide range of movement |
| Organisms can be divided into near-identical halves when a plane is passed at any angle through the central axis | Organisms’ can only be divided into near-identical halves along a single plane (the sagittal plane) |
| Organisms usually don’t have a head at the front of the body | Organisms have a distinct head at the front of the body |
Radial vs. Bilateral Symmetry
What is Radial Symmetry?
Similar bodily parts are clustered along a central axis in radially symmetrical organisms. They have left and right sides, but no top or bottom surfaces. When a plane is run across the central axis of an organism with radial symmetry, it may be split into two nearly equal halves.
Animals with radial symmetry have their generally cylindrical body components organised together or radiating from the centre axis. The oral (mouth-containing) side as well as the aboral side (the side without a mouth) of such creatures are the top and bottom halves, respectively (the side with no mouth).
Radial vs. Bilateral Symmetry
Examples of Radial Symmetry
In the animal world, there are just a few instances of radial symmetry. Radial symmetry is seen in animals belonging to the phylum Ctenophora or Cnidaria.
The comb jellies are a class of invertebrate organisms known as Ctenophora. They can be identified by their rows of swimming cilia, often called “combs.” The largest creatures reported to use cilia for movement are comb jellies, with sizes ranging from 1 millimetre (0.4 inch) to 1.5 meters (4 ft 11 in).
Several radially symmetrical jellyfish, corals, and sea anemones are members of the class Cnidaria. Both sea urchins and sea cucumbers are members of the phylum Echinoderm. They are examples of creatures with radial symmetry. Many flowers, outside of the animal realm, have radial symmetry.
Sedentary or slow-moving animals are common among radially symmetrical species. Their radial symmetry helps organisms feel their environment across all angles without excessive movement.
Radial vs. Bilateral Symmetry
What is Bilateral Symmetry?
Bilateral symmetry exists in organisms that may be divided into two parts along the sagittal plane. The sagittal plane divides the organism into two nearly identical halves that traverse the midline from front to back.
Examples of bilateral symmetry in animals include the front and back (dorsal and ventral), head and tail (anterior and posterior), and left and right sides. Bilaterally symmetrical animals have substantially more activity than radially symmetrical ones, permitting more complex connections between their surroundings and other organisms.
Radial vs. Bilateral Symmetry
Examples of Bilateral Symmetry
The most frequent kind of symmetry is bilateral symmetry, which may be found across the animal kingdom. Bilateral symmetry may be found in every creature, with distinguishing left and right, front and rear, and head and tail/bottom.
Bilaterally symmetrical organisms include humans and also other vertebrates, insects, spiders, crustaceans, snails, spiders, octopus, sea stars, clams, flatworms, common worms, and the larval stage of sea urchins.
Asymmetrical Animals
Asymmetrical creatures lack symmetry in their body patterns. Asymmetry is uncommon in the animal kingdom, with sponges being the lone exception (which belong to the phylum Porifera). Flounders are a kind of fish with both eyes solely on a single side of its head, are another example of asymmetry. However, since the fish’s larval stage is bilaterally symmetrical, this asymmetry does not appear until they reach maturity.
