Biological symmetry illustrates how an organism’s body parts are spatially organized around a point or axis. This organization provides a framework for the animal’s anatomy. Symmetry in nature is an approximate repetition of shapes or parts within a body plan. Across the animal kingdom, two main patterns dominate: bilateral symmetry, where an organism has distinct left and right sides, and radial symmetry. Radial symmetry represents one of the earliest forms of body organization found in multicellular animals.
Defining Radial Symmetry
Radial symmetry describes a body plan where structures are arranged concentrically around a central axis, much like the spokes of a wheel. The body typically takes the form of a cylinder or a bowl, with a main axis that extends from the mouth-bearing end (oral end) to the opposite aboral end. The organism can be divided into several similar halves by multiple planes, provided the cut passes through this central axis. This arrangement means the organism possesses a top and bottom surface but lacks distinct left and right sides, or a defined head and tail.
This form of organization can vary in the number of repeating parts around the central point. True radial symmetry allows for division into identical parts along many planes. However, some animals exhibit specialized forms like pentaradial symmetry, where the body is arranged in five or multiples of five parts. This pentamerism is a prominent feature in one entire group of marine animals.
Another distinct variation is biradial symmetry, which is a combination of radial and bilateral features. Biradially symmetrical organisms can only be divided into equal halves along two specific planes, rather than the multiple planes seen in true radial forms. This difference often arises from the presence of a single internal structure, such as a pharynx, that imposes a slight bilateral element onto the otherwise radial body.
Functional Significance of Radial Body Plans
The radial body plan is closely linked to the lifestyle and movement of the organisms that possess it. This type of symmetry is especially advantageous for animals that are either sessile, meaning they are fixed in one place, or pelagic, meaning they are slow-moving or drift freely in the water column. For a stationary animal, a radial arrangement allows it to interact with its environment equally from all directions.
A primary advantage is the ability for 360-degree environmental sensing, allowing the organism to detect potential food, predators, or stimuli from any side without the need to turn. This equal distribution of sensory and feeding structures, such as tentacles around a central mouth, makes it efficient for an animal waiting for food to drift by.
Radial symmetry presents a stark contrast to bilateral symmetry, which is found in the vast majority of the animal kingdom. Bilateral symmetry favors directional, rapid locomotion because it allows for a streamlined body and the concentration of sensory organs at the leading end, a process known as cephalization. The radial body plan, however, prioritizes a balanced, non-directional response to the surrounding environment over focused, forward movement.
Organisms Exhibiting Radial Symmetry
Radial symmetry is prominently displayed in two major animal phyla, representing different evolutionary pathways. The phylum Cnidaria, which includes jellyfish, sea anemones, and corals, exhibits primary radial symmetry. In these animals, the body parts, such as stinging tentacles, radiate outwards from the central digestive cavity, directly supporting their sessile or free-floating existence.
The second major group is the phylum Echinodermata, which includes sea stars, sea urchins, and sea cucumbers. These animals display a specialized form called secondary radial symmetry. While the adults are distinctly pentaradial, with five arms or sections radiating outward, their larval stage is bilaterally symmetrical.
This shift suggests that echinoderms evolved from bilaterally symmetrical ancestors, adopting the radial body plan later as an adaptation to their bottom-dwelling lifestyle. The five-part symmetry of the adult echinoderm is reflected in the arrangement of their internal water vascular system and the placement of their tube feet. This structural organization allows them to effectively cover the area immediately surrounding them.