The animal kingdom exhibits three main body plans: asymmetry, bilateral symmetry, or radial symmetry. Asymmetry, seen in sponges, means the animal has no plane of symmetry. Bilateral symmetry allows for a single line of division into two mirror-image halves, and is found in the vast majority of animals, including humans, fish, and insects. Radial symmetry is far less common and is specialized to specific ecological niches in the marine environment.
Defining the Radial Body Plan
Radial symmetry describes a body plan where structures are arranged uniformly around a central axis. An animal with this structure can be divided into two near-identical halves by slicing through its center along any number of planes. Unlike bilaterally symmetrical animals, which have distinct anterior (head) and posterior (tail) ends, radially symmetrical animals lack a defined front, back, left, or right side. They possess only a top (oral side, containing the mouth) and a bottom (aboral side).
The Primary Group: Cnidaria
The phylum Cnidaria represents the most prominent animal group where radial symmetry is the fundamental body plan. This phylum includes marine organisms such as jellyfish, sea anemones, and corals. The polyp form (e.g., sea anemone) is cylindrical and fixed to a substrate, while the medusa form (e.g., jellyfish) is free-floating and bell-shaped. Both forms organize body parts radiating outward from the central axis.
This arrangement is advantageous for their sessile (fixed) or planktonic (drifting) lifestyles. A sea anemone’s tentacles are arranged around the mouth, allowing it to capture prey approaching from any direction. The radial structure of a jellyfish allows it to efficiently drift and use its stinging cells, called cnidocytes, to respond to stimuli on all sides. This symmetry is effective because these animals do not actively chase food or flee predators in a single direction.
Secondary Radial Symmetry in Echinoderms
The phylum Echinodermata, which includes sea stars, sea urchins, and sea cucumbers, also displays radial symmetry. This symmetry is considered secondary because it develops later in life. The larval stage of an echinoderm is bilaterally symmetrical and free-swimming, reflecting a common ancestry with other bilateral animals.
During metamorphosis, the echinoderm body undergoes a transformation, developing a specialized form of radial symmetry called pentaradial symmetry, or five-fold symmetry. The adult body is organized into five equal parts radiating from the center, such as the five arms of a sea star. This developmental shift is an adaptation to a bottom-dwelling, slow-moving existence. The internal water vascular system, which powers their tube feet, also follows this five-part radial pattern.
Ecological Implications of Radial Symmetry
Radial symmetry is an adaptation to aquatic environments and specific modes of life. This body plan is effective for organisms that are permanently fixed or that drift in the water column. Since food and potential threats can approach from any angle, a radially symmetrical body ensures equal responsiveness in all directions, maximizing survival and feeding.
The radial design is linked to a lack of cephalization, which is the concentration of nervous tissue and sense organs into a distinct head region. Radially symmetrical animals do not require a centralized brain or complex sensory structures because they lack a primary direction of movement. Instead, their sensory cells and nervous system are distributed uniformly around the body periphery to effectively monitor their surroundings.