Biological symmetry describes the balanced distribution of body parts or shapes within an organism, referring to a correspondence of parts in size, shape, and relative position. While mathematical symmetry implies exact duplication, biological symmetry is often approximate, meaning parts are similar but not perfectly identical. The presence and type of symmetry in an organism’s body plan relate to its lifestyle and evolutionary adaptations.
Symmetry Along a Central Axis
Bilateral symmetry is a common body plan where an organism can be divided into two mirror-image halves along a single plane, known as the sagittal plane. This plane splits the organism into distinct left and right sides, a dorsal (top) and ventral (bottom) side, and an anterior (head) and posterior (tail) end. This arrangement is prevalent across the animal kingdom, including humans, insects, fish, and most mammals.
This form of symmetry offers significant advantages for active, motile organisms. The streamlined body shape facilitates efficient, directed movement, allowing animals to propel forward and reduce drag.
The development of a head region, a process called cephalization, is closely linked with bilateral symmetry. Cephalization involves the concentration of sensory organs, such as eyes and antennae, and nerve cells at the anterior end of the body. This concentration of sensory structures and nervous tissue at the leading end allows organisms to encounter and respond to environmental stimuli more effectively. For example, a fish’s bilateral symmetry and cephalized head enable it to swiftly navigate water, detect prey, and escape predators.
Symmetry Around a Central Point
Radial symmetry describes a body plan where an organism’s parts are arranged around a central axis, allowing multiple planes of symmetry to divide it into similar halves. This means the organism has a top (oral) and bottom (aboral) surface but lacks distinct left and right sides or a clear head and tail. This type of symmetry is characteristic of many sessile (stationary) or slow-moving aquatic organisms.
Organisms with radial symmetry, such as jellyfish, sea anemones, and adult starfish, can interact with their environment from all directions, detecting and responding to stimuli, capturing food, or defending themselves from any angle. For instance, the tentacles of a sea anemone or jellyfish, arranged radially around a central mouth, can efficiently capture prey approaching from any direction.
A specific form of radial symmetry is pentamerous radial symmetry, where body parts are arranged in five sections around a central point. This is a defining characteristic of adult echinoderms, including starfish and sea urchins, which typically display five arms or rows of body parts.
Variations and Absence of Symmetry
Not all organisms exhibit clear bilateral or radial symmetry; some display variations or a complete absence of symmetry. Asymmetry occurs when an organism lacks any definite plane of symmetry, meaning its body parts are irregularly arranged. Sponges are a prime example of asymmetrical animals, often displaying diverse and irregular shapes.
Some snails also exhibit asymmetry, particularly in the coiling pattern of their shells. Even within bilaterally symmetrical animals, internal organs like the human heart or liver are often asymmetrically positioned.
Spherical symmetry is a distinct form where any plane passing through the center of the organism divides it into two identical halves. This body plan is found in minute, simple organisms, such as the colonial alga Volvox and some single-celled protozoans like Radiolaria. These organisms are shaped like a sphere, with parts arranged concentrically around a central point, and they lack distinct ends or sides. Biradial symmetry represents a combination of radial and bilateral features, where an organism has two distinct planes of symmetry. While outwardly appearing radial, these organisms possess certain internal or external features that restrict the number of planes of symmetry to two. Comb jellies (ctenophores) are the most common biological examples of biradially symmetrical animals, possessing two tentacles or comb rows that create this unique arrangement.