Symmetry (biology)

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The elaborate patterns on the wings of butterflies are one example of biological symmetry.

Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of symmetry, either radial symmetry or bilateral symmetry. A small minority exhibit no symmetry (are asymmetric).

In nature and biology, symmetry is approximate. For example, plant leaves, while considered symmetric, will rarely match up exactly when folded in half.

Radial symmetry

Radial symmetry

Organisms with radial symmetry (sometimes called line symmetry) have body parts arranged in a regular, repeating pattern around a central axis (like a wagon wheel) or are completely symmetrical about a central axis (like a dinner plate).

These organisms resemble a pie where several cutting planes produce roughly identical pieces. An organism with radial symmetry exhibits no left or right sides. They have a top and a bottom (dorsal and ventral surface) only.

Animals

Most radially symmetric animals are symmetrical about an axis extending from the center of the oral surface, which contains the mouth, to the center of the opposite, or aboral, end. This type of symmetry is especially suitable for sessile animals such as the sea anemone, floating animals such as jellyfish, and slow moving organisms such as sea stars (see special forms of radial symmetry). Animals in the phyla cnidaria and echinodermata exhibit radial symmetry.

Plants

Many flowers, such as dandelions and daffodils, are radially symmetric. Roughly identical petals, sepals, and stamen form at regular intervals around the center of the flower.

Special forms of radial symmetry

Tetramerism

Many jellyfish have four radial canals and thus exhibit tetramerous radial symmetry

Pentamerism

This variant of radial symmetry (also called pentaradial and pentagonal symmetry) arranges roughly equal parts around a central axis at orientations of 72° apart.

• Animals

Members of the phyla echinodermata (like starfish) arrange parts around the axis of the mouth in five equal sectors. The radiolarians demonstrate a remarkable array of pentamerism forms. Examples include the Pentaspheridae, the Pentinastrum group of general in the Euchitoniidae, and Cicorrhegma (Circoporidae).

• Plants

Flowering plants demonstrate symmetry of five more frequently than any other form.

Around 1510–1516 A.D., Leonardo da Vinci determined that in many plants a sixth leaf stands above the first. This arrangement later became known as 2/5 phyllotaxy, a system where repetitions of five leaves occur in two turns of the axis. This is the most common of all patterns of leaf arrangement.

Bilateral symmetry

Bilateral symmetry

In Bilateral symmetry (also called plane symmetry), only one plane, called the sagittal plane, will divide an organism into roughly mirror image halves (with respect to external appearance only). Thus there is approximate reflection symmetry. Often the two halves can meaningfully be referred to as the right and left halves, e.g. in the case of an animal with a main direction of motion in the plane of symmetry.

Animals

Most animals are bilaterally symmetric, including humans (see also facial symmetry), and belong to the group Bilateria. The oldest known bilateral animal is the Vernanimalcula.

Bilateral symmetry permits streamlining, favors the formation of a central nerve center, contributes to cephalization, and promotes actively moving organisms.

Plants

Flowers such as orchids and sweet peas are bilaterally symmetrical. The leaves of most plants are also bilaterally symmetrical.