# Symmetry (biology)

"Bilateral symmetry" redirects here. For bilateral symmetry in mathematics, see reflection symmetry.
File:Commonbuckeye.JPG
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.

File:Haeckel Actiniae.jpg
These sea anemones display radial symmetry. (Plate from Ernst Haeckel's Kunstformen der Natur).

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. Radial also means there is only one plane in which symmetry exists.

#### 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 starfish (see special forms of radial symmetry). Animals in the phyla cnidaria and echinodermata exhibit radial symmetry. Cnidarians are one of the simplest animal forms on this planet.

#### Plants

Many flowers are radially symmetric (also known as actinomorphic). Roughly identical petals, sepals, and stamen occur at regular intervals around the center of the flower.

### Special forms of radial symmetry

#### Tetramerism

Many jellyfish have four radial root canals and thus exhibit tetramerous radial symmetry. This form of radial symmetry means it can be divided into 4 equal parts.

#### 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 (such as starfish and sea urchins) have parts arranged around the axis of the mouth in five equal sectors. Being bilaterian animals however, they initially develop biradially as larvae, then gain pentaradial symmetry later on. 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
File:Sterappel dwarsdrsn.jpg
Apple cut horizontally, showing pentamerism

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.

Various fruits also demonstrate pentamerism, a good example of which is seen in the arrangement of the seed carpels in an apple.

#### Hexamerism and octamerism

Corals and sea anemones (class Anthozoa) are divided into two groups based on their symmetry. The most common corals in the subclass Hexacorallia have a hexameric body plan; their polyps have sixfold internal symmetry and the number of their tentacles is a multiple of six.

Corals belonging to the subclass Octocorallia have polyps with eight tentacles and octameric radial symmetry.

## Bilateral symmetry

File:Leaf 1 web.jpg
A leaf showing 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, see situs solitus). 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. Bilateral symmetry is an aspect of both chordates and vertebrates.

### Plants

Flowers such as members of the orchid and pea families are bilaterally symmetrical (also known as zygomorphic). The leaves of most plants are also bilaterally symmetrical.

## Asymmetry

The notable exception among animals is the phylum Porifera (sponges) which have no symmetry.