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Symmetry (biology)

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"Bilateral symmetry" redirects here.

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.

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 symmetryEdit

File:Haeckel Actiniae.jpg

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.


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. Radial also means it only has one line of symmetry.

Special forms of radial symmetryEdit


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.


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).

Hexamerism and octamerismEdit

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 six-fold 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 symmetryEdit

File:Leaf 1 web.jpg

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.


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.


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

See alsoEdit


  • Fact Monster
  • Heads, Michael. "Principia Botanica: Croizat's Contribution to Botany." Tuatara 27.1 (1984): 26-48.
  • Zoology a website by the Monaco educational service

External linksEdit

{{enWP|Symmetry (biology)}

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