Symmetry in biology



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 or "spherical 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
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
Symmetry is important in the taxonomy of animals; animals with bilateral symmetry are classified in the taxon Bilateria, which is generally accepted to be a clade of the kingdom Animalia. Bilateral symmetry means capable of being halved into two equal parts so that one part is a mirror image of the other. The line of symmetry lies dorso-ventrally and anterior-posteriorly. 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 (although many sea anemones and some corals exhibit bilateral symmetry defined by a single structure, the siphonoglyph) (see Willmer, 1990). The echinodermata, however, exhibit bilateral symmetry in their larvae, and are thus classed as bilaterians.

Plants
Many flowers and plants are radially symmetric (also known as actinomorphic). Roughly identical petals, sepals, and stamen occur at regular intervals around the center of the flower. Cases where otherwise cylindrical plant shapes are transformed into helices are described by the term helical growth.

Tetramerism
Many jellyfish have four 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.

Members of the phyla Echinodermata (such as sea stars, sea urchins, and sea lilies) 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).
 * Animals

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

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.

Spherical Symmetry
Spherical symmetry occurs in an organism if it is able to be cut into two identical halves through any cut that runs through the organism's center.

Biradial symmetry
Biradial is a combination of radial and bilateral symmetry. Ctenophores exhibit biradial symmetry. Some of the phyla formerly classified as aschelminth have anterior and radial portions.

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. Most bilateral animals have an identical shape on either side, as if cut by a mirror.

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 superficially bilaterally symmetrical. A careful examination of leaf vein patterns often shows imperfect bilateral symmetry. Also, the pattern of leaves on a branch or stem may often show glide symmetry, with left, right alternation, rather than perfect bilateral symmetry. Cases where otherwise bilateral plant organs are transformed into seemingly helical shapes are known under the term helical growth.

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