Symmetry in biology
Symmetry in biology is the balanced distribution of duplicate body parts or shapes. In nature and biology, symmetry is approximate. For example, plant leaves, while considered symmetric, rarely match up exactly when folded in half. Symmetry creates a class of patterns in nature, where the near-repetition of the pattern element is by reflection or rotation. The body plans of most multicellular organisms exhibit some form of symmetry, whether radial symmetry, bilateral symmetry or "spherical symmetry". A small minority, notably the sponges, exhibit no symmetry (are asymmetric).
Radially symmetric organisms resemble a pie where several cutting planes produce roughly identical pieces. Such an organism exhibits no left or right sides. They have a top and a bottom (dorsal and ventral surface) only.
Symmetry has been important historically in the taxonomy of animals; animals with radial symmetry were classified in the taxon Radiata, which is now generally accepted to be a polyphyletic assemblage of different phyla of the kingdom Animalia. 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. The echinodermata however have bilaterally symmetric larvae, and are thus classed as bilaterians.
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 flowers. Cases where otherwise cylindrical plant shapes are transformed into helices are described by the term helical growth.
Many viruses have radial symmetries, their coats being composed of a relatively small number of protein molecules arranged in a regular pattern to form polyhedrons, spheres, or ovoids.
Special forms of radial symmetry
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.
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 phylum 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 pentameric forms. Examples include the Pentaspheridae, the Pentinastrum group in the Euchitoniidae, and Cicorrhegma (Circoporidae).
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.
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. The octopus, however, has bilateral symmetry, despite its eight arms.
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. Organisms which approximate spherical symmetry include the freshwater green alga Volvox.
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.
At cellular level, bilateral multicellular organisms have been computationally modeled where it is shown that cell orientation plays a central role in the development of bilateral symmetry. Interestingly, cell orientation switching in early founder cells can lead to inside-out growth. This has been postulated as a form of evolutionary saltation allowing for the formation of the internal skeleton from a primitive organism with an external skeleton.
Animals that are bilaterally symmetric have mirror symmetry in the sagittal plane, which divides the body vertically into left and right halves, with one of each sense organ and limb pair on either side. Most animals are bilaterally symmetric, including humans (see also facial symmetry). The oldest known bilateral animal is Vernanimalcula from the Ediacaran deposits.
When an organism normally moves in one direction, it inevitably has a front or head end. This end encounters the environment before the rest of the body as the organism moves along, so sensory organs such as eyes tend to be clustered there, and similarly it is the likely site for a mouth as food is encountered. A distinct head, with sense organs connected to a central nervous system, therefore tends to develop (cephalization). Given a direction of travel which creates a front/back difference, and gravity which creates a dorsal/ventral difference, left and right are unavoidably distinguished. Finally, given forward motion, a plane of symmetry, for example with the same number of legs on both sides, is to be expected. Bilateral symmetry also permits streamlining. Bilateral symmetry is found in most phyla including Platyhelminthes (flatworms), Arthropoda (e.g. insects), Annelida (e.g. earthworms), Mollusca (e.g. squids, though not in helically coiled snails) and Chordata (e.g. fish and mammals).
The phylum Echinodermata, which includes starfish, sea urchins and sand dollars, is unique among animals in having bilateral symmetry at the larval stage, but fivefold symmetry (a special type of radial symmetry) as adults.
Bilateral symmetry is not easily broken. In experiments using the fruit fly, Drosophila, in contrast to other traits (where laboratory selection experiments always yield a change), right or left-sidedness in eye size, or eye facet number, wing-folding behavior (left over right) show a lack of response. However Gynandromorphs break the symmetry of reproductive organs, while still being generally bilateral.
There is evidence that females of some species select for symmetry, presumed by biologists to be a mark (technically a "cue") of fitness. Female barn swallows, a species where adults have long tail streamers, prefer to mate with males that have the most symmetrical tails.
Flowers such as members of the orchid and pea families are bilaterally symmetrical (zygomorphic). The leaves of most plants are also superficially bilaterally symmetrical. A careful examination of leaf vein patterns often shows imperfect bilateral symmetry.
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 described as helical growth.
Biradial symmetry is a combination of radial and bilateral symmetry, as in the Ctenophores. Here, the body components are arranged with similar parts on either side of a central axis, and each of the four sides of the body is identical to the opposite side but different from the adjacent side.
Not all animals are symmetric. The phylum Porifera (sponges) have no symmetry. The approximately 400 species of flatfish also lack symmetry as adults, though the larvae are bilaterally symmetrical. Adult flatfish rest on one side, and the eye that was on that side has migrated round to the other (top) side of the body.
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