Symmetry (biology)
Symmetry in biology is the balanced arrangement of body parts or shapes around a central point or axis. That is, the size, shape, and relative location on one side of a dividing line mirrors the size, shape, and relative location on the other side.
In nature, there are three basic kinds of symmetry: spherical, radial, or bilaterial. The body plans of most multicellular organisms exhibit one of these forms. A small minority exhibit no symmetry (are asymmetric).
In biology, symmetry is approximate. For example, plant leaves, while considered symmetric, will rarely match up exactly when folded in half. Furthermore, symmetry may refer only to the external form and not the internal anatomy.
Spherical symmetry
In spherical symmetry, any plane that passes through the the center of the object divides the form into two identical halves that are mirror images of each other. Such objects are shaped like spheres or globes. For example, a round ball exhibits spherical symmetry at some point in their lives. (The sea cucumber, an echinoderm, exhibits bilaterial symmetry as an adult.)
In nature, spherican symmetry is exhibited by the external form of many colonial algae, such as Volvox.
Radial symmetry
In radial symmetry, all planes passing through a central axis (normally vertical) divides the form into two identical halves that are mirror images of each other. Such a form will have distinct ends (usually top and bottom) and any plane that passes through its longitudinal axis (a line from end to end through the center) will create two similar halves (Towle 1989).
These organisms resembles 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 surface only.
Animals with radial symmetry. 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, at least some time during their life cycle. Sea cucumbers, an echinoderm, demonstrate bilaterial symmetry as adults.
Plants with radial symmetry. Many flowers, such as buttercups and daffodils, 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 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. Members of the phyla echinodermata (like starfish) arrange parts around the axis of the forms. Examples include the Pentaspheridae, the Pentinastrum group of genera in the Euchitoniidae, and Cicorrhegma (Circoporidae). Like other echinoderms, sea urchins have five-fold symmetry(pentamerism). The pentamerous symmetry is not obvious at a casual glance, but is easily seen in the dried shell of the urchin. Flowering plants demonstrate symmetry of five more frequently than any other form.
Around 1510–1516 C.E., 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
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. An example would be an airplane, whereby a plane passing through the center of the plane from tip to tail would divide the plane into two equal parts (on external surface).
Most animals are bilaterally symmetric, including humans, 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 (concentration of nerve tissue and sense organs in the head), and promotes actively moving organisms, generally in the direction of the head. Bilateral symmetry is found in insects, spiders, worms and many other invertebrates, as well as is an an aspect of vertebrates.
Flowers such as orchids and sweet peas are bilaterally symmetrical (also known as zygomorphic). The leaves of most plants are also bilaterally symmetrical.
Asymmetry
Within animals, only one group exhibits true asymmetry, the Porifera phylum (sponges).
See also
- Floral symmetry
- Supernumerary body part
ReferencesISBN links support NWE through referral fees
- 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 links
- Live Science.com article called "Symmetry in Nature: Fundamental Fact or Human Bias?" By Ker Than
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