- "Bilateral symmetry" redirects here. For bilateral symmetry in mathematics, see reflection symmetry
Reflection symmetry, line symmetry, mirror symmetry, mirror-image symmetry, or bilateral symmetry is symmetry with respect to reflection....
.
Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most
multicellular organismMulticellular organisms are organisms consisting of more than one cell, and having differentiated cells that perform specialized functions in the organism...
s exhibit some form of
symmetrySymmetry generally conveys two primary meanings. The first is an imprecise sense of harmonious or aesthetically pleasing proportionality and balance; such that it reflects beauty or perfection...
, either
radial symmetry or
bilateral symmetry or "spherical symmetry". A small minority exhibit no symmetry (are asymmetric).
In
natureNature, in the broadest sense, is equivalent to the natural world, physical world, or material world. "Nature" refers to the phenomena of the physical world, and also to life in general...
and
biologyBiology is the natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy...
, 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
planeIn mathematics, a plane is a flat surface. Planes can arise as subspaces of some higher dimensional space, as with the walls of a room, or they may enjoy an independent existence in their own right, as in the setting of Euclidean geometry....
s produce roughly identical pieces. An organism with radial symmetry exhibits no left or right sides. They have a top and a bottom (
dorsal and ventralStandard anatomical terms of location are employed in science which deal with the anatomy of animals to avoid ambiguities which might otherwise arise. They are not language-specific, and thus require no translation...
surface) only.
Animals
Symmetry is important in the
taxonomyTaxonomy is the practice and science of classification. The word finds its roots in the Greek , taxis and , nomos...
of animals; animals with bilateral symmetry are classified in the taxon
BilateriaThe bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back. The bilateralia are a subregnum of animals, including the majority...
, which is generally accepted to be a
cladeA clade is a term used in modern alpha taxonomy, the scientific classification of living and fossil organisms, to describe a monophyletic group, defined as a group consisting of a single common ancestor and all its descendants.[The term "monophyletic group" is used in this article...]
of the
kingdomIn biological taxonomy, kingdom and/or regnum is a taxonomic rank in either the highest rank, or the rank below domain. Each kingdom is divided into smaller groups called phyla...
AnimalAnimals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously...
ia. 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
sessileIn zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a rock, or the hull of a ship in the case of barnacles. Corals lay down their own substrate....
animals such as the
sea anemoneSea anemones are a group of water dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower...
, floating animals such as
jellyfishJellyfish are free-swimming members of the phylum Cnidaria. Jellyfish have several different morphologies that represent several different cnidarian classes including the Scyphozoa , Staurozoa , Cubozoa , and Hydrozoa Jellyfish (also known as jellies or sea jellies) are free-swimming members of...
, and slow moving organisms such as
sea starSea stars are echinoderms belonging to the class Asteroidea. The names "sea star" and "starfish" are sometimes differentiated, with "starfish" used in a broader sense to include the closely related brittle stars, which make up the class Ophiuroidea, as well as excluding sea stars which do not have...
s (see special forms of radial symmetry). Animals in the phyla
cnidariaCnidaria is a phylum containing over 10,000 species of animals found exclusively in aquatic, mostly marine, environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey...
and
echinodermEchinoderm, there are seven main classes of Echinoderms which are brittle stars, basket stars, sea urchins, sand dollars, sea lilies, feather stars, and sea cucumbers...
ata exhibit radial symmetry (although many sea anemones and some corals exhibit bilateral symmetry defined by a single structure, the
siphonoglyphThe siphonoglyph is a ciliated groove at one or both ends of the mouth of sea anemones and some corals. The siphonoglyph extends into a pharynx and is used to create currents of water into the pharynx. These water currents are important for respiration and maintenance of internal pressure...
) (see Willmer, 1990). The echinodermata, however, exhibit bilateral symmetry in their larvae, and are thus classed as bilaterians.
Plants
Many
flowerA flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to mediate the union of male sperm with female ovum in order to produce seeds...
s are radially symmetric (also known as actinomorphic). Roughly identical
petalA petal is one member or part of the corolla of a flower. The corolla is the name for all of the petals of a flower; the inner perianth whorl, term used when this is not the same in appearance as the outermost whorl and is used to attract pollinators based on its bright color...
s,
sepalA sepal is a part of the flower of angiosperms . Sepals in most flowers are green and lie under the more conspicuous petals. As a collective unit the sepals form a calyx, whereas the collection of petals is called the corolla...
s, and
stamenThe stamen is the male organ of a flower. Each stamen generally has a stalk called the filament , and, on top of the filament, an anther , and pollen sacs,...
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 growthThe term helical growth describes the expansion of fungal, algal or higher plant cells or organs leading to a twisted cell or organ shape. Helical growth results in the breaking of symmetry . Resulting shapes may be left-handed or right-handed. Helical growth can arise naturally The term helical...
.
Tetramerism
Many
jellyfishJellyfish are free-swimming members of the phylum Cnidaria. Jellyfish have several different morphologies that represent several different cnidarian classes including the Scyphozoa , Staurozoa , Cubozoa , and Hydrozoa Jellyfish (also known as jellies or sea jellies) are free-swimming members of...
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 starSea stars are echinoderms belonging to the class Asteroidea. The names "sea star" and "starfish" are sometimes differentiated, with "starfish" used in a broader sense to include the closely related brittle stars, which make up the class Ophiuroidea, as well as excluding sea stars which do not have...
s,
sea urchinSea urchins or urchins are small, spiny, globular animals that compose part of class Echinoidea. They are found in oceans all over the world. Their shell, or "test", is round and spiny, typically from 3 to 10 cm across. Common colors include black and dull shades of green, olive, brown, purple,...
s, and sea lilies) have parts arranged around the axis of the mouth in five equal sectors. Being
bilateriaThe bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back. The bilateralia are a subregnum of animals, including the majority...
n animals, however, they initially develop biradially as larvae, then gain pentaradial symmetry later on. The
radiolarianRadiolarians are amoeboid protozoa that produce intricate mineral skeletons, typically with a central capsule dividing the cell into inner and outer portions, called endoplasm and ectoplasm. They are found as zooplankton throughout the ocean, and their skeletal remains cover large portions of the...
s demonstrate a remarkable array of pentamerism forms. Examples include the Pentaspheridae, the Pentinastrum group of general in the Euchitoniidae, and Cicorrhegma (Circoporidae).
Flowering plantThe flowering plants or angiosperms are the most diverse group of land plants. The flowering plants and the gymnosperms are the only extant groups of seed plants...
s demonstrate symmetry of five more frequently than any other form.
Around 1510–1516 A.D.,
Leonardo da VinciLeonardo di ser Piero da Vinci was an Italian polymath, scientist, mathematician, engineer, inventor, anatomist, painter, sculptor, architect, botanist, musician and writer....
determined that in many plants a sixth leaf stands above the first. This arrangement later became known as 2/5
phyllotaxyIn botany, phyllotaxis or phyllotaxy is the arrangement of the leaves on the stem of a plant .- Pattern structure :...
, 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
CoralCorals are marine organisms from the class Anthozoa and exist as small sea anemone-like polyps, typically in colonies of many identical individuals...
s and sea anemones (class
AnthozoaAnthozoa is a class within the phylum Cnidaria that contains the sea anemones and corals. Unlike other cnidarians, anthozoans do not have a medusa stage in their development. Instead, they release sperm and eggs that form a planula, which attaches to some substrate on which the cnidarian grows...
) are divided into two groups based on their symmetry. The most common corals in the subclass
HexacoralliaZoanthids are an order of cnidarians commonly found in coral reefs, the deep sea and many other marine environments around the world. These animals come in a variety of different colonizing formations and in numerous colors...
have a
hexameric body plan; their
polypA polyp in zoology is one of two forms found in the phylum Cnidaria, the other being the medusa. Polyps are approximately cylindrical in shape and elongated at the axis of the body...
s have sixfold internal symmetry and the number of their
tentacleTentacles can refer to the elongated flexible organs that are present in some animals, especially invertebrates, and sometimes to the hairs of the leaves of some insectivorous plants. Usually, they are used for feeding, feeling and grasping...
s is a multiple of six.
Corals belonging to the subclass
OctocoralliaOctocorallia is a subclass of Anthozoa comprising ~3,000 species of water-based organisms formed of colonial polyps with 8-fold symmetry. It includes the blue coral, soft corals, sea pens, and gorgonians within three orders: Alcyonacea, Helioporacea, and Pennatulacea...
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 is an example of biradial symmetry.
Bilateral symmetry
In bilateral symmetry (also called plane symmetry), only one plane, called the
sagittal planeA sagittal plane of the human body is an imaginary plane that travels vertically from the top to the bottom of the body along the Y axis, dividing it into left and right portions.-Variations:Examples include:...
, will divide an organism into roughly
mirror imageA mirror image is a reflected duplication that appears identical but in reverse. As an optical effect it results from reflection off of substances such as a mirror or water...
halves (with respect to external appearance only, see
situs solitusSitus solitus refers to the normal position of thoracic and abdominal organs. Anatomically, this means that the heart is on the left with the pulmonary atrium on the right and the systemic atrium on the left along with the cardiac apex...
). Thus there is approximate
reflection symmetryReflection symmetry, line symmetry, mirror symmetry, mirror-image symmetry, or bilateral symmetry is symmetry with respect to reflection....
. 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 symmetrySymmetry, especially facial symmetry, is one of a number of aesthetic traits, including averageness and youthfulness, associated with health, physical attractiveness and beauty of a person or non-human animal according to the authors of Facial Attractiveness: Gillian Rhodes, Leslie A. Zebrowitz...
), and belong to the group
BilateriaThe bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back. The bilateralia are a subregnum of animals, including the majority...
. The oldest known bilateral animal is the
VernanimalculaVernanimalcula guizhouena is a fossil believed by some to represent the earliest known member of the Bilateria . It is known from deposits dating to . The fossils are between 0.1 and 0.2 mm across...
. Most bilateral animals have an identical shape on either side, as if cut by a mirror.
Bilateral symmetry permits
streamliningStreamline may refer to:*Streamlines, streaklines, and pathlines, in fluid flows. See also: Drag **Streamliner, any vehicle shaped to be less resistant to air...
, favors the formation of a central nerve center, contributes to
cephalizationCephalization is an evolutionary trend, whereby nervous tissue, over many generations, becomes concentrated toward one end of an organism. This process eventually produces a head region with sensory organs....
, and promotes actively moving organisms. Bilateral symmetry is an aspect of both chordates and vertebrates.
Plants
Flowers such as members of the
orchidOrchidaceae, the Orchid family, is the largest family of the flowering plants . Its name is derived from the genus Orchis....
and
peaFabaceae or Leguminosae is a large and economically important family of flowering plants, which is commonly known as the legume family, pea family, bean family or pulse family. The name 'Fabaceae' comes from the defunct genus Faba, now included into Vicia...
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 growthThe term helical growth describes the expansion of fungal, algal or higher plant cells or organs leading to a twisted cell or organ shape. Helical growth results in the breaking of symmetry . Resulting shapes may be left-handed or right-handed. Helical growth can arise naturally The term helical...
.
Asymmetry
The notable exception among animals is the
phylumIn biology, a phylum ["Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states.] is a taxonomic rank below Kingdom and above Class...
Porifera (sponges) which have no symmetry.
External links