Symmetry (biology)
Encyclopedia
"Bilateral symmetry" redirects here. For bilateral symmetry in mathematics, see reflection symmetry
Reflection symmetry
Reflection symmetry, reflectional symmetry, line symmetry, mirror symmetry, mirror-image symmetry, or bilateral symmetry is symmetry with respect to reflection. That is, a figure which does not change upon undergoing a reflection has reflectional symmetry.In 2D there is a line of symmetry, in 3D a...

.


Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organism
Multicellular organism
Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms. Most life that can be seen with the the naked eye is multicellular, as are all animals and land plants.-Evolutionary history:Multicellularity has evolved independently dozens of times...

s exhibit some form of symmetry
Symmetry
Symmetry 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 nature
Nature
Nature, 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 biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

, 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 plane
Plane (mathematics)
In mathematics, a plane is a flat, two-dimensional surface. A plane is the two dimensional analogue of a point , a line and a space...

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 ventral
Anatomical terms of location
Standard anatomical terms of location are designations employed in science that deal with the anatomy of animals to avoid ambiguities that might otherwise arise. They are not language-specific, and thus require no translation...

 surface) only.

Animals

Symmetry is important in the taxonomy
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...

 of animals; animals with radial symmetry are classified in the taxon Radiata
Radiata
The Radiata are the radially symmetric animals of the Eumetazoa subkingdom. The term Radiata has had various meanings in the history of classification...

, which is generally accepted to be a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 of the kingdom
Kingdom (biology)
In biology, kingdom is a taxonomic rank, which is either the highest rank or in the more recent three-domain system, the rank below domain. Kingdoms are divided into smaller groups called phyla or divisions in botany...

 Animal
Animal
Animals are a major group of 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 and...

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 sessile
Sessility (zoology)
In 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 part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...

 animals such as the sea anemone
Sea anemone
Sea anemones are a group of water-dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Zoantharia. Anthozoa often have large polyps that allow for digestion of larger...

, floating animals such as jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...

, and slow moving organisms such as sea star
Sea star
Starfish or sea stars are echinoderms belonging to the class Asteroidea. The names "starfish" and "sea star" essentially refer to members of the class Asteroidea...

s (see special forms of radial symmetry). Animals in the phyla cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...

 and echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....

ata exhibit radial symmetry (although many sea anemones and some corals exhibit bilateral symmetry defined by a single structure, the siphonoglyph
Siphonoglyph
The 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
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...

, and are thus classed as bilaterians.

Plants

Many flowers and plants are radially symmetric (also known as actinomorphic). Roughly identical petal
Petal
Petals are modified leaves that surround the reproductive parts of flowers. They often are brightly colored or unusually shaped to attract pollinators. Together, all of the petals of a flower are called a corolla. Petals are usually accompanied by another set of special leaves called sepals lying...

s, sepal
Sepal
A sepal is a part of the flower of angiosperms . Collectively the sepals form the calyx, which is the outermost whorl of parts that form a flower. Usually green, sepals have the typical function of protecting the petals when the flower is in bud...

s, and stamen
Stamen
The stamen is the pollen producing reproductive organ of a flower...

 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
Helical growth
The 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 jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...

 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.
  • Animals

Members of the phylum Echinodermata (such as sea star
Sea star
Starfish or sea stars are echinoderms belonging to the class Asteroidea. The names "starfish" and "sea star" essentially refer to members of the class Asteroidea...

s, sea urchin
Sea urchin
Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...

s, and sea lilies) have parts arranged around the axis of the mouth in five equal sectors. Being bilateria
Bilateria
The 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...

n animals, however, they initially develop biradially as larvae, then gain pentaradial symmetry later on. The radiolarian
Radiolarian
Radiolarians 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 pentameric forms. Examples include the Pentaspheridae, the Pentinastrum group of general in the Euchitoniidae, and Cicorrhegma (Circoporidae).
  • Plants

Flowering plant
Flowering plant
The flowering plants , also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies...

s demonstrate symmetry of five more frequently than any other form.

Around 1510–1516 A.D., Leonardo da Vinci
Leonardo da Vinci
Leonardo di ser Piero da Vinci was an Italian Renaissance polymath: painter, sculptor, architect, musician, scientist, mathematician, engineer, inventor, anatomist, geologist, cartographer, botanist and writer whose genius, perhaps more than that of any other figure, epitomized the Renaissance...

 determined that in many plants a sixth leaf stands above the first. This arrangement later became known as 2/5 phyllotaxy
Phyllotaxis
In botany, phyllotaxis or phyllotaxy is the arrangement of leaves on a plant stem .- 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

Coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...

s and sea anemones (class Anthozoa
Anthozoa
Anthozoa 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 Hexacorallia
Zoantharia
Zoanthids 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 polyp
Polyp
A 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 tentacle
Tentacle
A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...

s is a multiple of six.

Corals belonging to the subclass Octocorallia
Alcyonaria
Octocorallia 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 exhibit biradial symmetry. Some of the phyla formerly classified as aschelminth
Aschelminth
The Aschelminthes , closely associated with the Platyhelminthes, are an obsolete phylum of pseudocoelomate and other similar animals that are no longer considered closely related and have been promoted to phyla in their own right...

 have anterior and radial portions.

In this type of symmetry, the body components of an animal are arranged such that similar parts are located to 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.

Bilateral symmetry

In bilateral symmetry (also called plane symmetry), only one plane, called the sagittal plane
Sagittal plane
Sagittal plane is a vertical plane which passes from front to rear dividing the body into right and left sections.-Variations:Examples include:...

, will divide an organism into roughly mirror image
Mirror image
A mirror image is a reflected duplication of an object that appears identical but reversed. As an optical effect it results from reflection off of substances such as a mirror or water. It is also a concept in geometry and can be used as a conceptualization process for 3-D structures...

 halves (with respect to external appearance only, see situs solitus
Situs solitus
Situs 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 symmetry
Reflection symmetry
Reflection symmetry, reflectional symmetry, line symmetry, mirror symmetry, mirror-image symmetry, or bilateral symmetry is symmetry with respect to reflection. That is, a figure which does not change upon undergoing a reflection has reflectional symmetry.In 2D there is a line of symmetry, in 3D a...

. 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

Animals that are bilaterally symmetric are put in the group Bilateria
Bilateria
The 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...

. Bilateral symmetry means capable of being split into two equal parts so that one part is a mirror image of the other. The plane of symmetry lies dorso-ventrally and anterior-posteriorly.

Most animals are bilaterally symmetric, including humans (see also facial symmetry
Facial symmetry
Facial symmetry, is one element of bodily symmetry, including fluctuating asymmetry. Along with traits such as averageness and youthfulness it influences judgements of aesthetic traits of physical attractiveness and beauty, and is associated with fitness-linked traits including health, It is also...

). The oldest known bilateral animal is the Vernanimalcula
Vernanimalcula
Vernanimalcula 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 bisected by a mirror.
Bilateral symmetry permits streamlining, favors the formation of a central nerve center, contributes to cephalization
Cephalization
Cephalization is considered 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.

One unique animal in the group Bilateria
Bilateria
The 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...

 is the phylum Echinodermata which includes starfish, sea urchin
Sea urchin
Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...

 and sand dollars. All these animals show bilateral symmetry in their larval stage, but pentalateral symmetry (a special type of radial symmetry) as adults.

Bilateral symmetry is not easily broken. In experiments using the fruit fly, Drosophila
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...

, in contrast of 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.

Plants

Flowers such as members of the orchid
Orchidaceae
The Orchidaceae, commonly referred to as the orchid family, is a morphologically diverse and widespread family of monocots in the order Asparagales. Along with the Asteraceae, it is one of the two largest families of flowering plants, with between 21,950 and 26,049 currently accepted species,...

 and pea
Fabaceae
The Fabaceae or Leguminosae, commonly known as the legume, pea, or bean family, is a large and economically important family of flowering plants. The group is the third largest land plant family, behind only the Orchidaceae and Asteraceae, with 730 genera and over 19,400 species...

 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
Helical growth
The 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 phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....

 Porifera (sponges) which have no symmetry. The approximately 400 species of flatfish
Flatfish
The flatfish are an order of ray-finned fish, also called the Heterosomata, sometimes classified as a suborder of Perciformes. In many species, both eyes lie on one side of the head, one or the other migrating through and around the head during development...

also lack symmetry as adults, though the larvae are bilaterally symmetrical.

External links

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