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In biology, symmetry is a feature of certain organisms in which there is a regularity of parts in a plane or around an axis . An organism that is symmetrical, that is, that exhibits symmetry, has an even distribution of duplicate parts on each side of the axis. It may not necessarily be an exact duplicate, but rather a rough repeat. An example of symmetry in organisms is bilateral symmetry.
Bilateral symmetry concept
Bilateral symmetry, also known as planar symmetry, is a form of symmetry in which opposite sides of an organism are similar . The external appearance is the same on the left and right sides in a sagittal plane. The sagittal plane is the body plane of most animals, including humans.
When an organism is bilaterally symmetrical, the body plane is divisible into nearly equal mirror halves in a sagittal plane. However, internal body parts, such as organs, are not necessarily symmetrical.
Animals with bilateral symmetry belong to the taxonomic clade Bilateria. Animals in this clade (grouping of species with a common ancestor), which are also known as bilaterians, have similar left and right sides, and are distinguished from those with another form of symmetry such as radial symmetry (http://www .yubrain.com/ciencia/biologia/la-simetria-radial-en-la-vida-marina/), as well as those that lack symmetry (asymmetry).
Characteristics of a body with bilateral symmetry
Animals with bilateral symmetry have a head, part of the body also known as the anterior or ventral part, that is, the front. They also have a tail, known as the back or dorsal part, and left and right sides.
All vertebrate animals, except those with radial symmetry, are bilaterally symmetric. The evolution of bilateral symmetry and thus the formation of the anterior and posterior ends (head and tail) promoted a phenomenon called cephalization, which refers to the accumulation of an organized nervous system at the anterior end of the animal.
Unlike radial symmetry, which is better suited for stationary or limited-motion lifestyles, bilateral symmetry allows for streamlined, directional movement. These characteristics help animals with bilateral symmetry to expend less energy during movements on land, in the air, and in water.
In evolutionary terms, this simple form of symmetry promoted active mobility and increased sophistication in resource-seeking and predator-prey relationships.
Some examples of bilateral symmetry can be seen in butterflies, mollusks, and humans.
Secondary radial symmetry
Animals of the Echinodermata phylum, such as starfish, clipeasteroids or sand dollars, and sea urchins, show radial symmetry as adults, but their larval stages are bilaterally symmetrical. This is called secondary radial symmetry. Animals with this feature are thought to have evolved from animals with bilateral symmetry.
Bilateral symmetry in plants
Bilateral symmetry is also found in the plant kingdom, however overall symmetry is less important here than among animals. These forms of symmetry are most important in the structure of flowers, which are the fertilization points of angiosperms.
Unlike the animal kingdom, where radially symmetric organisms developed from a nascent bilateral structure, the opposite is true in plants. Many plant phyla have gradually shifted from being radially to bilaterally symmetrical. Much of this is the result of form following function: plants with bilateral symmetry are able to signal to a given pollinator the direction of the fertilizing organs of flowers.
A good way to judge flower symmetry is to classify flowers into the following categories:
- Fused petals equals radial symmetry.
- Free and fully open petals are almost always radially symmetrical.
- Free but closed petals are almost always plants with bilateral symmetry.
Sources
- Photo by Alfred Schrock on Unsplash .
- Natural History Museum. Bilateral Symmetry .
- Nielsen, N. (2016). What it Means to be Bilaterally Symmetrical.
- Prosser, W. A. (2012). Animal Body Plans and Movement: Symmetry in Action . Science decoded.
