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What Is Bilateral Symmetry?

By John Markley
Updated May 21, 2024
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Bilateral symmetry, sometimes called reflection symmetry or mirror symmetry, means that an object or shape has two halves that are mirror images of each other. For example, the capital letter T and an isosceles triangle are bilaterally symmetrical. The term is most commonly used in biology, where it refers to organisms that have two symmetrical halves when divided along only one plane. This is called called the saggital plane, an imaginary vertical plane that goes through the center of the organism's body from front to back, splitting it into left and right sides. Most animal species have bilateral symmetry, including humans, and plants often have particular structures that are bilaterally symmetrical.

When the term "bilateral symmetry" is used in reference to living beings rather than geometric forms, the symmetry is only approximate, because the process of biological growth is not perfectly precise, and no organism is perfectly symmetrical. The placement and structure of internal organs also does not need to be perfectly symmetrical for an organism to be considered bilaterally symmetrical. The human liver is located on the right side of the abdominal cavity, for instance, and it is perfectly normal for a person to have asymmetries such as one arm or leg that is slightly longer than the other.

Bilateral symmetry is distinguished from radial symmetry, which refers to forms that can be divided into roughly symmetrical halves along more than one plane. Starfish and jellyfish are examples of radially symmetrical animals. Bilaterally symmetrical organisms, because they are symmetrical on only one plane, have distinct fronts and backs, but radially symmetric organisms do not. Many plants have parts that are bilaterally symmetrical, such as the leaves of trees.

Animal species that have bilateral symmetry are collectively referred to as bilateria. This is a huge category that includes the majority of animal phyla and encompasses an enormously diverse array of organisms, ranging from flatworms to human beings. The oldest confirmed fossil of an organism in this group is believed to be more than 500 million years old.

Bilateral symmetry provides a number of advantages for organisms. It allows an organism to be more streamlined and thus faster-moving. Indeed, it should be noted that animals species that do not have bilateral symmetry are either very slow-moving, as is the case with starfish, or completely sessile, as in the case of organisms such as sponges and see anemones.

A species that has a body specialized to move on a particular axis, rather than equally in all directions, can also concentrate the majority of its sensory organs and associated nervous tissue in the front of its body instead of needing to spread them in all directions. This specialization of the foremost part of the organism's body, called cephalization, can lead to the evolution of more complex behavior and greater intelligence as nervous tissue becomes centralized. All organisms with more than a very rudimentary nervous system are bilateria. The complex brains of humans and other highly intelligent animals have their evolutionary origins in this process.

Frequently Asked Questions

What is bilateral symmetry in animals?

Bilateral symmetry refers to a body plan in which an organism can be divided into two equal halves along a single plane, typically down the middle, resulting in mirror-image sides. This is a common trait in the animal kingdom, including humans, and is associated with efficient movement and the development of a head region with sensory organs.

Why is bilateral symmetry important in nature?

Bilateral symmetry is crucial for mobility and sensory orientation. It allows for streamlined movement and forward progression, which is advantageous for hunting, escaping predators, and exploring environments. According to evolutionary biology, this symmetry has evolved to help organisms better adapt to their surroundings and increase their chances of survival.

Can you give examples of animals with bilateral symmetry?

Most animals exhibit bilateral symmetry, including humans, fish, birds, and mammals. For instance, a tiger's body can be split down the middle to yield two mirror-image halves. Even invertebrates like butterflies and bees display this symmetry, with their left and right wings being identical in shape and size.

How does bilateral symmetry differ from radial symmetry?

Bilateral symmetry involves a single plane of symmetry creating two mirrored halves, while radial symmetry involves multiple planes of symmetry passing through a central axis, as seen in starfish or jellyfish. Radial symmetry is typically found in sessile or slow-moving animals, whereas bilateral symmetry is associated with active, mobile organisms.

Are there any disadvantages to bilateral symmetry?

While bilateral symmetry offers many advantages, it can also present challenges. For example, injury to one side of the body can result in loss of symmetry, affecting mobility and function. Additionally, the evolution of a centralized nervous system can make bilaterally symmetrical animals more susceptible to head injuries with severe consequences.

How does bilateral symmetry relate to the evolution of the brain?

Bilateral symmetry is closely linked to the evolution of a centralized nervous system and brain. This body plan allows for the development of a distinct head region, or cephalization, which houses the brain and sensory organs. This configuration enhances directional movement and complex behaviors, contributing to the evolutionary success of bilaterally symmetrical animals.

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