Bilateral Symmetry

Definition and Examples in Marine Life

Seal Pup, Canada
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Bilateral symmetry is a body plan in which the body can be divided into mirror images along a central axis.

In this article, you can learn more about symmetry, advantages of bilateral symmetry and examples of marine life that exhibit bilateral symmetry.

What Is Symmetry?

Symmetry is the arrangement of shapes or body parts so that they are equal on each side of a dividing line. In an animal, this describes the way its body parts are arranged around a central axis. 

There are several types of symmetry found in marine organisms. The two main types are bilateral symmetry and radial symmetry, but organisms may also exhibit pentaradial symmetry or biradial symmetry. Some organisms are asymmetrical. Sponges are the only asymmetrical marine animal.

Definition of Bilateral Symmetry

Bilateral symmetry is the arrangement of body parts into left and right halves on either side of a central axis. When an organism is bilaterally symmetrical, you can draw an imaginary line (this is called the sagittal plane) from the tip of its snout to the tip of its back end, and on either side of this line would be halves that are mirror images of each other.

In a bilaterally symmetrical organism, only one plane can divide the organism into mirror images. This can also be called left/right symmetry. The right and left halves aren't exactly the same. For example, the right flipper of a whale may be a little larger or differently shaped than the left flipper. 

Many animals, including humans, exhibit bilateral symmetry. For example, the fact that we have an eye, arm, and leg in about the same place on each side of our bodies makes us bilaterally symmetrical.

Bilateral Symmetry Etymology

The term bilateral can be traced to the Latin bis ("two") and latus ("side"). The word symmetry comes from the Greek words syn ("together") and metron ("meter").

Characteristics of Animals That Are Bilaterally Symmetrical

Animals that exhibit bilateral symmetry typically have head and tail (anterior and posterior) regions, a top and a bottom (dorsal and ventral) and left and right sides. Most have a complex brain that is located in the head, which is part of a well-developed nervous system and may even have right and left sides. They also usually have eyes and a mouth located in this region.

In addition to having a more developed nervous system, bilaterally symmetrical animals can move more quickly than animals with other body plans. This bilaterally symmetrical body plan may have evolved to help animals better find food or escape predators. Also, having a head and tail region means that waste is eliminated in a different region from where food is eaten - definitely a perk for us! 

Animals with bilateral symmetry also have better eyesight and hearing than those with radial symmetry.

Examples of Bilateral Symmetry

Humans and many other animals exhibit bilateral symmetry. In the ocean world, most marine creatures, including all vertebrates and some invertebrates exhibit bilateral symmetry. Following are examples of marine life profiled on this site that exhibit bilateral symmetry:

References and Further Information

  • Morrissey, J.F. and J.L. Sumich. 2012. Introduction to the Biology of Marine Life (10th Edition). Jones & Bartlett Learning. 467pp.
  • Natural History Museum. Bilateral Symmetry. Accessed June 16, 2015.
  • Prosser, W. A. M. 2012. Animal Body Plans and Movement: Symmetry in Action. Decoded Science. Accessed February 28, 2016.
  • University of California Museum of Paleontology. Bilateral (left/right) Symmetry. Understanding Evolution. Accessed February 28, 2016. 
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Your Citation
Kennedy, Jennifer. "Bilateral Symmetry." ThoughtCo, Aug. 26, 2020, Kennedy, Jennifer. (2020, August 26). Bilateral Symmetry. Retrieved from Kennedy, Jennifer. "Bilateral Symmetry." ThoughtCo. (accessed March 27, 2023).