What Is Bilateral Symmetry?

How It Is Used In Classifying Marine Organisms

Close-Up Of Turtle In Sea
Sea Turtles are an example of an animal that has bilateral symmetry. Gabriel Visintin / EyeEm / Getty Images

Bilateral symmetry is the arrangement of an organism's body parts into left and right halves on either side of a central axis, or plane. Essentially, if you draw a line from the head to the tail of an organism -- or a plane -- both sides are mirror images. In that case, the organism exhibits bilateral symmetry. Bilateral symmetry is also known as plane symmetry as one plane divides an organism into mirrored halves.

The term "bilateral" has roots in Latin with bis ("two") and latus ("side"). The word "symmetry" is derived from the Greek words syn ("together") and metron ("meter").

Most animals on the planet exhibit bilateral symmetry. This includes human beings, as our bodies can be cut down the middle and have mirrored sides. In the marine biology field, many students will study this when they begin learning about classifying marine life.

Bilateral vs. Radial Symmetry

Bilateral symmetry differs from radial symmetry. In that case, the radially symmetric organisms are similar to a pie shape, where each piece is nearly identical though they do not have left or right sides; instead, they have a top and bottom surface.

Organisms that exhibit radial symmetry include aquatic cnidarians, including corals. It also includes jellyfish and sea anemones. Dchinoderms are another group that includes sand dollars, sea urchins, and starfish; meaning they have a five-point radial symmetry. 

Traits of Bilaterally Symmetrical Organisms

Organisms that are bilaterally symmetrical demonstrate a head and a tail (anterior and posterior) regions, a top and a bottom (dorsal and ventral), as well as left and right sides. Most of these animals have a complex brain in their heads, which are part of their nervous systems. Typically, they move more quickly than animals that do not demonstrate bilateral symmetry. They also tend to have improved eyesight and hearing capabilities compared to those that have radial symmetry.

Mostly all marine organisms, including all vertebrates and some invertebrates are bilaterally symmetrical. This includes marine mammals such as dolphins and whales, fish, lobsters, and sea turtles. Interestingly, some animals have one type of body symmetry when they are first life forms, but they develop differently as they grow. 

There is one marine animal that does not display symmetry at all: Sponges. These organisms are multicellular but are the only classification of animals that are asymmetrical. They do not show any symmetry at all. That means there is no place in their bodies where you could drive a plane into cut them in half and see mirrored images. 

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Kennedy, Jennifer. "What Is Bilateral Symmetry?" ThoughtCo, Aug. 27, 2020, thoughtco.com/what-is-bilateral-symmetry-3970965. Kennedy, Jennifer. (2020, August 27). What Is Bilateral Symmetry? Retrieved from https://www.thoughtco.com/what-is-bilateral-symmetry-3970965 Kennedy, Jennifer. "What Is Bilateral Symmetry?" ThoughtCo. https://www.thoughtco.com/what-is-bilateral-symmetry-3970965 (accessed March 26, 2023).