Bilateral Symmetry: Definition, Examples & Advantages

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  • 0:00 Defining Symmetry
  • 1:18 Examples of Bilateral Symmetry
  • 2:01 Advantages of…
  • 3:10 Lesson Summary
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Lesson Transcript
Instructor: Adrianne Baron

Adrianne has a master's degree in cancer biology and has taught high school and college biology.

We will learn all about bilateral symmetry and look at some examples. The advantages of bilateral symmetry will also be discussed. Then, you can take the quiz to check your knowledge.

Defining Symmetry

In order to fully define bilateral symmetry, we need to first define symmetry. Symmetry has to do with the orientation of an organism based on a plane or around an axis. Considering the different shapes and orientations of various organisms, scientists have come up with three basic types of symmetry.

The three types of body symmetry
Diagram showing the different types of symmetry

The first type is radial symmetry. With this type of symmetry, the body plan is based around an axis. In other words, the body is oriented so that it radiates out from an imaginary line through the center of the organism. These organisms have a top and bottom, but they don't have a left and right side or a front and back side. A couple examples of radial symmetry are starfish, jellyfish, and sea anemones.

Picture of sea anemone

There are some organisms that don't display any symmetry at all. These are classified as being asymmetrical. The only animals that really belong to this classification are sponges.

Picture of sponge

The last type of symmetry is the bilateral symmetry. Bilateral symmetry is when the body plan can be divided along a plane that splits the animal's body into right and left sides that are mirror images of each other. Let's look at this type of symmetry a little more.

Examples of Bilateral Symmetry

So, now you may be trying to think of the different animals that display bilateral symmetry. Well, guess what? You are the first example that we're going to discuss. Yep, we humans are an example of bilateral symmetry. Go ahead and take a look in the mirror and see for yourself. We could draw a line straight down the middle of your body, right through your nose, and divide you into right and left mirror images. Even your brain can be divided into equal right and left sides. Okay, you get the picture.

The brain shows bilateral symmetry.
Diagram of brain showing bilateral symmetry

Let's look at another example. Got a pet dog or cat? They also exhibit bilateral symmetry. Other examples that you may not have thought about are sharks, butterflies, and ants.

Advantages of Bilateral Symmetry

So, there are actually some real advantages to having bilateral symmetry. The fact that we have two eyes and ears means that we can see and hear more than most animals with radial symmetry. Bilateral symmetry also caused the formation of a head and tail area. This means that things can go in one end and come out the other, as opposed to everything having to use the same opening. Without getting into too much detail, let's just say that I'm sure we're all pretty happy about that!

Another advantage is that bilateral symmetry allowed for the development of a more thorough nervous system that can control the body which is located in the head region.

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