# Radial Symmetry in Biology: Definition & Examples

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• 0:01 Definition of Radial Symmetry
• 0:47 Examples of Radial Symmetry
• 2:03 Lesson Summary

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Lesson Transcript
Instructor: Meredith Mikell
One way that animals are broadly categorized is based on their body symmetry. Radial symmetry is one type, and here, we will explore what it is and what organisms exhibit it. Once you get to the end of the lesson, you can test your knowledge with a brief quiz.

Next time you look in the mirror, try drawing an imaginary line right down the middle of your face, as if to divide it into a left and right side. Chances are, it looks about the same on both sides, except for some moles, freckles, and scars. This is your face's bilateral symmetry, equal on both sides, down the middle. But not all organisms have this type of symmetry; in fact, some, like sponges, have no symmetry at all.

Another common type of symmetry is radial symmetry, in which an organism can be divided equally about a central point, much like a pie cut into equal parts. Radial symmetry can be divided equally many times around this central point. It just depends how many pieces of pie you want! One slice represents one equal segment of the whole.

In the plant kingdom, it is common to see radial symmetry in flowers, though the rest of the plant may exhibit other symmetry, or no symmetry at all. Flower petals tend to extend from a central axis into a radial pattern. Fruits frequently have radial symmetry. Think of an orange or apple that has been cut into wedges. The seeds within the fruit are distributed in a radial pattern.

In the animal kingdom, there are two broad phyla that exhibit radial symmetry:

One of these is cnidarians, which include jellyfish, anemones, and corals. Jellyfish exhibit radial symmetry in four points around its center.

Also, echinoderms, such as sea stars, urchins, and sea cucumbers. Sea stars have what is called a pentamerous radial symmetry, meaning it is divided in five ways around its central point.

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