Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

You may feel like you understand symmetry in biology, but I am going to throw a wrench into your understanding with biradial symmetry. This lesson will introduce this term and will discuss how it fits in with bilateral and radial symmetry.

## Types of Symmetry

You've probably heard of bilateral symmetry and radial symmetry, but what about biradial symmetry? Nope, this isn't some made-up up word, it's kind of a bilateral and radial symmetry intermediate. Even the name is an intermediate: Bi+radial. Before we look at biradial symmetry, let's take a closer look at bilateral and radial symmetry.

Bilateral symmetry is a type of symmetry where the organism can be divided down the middle, and the two halves are mirror images of each other. The word bilateral means two sides. In bilateral symmetry one plane divides the organism into two parts. You, by the way, have bilateral symmetry.

Radial symmetry occurs when the symmetry exists around a center and the organism can be divided into similar pieces (like a cake). More specifically, more than two planes will divide the orgasm into identical parts. Jellyfish and sea anemones are some animals with this body plan.

And now the one you've been waiting for: biradial symmetry, which is when the organism can be divided up into equal parts, but only with two planes. It is different than radial symmetry, because two planes divide the organism, but not more than two. Comb jellies are an example of an organism with biradial symmetry.

Now that you have an idea of what biradial symmetry is, let's take a moment to explore it in a little more detail. Some scientists believe that animals with bilateral symmetry evolved from those with radial symmetry. Why? Radial symmetry is great for animals that don't move a whole lot, or depend on water to help them move. But being bilateral is better for organisms that want to move in a specific direction. As organisms evolved, certain environments selected for a bilateral body plan over a radial plan. Today around 99% of animals have bilateral symmetry, so it is assumed it provides advantages that radial symmetry does not provide (i.e. being able to go where you want to go).

Some scientists believe that the biradial animals are an intermediate held over from the transition from radial to bilateral symmetry. The ctenophore phylum, or the comb jellies, exhibit this type of symmetry. This group of animals lives in marine environments and propel themselves using cilia combs, which are tiny hair like structures usually associated with bacteria and not animals.

There are many species within this phylum, with varying body shapes and sizes. If you were to divide the comb jelly in half from mouth to their sensory organ (called the statocyst), they would show radial symmetry, however the lower portions of the comb jelly have tentacles that negate radial symmetry (i.e. the animal could not be divided up like a pie with similar pieces).

If you had to choose a side for comb jellies (either radial or bilateral), they are probably more like the radially symmetrical animals over the bilateral animals. While some can swim at 5 centimeters per second, they also depend upon the water current to move them.

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