Body Symmetry in Nematodes

Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

The phylum Nematoda is a diverse and abundant one on Earth. In this lesson, you'll learn about the type of body symmetry that these roundworms exhibit.

What are Nematodes?

With over 25,000 named species, my guess is you've at least come across a nematode once or twice in your life. These guys are commonly known as roundworms. Some are free-living animals, but many of them are parasitic, meaning that they live off of other organisms.

Nematodes live in all different environments, and many are parasitic pests.

Nematodes inhabit all different types of environments, too. Some live in the soil, performing the important function of decomposing dead and decaying material. Others live in freshwater and marine environments. And, as we already discussed, some live within other organisms.

Nematode Body Plans

Although there is great diversity among nematodes, their body plans are actually quite similar. Some exist as very small organisms (only a millimeter or two), while others may be more than 7 meters long! But in general they have some things in common. For example, their body symmetry. Nematodes have what is called bilateral symmetry ('bi' means 'two'). This means that their bodies can be divided into two halves that are mirror images of each other.

This is very much like you and me. If you take a good look at your own body, you will notice that your right and left sides are mirror images of each other as well, and we see this same symmetry in the nematode worms.

Bilaterally symmetric organisms have two halves that are mirror images of each other.
bilateral symmetry

This might be hard to imagine in a roundworm, though, because there are no arms, legs, or any other external structures like this to cue you in to the two identical halves. But if you were to cut that nematode right down the middle, you would certainly find that each inside half was identical to the other. For example, there are four nerve cords that run longitudinally down the length of nematodes. One on the back, one on the belly, and one on each side. Cut the worm in half and you'd see these nerve cords mirrored on each side.

Like the spokes of a wheel, radially symmetric structures are the same on all sides.
radially symmetric

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