Bilateral Symmetry

Thomy: Whenever I look at something my nose gets in the way. Why do I always see my nose?

Me: That's a big question. What do you think Squeaker sees?

Thomy: Oh, his nose is way out there.

Me: Yep. He has a long snout and his nose is on the end. What direction does he travel?

Thomy: Straight.

Me: Does he move forward or backward?

Thomy: Forward.

Me: What would happen if his nose was on his butt?

Thomy: He wouldn't be able to smell things.

Me: That's right. He would have to stop and turn around when he wanted to smell something. That's why animals like humans and dogs have their sense organs--like eyes, noses, ears, and mouths--in front where they can sense things as they move forward. These animals are bilaterally symmetrical. What does symmetrical mean?

Thomy: It means the same.

Me: Yes. Syn- means same or joint and -metry means measurement, like metrology or metric. So together they mean "same measurement". Animals that are bilaterally symmetrical are the same on the right side and the left side. What about a starfish? How does it move around?

Thomy: Starfish have eyes on their feet.

Me: That's right, they have an eye on the end of each arm and they move around in any direction so no matter where they go they have an eye out in front. Can you think of another animal that is not symmetrical or has a different kind of symmetry?

Thomy: A germ.

Me: That's true; many single-celled organisms are not symmetrical or have a different form of symmetry. Can you think of a multicellular animal that is not symmetrical or has a different form of symmetry?

Thomy: A sponge.

Me: Yeah, many sponges are not symmetrical or not perfectly symmetrical. They also don't have any sense organs or any nerves either. I think only jellyfish and other animals have nerves.