Examples of Symmetry in Phylum Platyhelminthes

Instructor: Joanne Abramson

Joanne has taught middle school and high school science for more than ten years and has a master's degree in education.

Almost all creatures in the animal kingdom show some kind of symmetry. Read on to learn what the Platyhelminthes are and what kind of symmetry they exhibit.

Biological Symmetry

Looking into a mirror, I'm sure you have noticed that you have two eyes and two ears directly across from each other. You also have one nose and a mouth down the center of your face. In fact, it's almost as if one side were a direct reflection of the other. (Though, admittedly, the reflection isn't perfect.) What you are seeing is a wonderful example of bilateral symmetry.

Flatworms, the organisms that make up phylum Platyhelminthes, are a very early and very basic form of multicellular life. As such, they really don't have a lot in common with us humans. One very obvious trait we do have in common, however, is that we both show bilateral symmetry!

Most animals show some type of biological symmetry, meaning that their bodies can be divided into nearly matching halves by drawing a line down the center. There are two primary types of symmetry found in the animal kingdom, radial and bilateral symmetry. (Animals that have no symmetry, such as sponges, are said to be asymmetrical.)

A comparison of symmetry found in the animal kingdom.
Diagram showing examples of radial symmetry, bilateral symmetry and asymmetry.

In radial symmetry, there are multiple ways to draw a line through the animal to create matching halves. This type of symmetry is found in animals such as starfish and jellyfish. In bilateral symmetry, there is only one way to create two matching halves. You do so by drawing a line from the anterior (head) end to the posterior (tail) end, thus creating two sides that seemingly reflect each other. 'Bilateral' literally means 'two sides': bi- means ''two'' and lateral means ''side''. A butterfly, a crab, a dog and many other forms of animal life show bilateral symmetry.

There are multiple evolutionary advantages to bilateral symmetry, which is why many forms of life, especially the most advanced forms, show it. Perhaps the easiest to understand is that it allows us to have more direct movement. Compare the scrambling of a starfish to the precise, straightforward steps of a cat. This comes in very handy when either looking for food or escaping from a predator. Additionally, bilateral symmetry allows us to have a head, and more importantly, a brain. This concentration of nervous tissue allows animals to be smarter and faster, as well as more complex. A larger brain is able to control and manage more specialized organ systems, allowing for more advanced and more specialized animals overall.

Symmetry in Phylum Platyhelminthes

Three different species of flatworms.
Photographs of three flatworm species.

The phylum Platyhelminthes consists of the flatworms, a very early branch of animal life. These worms do not have a circulatory or respiratory system. As such, they are very thin, or flat, so that each cell can respire (breathe) individually with the outside environment. Thus, they are flatworms! Flatworms, like humans, exhibit bilateral symmetry.

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