In this lesson, we’ll explore a group of organisms known as cnidarians. We’ll learn about their general features and also examine what type of symmetry they have.
Types of Symmetry
Have you ever tried to cut a cake or pie so that each person gets an equal piece? This is an example of trying to maintain symmetry. Symmetry is when something is divided into equal parts.The concept of symmetry can also be applied to organisms. Most animals exhibit either bilateral or radial symmetry.Bilateral symmetry is when the body or object can be divided into left and right sides that are mirror images of each other. The example below shows that the letter ‘M’ has bilateral symmetry.
When you cut the letter ‘M’ in half, you end up with exact copies on the left and right sides.Radial symmetry occurs when the parts of an animal or object are arranged around a central axis, and if they’re divided through the axis, they create equal parts on either side. The example below shows that the square has radial symmetry. If you draw a straight line through the center of the square in any direction, you’ll end up with two equal parts. The cake and pie from the beginning of the lesson are also examples of radial symmetry.
Body Symmetry of Cnidarians
Although cnidarians come in many shapes and sizes, they share a basic body plan. Cnidarians exhibit either the polyp or medusa body plan.
The polyp body plan is cylindrical with one end attached to a surface and the other end containing a mouth surrounded by tentacles. When cnidarians are in the polyp form, they attach to a surface and remain stationary.Some cnidarians, such as hydra, stay in this polyp form their whole lives, while other organisms start in the polyp form and transform into the medusa body plan. The medusa body plan is umbrella-shaped and has a mouth surrounded by tentacles on the underside. Organisms in the medusa stage, such as jellyfish, live free in the water and swim around.Based on both the polyp and medusa body plans, cnidarians are classified as having radial symmetry. All cnidarians have a central point or axis, and if you divide a cnidarian through this central axis, you’ll end up with equal parts on either side.
Even though cnidarians vary a great deal, they all have radial symmetry as a common characteristic. The images below show how a sea anemone and jellyfish have radial symmetry, while a lobster has bilateral symmetry.
The next time you cut slices of a pie, maybe you’ll think of the different types of symmetry. Bilateral symmetry is when the body or object can be divided into left and right sides that are mirror images of each other. Radial symmetry occurs when the parts of an animal or object are arranged around a central axis, and if they are divided through the axis, they create equal parts on either side.In this lesson, we explored a group of organisms known as cnidarians and discussed their body symmetry. In general, cnidarians are simple organisms found in saltwater that exhibit a polyp, or cylindrical, shape or a medusa, or umbrella, shape.
Members of the group include jellyfish, sea anemones, and hydra. They are a diverse group, but all organisms in this phylum have radial symmetry and can be divided into equal parts through a central axis – just like a pie!