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Animal Body Plans: Classifications and Features

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  • 0:03 What Are Body Plans?
  • 1:02 Types of Symmetry
  • 2:42 Body Cavities and Tissues
  • 4:00 Location of Body Structures
  • 5:29 In One End and Out the Other
  • 6:26 Lesson Summary
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Lesson Transcript
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 Animal Kingdom is diverse and complex. How do we make sense of it all? Scientists use body plans to classify and organize the animals of Earth, making it much easier to differentiate between them.

What Are Body Plans?

If you stop and think about it, you can probably come up with a long list of the different types of animals found on Earth. But there are so many more than that! I bet there are some that you have never even heard of or didn't even know were considered animals.

The diversity of animal life on Earth is indeed quite vast. Fortunately, there are ways to make sense of it. We do this by classifying animals based on certain body structures as well as how those body structures are combined to make a whole animal.

Together the organization and combination of body structures describes an animal's body plan. For example, you have legs, which is one of your body structures. But you are quite different from a crayfish, which also has legs! Our overall combination of body structures - our body plan - is not even close to that of a crayfish, even though we share some of the same structures. And this combination and organization of structures is how we differentiate between different types of animals.

Types of Symmetry

One of the most distinguishing features of animals is their body symmetry. This describes how an animal looks from one side to the other. Some animals have radial symmetry, which means that they look the same on all sides from the center. Think of a bike wheel or pumpkin pie - these also have radial symmetry. No matter where you slice it, each piece looks the same as it radiates outward from the center. Animals like sea anemones, corals and jellies have radial symmetry.

Most of the animals you put on your list probably have bilateral symmetry. This is when each side of the animal looks like a mirror image of the other side. Mammals (including humans) have bilateral symmetry - if you folded us in half, one side would fit perfectly over the other as a mirror image.

You know what else has bilateral symmetry? That crayfish from before! But don't worry, you still aren't going to turn into one any time soon. As we go on, you'll see that despite these similarities, there are important differences between us.

Symmetry is important in animal function, and it tells us quite a bit about the animal's lifestyle. Animals with bilateral symmetry tend to have their brains, mouths and other sense organs (like eyes and ears) up in their heads. This allows them to have mobility - the head goes through the environment first with the eyes and ears ready to take in all of the activity around them.

In contrast, radially symmetric animals are designed for a sedentary lifestyle. Because they are the same on all sides, they're ready for whatever comes at them - no matter the direction of attack!

Body Cavities and Tissues

Another important feature that differentiates animals is whether or not they have a true body cavity, called a coelom. This is a space between the body tissues and internal organs, and it allows for independent movement and growth of those organs. In animals that don't have a coelom, the internal organs are attached to the tissues of the body wall, so they are not independent.

Of course, some animals also fall somewhere in between. These animals have a pseudocoelom, which is a body cavity not completely lined by tissue. Pseudocoeloms work just like true coeloms, even though this name meaning 'false cavity' may suggest otherwise.

Most bilaterally symmetrical animals (including us) are coelomates. Animals, such as roundworms, fall into the pseudocoelomate group, while other worms, such as flatworms, do not have a coelom at all.

In addition to body cavities, we can also differentiate animals by whether or not they have true tissues. Animals that have true tissues are known as eumetazoans. What this means is that they have body tissues that are specialized, like cells that specifically form muscle, ligaments and different organs. You can see how this is an important way to differentiate animals from one another.

Location of Body Structures

Location, location, location! The location of an animal's body structures is by far one of the most important factors, so we need to be able to describe it appropriately.

Think of it as trying to describe a car. If you just say 'right side,' 'left side,' 'front' or 'back,' you are likely going to create more confusion than not. The right side could be either side, depending on where you are standing. So instead, we say 'driver's side' and 'passenger's side,' making it perfectly clear which side of the vehicle we're referring to.

An animal's body plan is no different. Saying 'right,' 'left,' 'up' or 'down' leaves much open to the viewer's interpretation, so instead we have more descriptive terms that clearly describe the location.

For an animal's head, we use the term anterior. The opposite end is posterior, which is the tail end. It might help to think of anterior starting with the letter 'A,' which is the beginning of the alphabet and also the beginning of the animal's body. Posterior has the word 'post' in it, which also means 'after' or 'end.'

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