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.'
For the back of the animal, we use the term dorsal. You can just think of a shark's dorsal fin, and I bet you won't have difficulty remembering this one! Opposite from dorsal on the front side is called ventral. This is where your mouth and nose are located, which can also be thought of as your breathing 'vents.'
In One End and Out the Other
Lastly, but definitely not least, we can classify animals based on how they develop a very important feature - the mouth! There are two ways this critical structure can form during embryonic development - first or second. Animals that develop their mouth before the anus during embryonic development are called protostomes, which literally means 'first mouth.' Contrary to this, during embryonic development deuterostomes form the anus first and mouth second. This is quite fitting, since deuterostome means 'second mouth.'
Remember how I promised you that you are in fact very different from a crayfish? Here's your proof! Crayfish are protostomes, while you are a deuterostome. So even though you both have bilateral symmetry, you both have legs and you both have true coeloms, there is no doubt you are different because this classification is so important that it trumps all the other ones!
Lesson Summary
The animal kingdom is a diverse and complex set of organisms. There are so many different types of animals on Earth that it would take you a long, long time to list them all. Because of this, it's important to organize such diversity appropriately. One of the best ways to do this is by classifying animals based on their body plans. This is the combination and organization of an animal's body structures.
Understanding body plans helps us see how even though some animals may share certain features, they can in fact be quite different. We use an animal's symmetry, body cavity, tissue type, structure location and embryonic development to classify it, which helps us understand not only that animal's role in the ecological world, but also how it may or may not be related to the many other animals it shares the Earth with.
Learning Outcomes
By the end of this lesson you should be able to:
- Determine if an animal has radial or bilateral symmetry
- Explain the difference between an animal with a coelom or pseudocoelom
- Describe an animal's body plan with the correct terminology
